1======================== 2ftrace - Function Tracer 3======================== 4 5Copyright 2008 Red Hat Inc. 6 7:Author: Steven Rostedt <srostedt@redhat.com> 8:License: The GNU Free Documentation License, Version 1.2 9 (dual licensed under the GPL v2) 10:Original Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton, 11 John Kacur, and David Teigland. 12 13- Written for: 2.6.28-rc2 14- Updated for: 3.10 15- Updated for: 4.13 - Copyright 2017 VMware Inc. Steven Rostedt 16- Converted to rst format - Changbin Du <changbin.du@intel.com> 17 18Introduction 19------------ 20 21Ftrace is an internal tracer designed to help out developers and 22designers of systems to find what is going on inside the kernel. 23It can be used for debugging or analyzing latencies and 24performance issues that take place outside of user-space. 25 26Although ftrace is typically considered the function tracer, it 27is really a framework of several assorted tracing utilities. 28There's latency tracing to examine what occurs between interrupts 29disabled and enabled, as well as for preemption and from a time 30a task is woken to the task is actually scheduled in. 31 32One of the most common uses of ftrace is the event tracing. 33Throughout the kernel is hundreds of static event points that 34can be enabled via the tracefs file system to see what is 35going on in certain parts of the kernel. 36 37See events.rst for more information. 38 39 40Implementation Details 41---------------------- 42 43See Documentation/trace/ftrace-design.rst for details for arch porters and such. 44 45 46The File System 47--------------- 48 49Ftrace uses the tracefs file system to hold the control files as 50well as the files to display output. 51 52When tracefs is configured into the kernel (which selecting any ftrace 53option will do) the directory /sys/kernel/tracing will be created. To mount 54this directory, you can add to your /etc/fstab file:: 55 56 tracefs /sys/kernel/tracing tracefs defaults 0 0 57 58Or you can mount it at run time with:: 59 60 mount -t tracefs nodev /sys/kernel/tracing 61 62For quicker access to that directory you may want to make a soft link to 63it:: 64 65 ln -s /sys/kernel/tracing /tracing 66 67.. attention:: 68 69 Before 4.1, all ftrace tracing control files were within the debugfs 70 file system, which is typically located at /sys/kernel/debug/tracing. 71 For backward compatibility, when mounting the debugfs file system, 72 the tracefs file system will be automatically mounted at: 73 74 /sys/kernel/debug/tracing 75 76 All files located in the tracefs file system will be located in that 77 debugfs file system directory as well. 78 79.. attention:: 80 81 Any selected ftrace option will also create the tracefs file system. 82 The rest of the document will assume that you are in the ftrace directory 83 (cd /sys/kernel/tracing) and will only concentrate on the files within that 84 directory and not distract from the content with the extended 85 "/sys/kernel/tracing" path name. 86 87That's it! (assuming that you have ftrace configured into your kernel) 88 89After mounting tracefs you will have access to the control and output files 90of ftrace. Here is a list of some of the key files: 91 92 93 Note: all time values are in microseconds. 94 95 current_tracer: 96 97 This is used to set or display the current tracer 98 that is configured. Changing the current tracer clears 99 the ring buffer content as well as the "snapshot" buffer. 100 101 available_tracers: 102 103 This holds the different types of tracers that 104 have been compiled into the kernel. The 105 tracers listed here can be configured by 106 echoing their name into current_tracer. 107 108 tracing_on: 109 110 This sets or displays whether writing to the trace 111 ring buffer is enabled. Echo 0 into this file to disable 112 the tracer or 1 to enable it. Note, this only disables 113 writing to the ring buffer, the tracing overhead may 114 still be occurring. 115 116 The kernel function tracing_off() can be used within the 117 kernel to disable writing to the ring buffer, which will 118 set this file to "0". User space can re-enable tracing by 119 echoing "1" into the file. 120 121 Note, the function and event trigger "traceoff" will also 122 set this file to zero and stop tracing. Which can also 123 be re-enabled by user space using this file. 124 125 trace: 126 127 This file holds the output of the trace in a human 128 readable format (described below). Opening this file for 129 writing with the O_TRUNC flag clears the ring buffer content. 130 Note, this file is not a consumer. If tracing is off 131 (no tracer running, or tracing_on is zero), it will produce 132 the same output each time it is read. When tracing is on, 133 it may produce inconsistent results as it tries to read 134 the entire buffer without consuming it. 135 136 trace_pipe: 137 138 The output is the same as the "trace" file but this 139 file is meant to be streamed with live tracing. 140 Reads from this file will block until new data is 141 retrieved. Unlike the "trace" file, this file is a 142 consumer. This means reading from this file causes 143 sequential reads to display more current data. Once 144 data is read from this file, it is consumed, and 145 will not be read again with a sequential read. The 146 "trace" file is static, and if the tracer is not 147 adding more data, it will display the same 148 information every time it is read. 149 150 trace_options: 151 152 This file lets the user control the amount of data 153 that is displayed in one of the above output 154 files. Options also exist to modify how a tracer 155 or events work (stack traces, timestamps, etc). 156 157 options: 158 159 This is a directory that has a file for every available 160 trace option (also in trace_options). Options may also be set 161 or cleared by writing a "1" or "0" respectively into the 162 corresponding file with the option name. 163 164 tracing_max_latency: 165 166 Some of the tracers record the max latency. 167 For example, the maximum time that interrupts are disabled. 168 The maximum time is saved in this file. The max trace will also be 169 stored, and displayed by "trace". A new max trace will only be 170 recorded if the latency is greater than the value in this file 171 (in microseconds). 172 173 By echoing in a time into this file, no latency will be recorded 174 unless it is greater than the time in this file. 175 176 tracing_thresh: 177 178 Some latency tracers will record a trace whenever the 179 latency is greater than the number in this file. 180 Only active when the file contains a number greater than 0. 181 (in microseconds) 182 183 buffer_percent: 184 185 This is the watermark for how much the ring buffer needs to be filled 186 before a waiter is woken up. That is, if an application calls a 187 blocking read syscall on one of the per_cpu trace_pipe_raw files, it 188 will block until the given amount of data specified by buffer_percent 189 is in the ring buffer before it wakes the reader up. This also 190 controls how the splice system calls are blocked on this file:: 191 192 0 - means to wake up as soon as there is any data in the ring buffer. 193 50 - means to wake up when roughly half of the ring buffer sub-buffers 194 are full. 195 100 - means to block until the ring buffer is totally full and is 196 about to start overwriting the older data. 197 198 buffer_size_kb: 199 200 This sets or displays the number of kilobytes each CPU 201 buffer holds. By default, the trace buffers are the same size 202 for each CPU. The displayed number is the size of the 203 CPU buffer and not total size of all buffers. The 204 trace buffers are allocated in pages (blocks of memory 205 that the kernel uses for allocation, usually 4 KB in size). 206 A few extra pages may be allocated to accommodate buffer management 207 meta-data. If the last page allocated has room for more bytes 208 than requested, the rest of the page will be used, 209 making the actual allocation bigger than requested or shown. 210 ( Note, the size may not be a multiple of the page size 211 due to buffer management meta-data. ) 212 213 Buffer sizes for individual CPUs may vary 214 (see "per_cpu/cpu0/buffer_size_kb" below), and if they do 215 this file will show "X". 216 217 buffer_total_size_kb: 218 219 This displays the total combined size of all the trace buffers. 220 221 buffer_subbuf_size_kb: 222 223 This sets or displays the sub buffer size. The ring buffer is broken up 224 into several same size "sub buffers". An event can not be bigger than 225 the size of the sub buffer. Normally, the sub buffer is the size of the 226 architecture's page (4K on x86). The sub buffer also contains meta data 227 at the start which also limits the size of an event. That means when 228 the sub buffer is a page size, no event can be larger than the page 229 size minus the sub buffer meta data. 230 231 Note, the buffer_subbuf_size_kb is a way for the user to specify the 232 minimum size of the subbuffer. The kernel may make it bigger due to the 233 implementation details, or simply fail the operation if the kernel can 234 not handle the request. 235 236 Changing the sub buffer size allows for events to be larger than the 237 page size. 238 239 Note: When changing the sub-buffer size, tracing is stopped and any 240 data in the ring buffer and the snapshot buffer will be discarded. 241 242 free_buffer: 243 244 If a process is performing tracing, and the ring buffer should be 245 shrunk "freed" when the process is finished, even if it were to be 246 killed by a signal, this file can be used for that purpose. On close 247 of this file, the ring buffer will be resized to its minimum size. 248 Having a process that is tracing also open this file, when the process 249 exits its file descriptor for this file will be closed, and in doing so, 250 the ring buffer will be "freed". 251 252 It may also stop tracing if disable_on_free option is set. 253 254 tracing_cpumask: 255 256 This is a mask that lets the user only trace on specified CPUs. 257 The format is a hex string representing the CPUs. 258 259 set_ftrace_filter: 260 261 When dynamic ftrace is configured in (see the 262 section below "dynamic ftrace"), the code is dynamically 263 modified (code text rewrite) to disable calling of the 264 function profiler (mcount). This lets tracing be configured 265 in with practically no overhead in performance. This also 266 has a side effect of enabling or disabling specific functions 267 to be traced. Echoing names of functions into this file 268 will limit the trace to only those functions. 269 This influences the tracers "function" and "function_graph" 270 and thus also function profiling (see "function_profile_enabled"). 271 272 The functions listed in "available_filter_functions" are what 273 can be written into this file. 274 275 This interface also allows for commands to be used. See the 276 "Filter commands" section for more details. 277 278 As a speed up, since processing strings can be quite expensive 279 and requires a check of all functions registered to tracing, instead 280 an index can be written into this file. A number (starting with "1") 281 written will instead select the same corresponding at the line position 282 of the "available_filter_functions" file. 283 284 set_ftrace_notrace: 285 286 This has an effect opposite to that of 287 set_ftrace_filter. Any function that is added here will not 288 be traced. If a function exists in both set_ftrace_filter 289 and set_ftrace_notrace, the function will _not_ be traced. 290 291 set_ftrace_pid: 292 293 Have the function tracer only trace the threads whose PID are 294 listed in this file. 295 296 If the "function-fork" option is set, then when a task whose 297 PID is listed in this file forks, the child's PID will 298 automatically be added to this file, and the child will be 299 traced by the function tracer as well. This option will also 300 cause PIDs of tasks that exit to be removed from the file. 301 302 set_ftrace_notrace_pid: 303 304 Have the function tracer ignore threads whose PID are listed in 305 this file. 306 307 If the "function-fork" option is set, then when a task whose 308 PID is listed in this file forks, the child's PID will 309 automatically be added to this file, and the child will not be 310 traced by the function tracer as well. This option will also 311 cause PIDs of tasks that exit to be removed from the file. 312 313 If a PID is in both this file and "set_ftrace_pid", then this 314 file takes precedence, and the thread will not be traced. 315 316 set_event_pid: 317 318 Have the events only trace a task with a PID listed in this file. 319 Note, sched_switch and sched_wake_up will also trace events 320 listed in this file. 321 322 To have the PIDs of children of tasks with their PID in this file 323 added on fork, enable the "event-fork" option. That option will also 324 cause the PIDs of tasks to be removed from this file when the task 325 exits. 326 327 set_event_notrace_pid: 328 329 Have the events not trace a task with a PID listed in this file. 330 Note, sched_switch and sched_wakeup will trace threads not listed 331 in this file, even if a thread's PID is in the file if the 332 sched_switch or sched_wakeup events also trace a thread that should 333 be traced. 334 335 To have the PIDs of children of tasks with their PID in this file 336 added on fork, enable the "event-fork" option. That option will also 337 cause the PIDs of tasks to be removed from this file when the task 338 exits. 339 340 set_graph_function: 341 342 Functions listed in this file will cause the function graph 343 tracer to only trace these functions and the functions that 344 they call. (See the section "dynamic ftrace" for more details). 345 Note, set_ftrace_filter and set_ftrace_notrace still affects 346 what functions are being traced. 347 348 set_graph_notrace: 349 350 Similar to set_graph_function, but will disable function graph 351 tracing when the function is hit until it exits the function. 352 This makes it possible to ignore tracing functions that are called 353 by a specific function. 354 355 available_filter_functions: 356 357 This lists the functions that ftrace has processed and can trace. 358 These are the function names that you can pass to 359 "set_ftrace_filter", "set_ftrace_notrace", 360 "set_graph_function", or "set_graph_notrace". 361 (See the section "dynamic ftrace" below for more details.) 362 363 available_filter_functions_addrs: 364 365 Similar to available_filter_functions, but with address displayed 366 for each function. The displayed address is the patch-site address 367 and can differ from /proc/kallsyms address. 368 369 dyn_ftrace_total_info: 370 371 This file is for debugging purposes. The number of functions that 372 have been converted to nops and are available to be traced. 373 374 enabled_functions: 375 376 This file is more for debugging ftrace, but can also be useful 377 in seeing if any function has a callback attached to it. 378 Not only does the trace infrastructure use ftrace function 379 trace utility, but other subsystems might too. This file 380 displays all functions that have a callback attached to them 381 as well as the number of callbacks that have been attached. 382 Note, a callback may also call multiple functions which will 383 not be listed in this count. 384 385 If the callback registered to be traced by a function with 386 the "save regs" attribute (thus even more overhead), a 'R' 387 will be displayed on the same line as the function that 388 is returning registers. 389 390 If the callback registered to be traced by a function with 391 the "ip modify" attribute (thus the regs->ip can be changed), 392 an 'I' will be displayed on the same line as the function that 393 can be overridden. 394 395 If a non ftrace trampoline is attached (BPF) a 'D' will be displayed. 396 Note, normal ftrace trampolines can also be attached, but only one 397 "direct" trampoline can be attached to a given function at a time. 398 399 Some architectures can not call direct trampolines, but instead have 400 the ftrace ops function located above the function entry point. In 401 such cases an 'O' will be displayed. 402 403 If a function had either the "ip modify" or a "direct" call attached to 404 it in the past, a 'M' will be shown. This flag is never cleared. It is 405 used to know if a function was every modified by the ftrace infrastructure, 406 and can be used for debugging. 407 408 If the architecture supports it, it will also show what callback 409 is being directly called by the function. If the count is greater 410 than 1 it most likely will be ftrace_ops_list_func(). 411 412 If the callback of a function jumps to a trampoline that is 413 specific to the callback and which is not the standard trampoline, 414 its address will be printed as well as the function that the 415 trampoline calls. 416 417 touched_functions: 418 419 This file contains all the functions that ever had a function callback 420 to it via the ftrace infrastructure. It has the same format as 421 enabled_functions but shows all functions that have every been 422 traced. 423 424 To see any function that has every been modified by "ip modify" or a 425 direct trampoline, one can perform the following command: 426 427 grep ' M ' /sys/kernel/tracing/touched_functions 428 429 function_profile_enabled: 430 431 When set it will enable all functions with either the function 432 tracer, or if configured, the function graph tracer. It will 433 keep a histogram of the number of functions that were called 434 and if the function graph tracer was configured, it will also keep 435 track of the time spent in those functions. The histogram 436 content can be displayed in the files: 437 438 trace_stat/function<cpu> ( function0, function1, etc). 439 440 trace_stat: 441 442 A directory that holds different tracing stats. 443 444 kprobe_events: 445 446 Enable dynamic trace points. See kprobetrace.rst. 447 448 kprobe_profile: 449 450 Dynamic trace points stats. See kprobetrace.rst. 451 452 max_graph_depth: 453 454 Used with the function graph tracer. This is the max depth 455 it will trace into a function. Setting this to a value of 456 one will show only the first kernel function that is called 457 from user space. 458 459 printk_formats: 460 461 This is for tools that read the raw format files. If an event in 462 the ring buffer references a string, only a pointer to the string 463 is recorded into the buffer and not the string itself. This prevents 464 tools from knowing what that string was. This file displays the string 465 and address for the string allowing tools to map the pointers to what 466 the strings were. 467 468 saved_cmdlines: 469 470 Only the pid of the task is recorded in a trace event unless 471 the event specifically saves the task comm as well. Ftrace 472 makes a cache of pid mappings to comms to try to display 473 comms for events. If a pid for a comm is not listed, then 474 "<...>" is displayed in the output. 475 476 If the option "record-cmd" is set to "0", then comms of tasks 477 will not be saved during recording. By default, it is enabled. 478 479 saved_cmdlines_size: 480 481 By default, 128 comms are saved (see "saved_cmdlines" above). To 482 increase or decrease the amount of comms that are cached, echo 483 the number of comms to cache into this file. 484 485 saved_tgids: 486 487 If the option "record-tgid" is set, on each scheduling context switch 488 the Task Group ID of a task is saved in a table mapping the PID of 489 the thread to its TGID. By default, the "record-tgid" option is 490 disabled. 491 492 snapshot: 493 494 This displays the "snapshot" buffer and also lets the user 495 take a snapshot of the current running trace. 496 See the "Snapshot" section below for more details. 497 498 stack_max_size: 499 500 When the stack tracer is activated, this will display the 501 maximum stack size it has encountered. 502 See the "Stack Trace" section below. 503 504 stack_trace: 505 506 This displays the stack back trace of the largest stack 507 that was encountered when the stack tracer is activated. 508 See the "Stack Trace" section below. 509 510 stack_trace_filter: 511 512 This is similar to "set_ftrace_filter" but it limits what 513 functions the stack tracer will check. 514 515 trace_clock: 516 517 Whenever an event is recorded into the ring buffer, a 518 "timestamp" is added. This stamp comes from a specified 519 clock. By default, ftrace uses the "local" clock. This 520 clock is very fast and strictly per cpu, but on some 521 systems it may not be monotonic with respect to other 522 CPUs. In other words, the local clocks may not be in sync 523 with local clocks on other CPUs. 524 525 Usual clocks for tracing:: 526 527 # cat trace_clock 528 [local] global counter x86-tsc 529 530 The clock with the square brackets around it is the one in effect. 531 532 local: 533 Default clock, but may not be in sync across CPUs 534 535 global: 536 This clock is in sync with all CPUs but may 537 be a bit slower than the local clock. 538 539 counter: 540 This is not a clock at all, but literally an atomic 541 counter. It counts up one by one, but is in sync 542 with all CPUs. This is useful when you need to 543 know exactly the order events occurred with respect to 544 each other on different CPUs. 545 546 uptime: 547 This uses the jiffies counter and the time stamp 548 is relative to the time since boot up. 549 550 perf: 551 This makes ftrace use the same clock that perf uses. 552 Eventually perf will be able to read ftrace buffers 553 and this will help out in interleaving the data. 554 555 x86-tsc: 556 Architectures may define their own clocks. For 557 example, x86 uses its own TSC cycle clock here. 558 559 ppc-tb: 560 This uses the powerpc timebase register value. 561 This is in sync across CPUs and can also be used 562 to correlate events across hypervisor/guest if 563 tb_offset is known. 564 565 mono: 566 This uses the fast monotonic clock (CLOCK_MONOTONIC) 567 which is monotonic and is subject to NTP rate adjustments. 568 569 mono_raw: 570 This is the raw monotonic clock (CLOCK_MONOTONIC_RAW) 571 which is monotonic but is not subject to any rate adjustments 572 and ticks at the same rate as the hardware clocksource. 573 574 boot: 575 This is the boot clock (CLOCK_BOOTTIME) and is based on the 576 fast monotonic clock, but also accounts for time spent in 577 suspend. Since the clock access is designed for use in 578 tracing in the suspend path, some side effects are possible 579 if clock is accessed after the suspend time is accounted before 580 the fast mono clock is updated. In this case, the clock update 581 appears to happen slightly sooner than it normally would have. 582 Also on 32-bit systems, it's possible that the 64-bit boot offset 583 sees a partial update. These effects are rare and post 584 processing should be able to handle them. See comments in the 585 ktime_get_boot_fast_ns() function for more information. 586 587 tai: 588 This is the tai clock (CLOCK_TAI) and is derived from the wall- 589 clock time. However, this clock does not experience 590 discontinuities and backwards jumps caused by NTP inserting leap 591 seconds. Since the clock access is designed for use in tracing, 592 side effects are possible. The clock access may yield wrong 593 readouts in case the internal TAI offset is updated e.g., caused 594 by setting the system time or using adjtimex() with an offset. 595 These effects are rare and post processing should be able to 596 handle them. See comments in the ktime_get_tai_fast_ns() 597 function for more information. 598 599 To set a clock, simply echo the clock name into this file:: 600 601 # echo global > trace_clock 602 603 Setting a clock clears the ring buffer content as well as the 604 "snapshot" buffer. 605 606 trace_marker: 607 608 This is a very useful file for synchronizing user space 609 with events happening in the kernel. Writing strings into 610 this file will be written into the ftrace buffer. 611 612 It is useful in applications to open this file at the start 613 of the application and just reference the file descriptor 614 for the file:: 615 616 void trace_write(const char *fmt, ...) 617 { 618 va_list ap; 619 char buf[256]; 620 int n; 621 622 if (trace_fd < 0) 623 return; 624 625 va_start(ap, fmt); 626 n = vsnprintf(buf, 256, fmt, ap); 627 va_end(ap); 628 629 write(trace_fd, buf, n); 630 } 631 632 start:: 633 634 trace_fd = open("trace_marker", O_WRONLY); 635 636 Note: Writing into the trace_marker file can also initiate triggers 637 that are written into /sys/kernel/tracing/events/ftrace/print/trigger 638 See "Event triggers" in Documentation/trace/events.rst and an 639 example in Documentation/trace/histogram.rst (Section 3.) 640 641 trace_marker_raw: 642 643 This is similar to trace_marker above, but is meant for binary data 644 to be written to it, where a tool can be used to parse the data 645 from trace_pipe_raw. 646 647 uprobe_events: 648 649 Add dynamic tracepoints in programs. 650 See uprobetracer.rst 651 652 uprobe_profile: 653 654 Uprobe statistics. See uprobetrace.txt 655 656 instances: 657 658 This is a way to make multiple trace buffers where different 659 events can be recorded in different buffers. 660 See "Instances" section below. 661 662 events: 663 664 This is the trace event directory. It holds event tracepoints 665 (also known as static tracepoints) that have been compiled 666 into the kernel. It shows what event tracepoints exist 667 and how they are grouped by system. There are "enable" 668 files at various levels that can enable the tracepoints 669 when a "1" is written to them. 670 671 See events.rst for more information. 672 673 set_event: 674 675 By echoing in the event into this file, will enable that event. 676 677 See events.rst for more information. 678 679 available_events: 680 681 A list of events that can be enabled in tracing. 682 683 See events.rst for more information. 684 685 timestamp_mode: 686 687 Certain tracers may change the timestamp mode used when 688 logging trace events into the event buffer. Events with 689 different modes can coexist within a buffer but the mode in 690 effect when an event is logged determines which timestamp mode 691 is used for that event. The default timestamp mode is 692 'delta'. 693 694 Usual timestamp modes for tracing: 695 696 # cat timestamp_mode 697 [delta] absolute 698 699 The timestamp mode with the square brackets around it is the 700 one in effect. 701 702 delta: Default timestamp mode - timestamp is a delta against 703 a per-buffer timestamp. 704 705 absolute: The timestamp is a full timestamp, not a delta 706 against some other value. As such it takes up more 707 space and is less efficient. 708 709 hwlat_detector: 710 711 Directory for the Hardware Latency Detector. 712 See "Hardware Latency Detector" section below. 713 714 per_cpu: 715 716 This is a directory that contains the trace per_cpu information. 717 718 per_cpu/cpu0/buffer_size_kb: 719 720 The ftrace buffer is defined per_cpu. That is, there's a separate 721 buffer for each CPU to allow writes to be done atomically, 722 and free from cache bouncing. These buffers may have different 723 size buffers. This file is similar to the buffer_size_kb 724 file, but it only displays or sets the buffer size for the 725 specific CPU. (here cpu0). 726 727 per_cpu/cpu0/trace: 728 729 This is similar to the "trace" file, but it will only display 730 the data specific for the CPU. If written to, it only clears 731 the specific CPU buffer. 732 733 per_cpu/cpu0/trace_pipe 734 735 This is similar to the "trace_pipe" file, and is a consuming 736 read, but it will only display (and consume) the data specific 737 for the CPU. 738 739 per_cpu/cpu0/trace_pipe_raw 740 741 For tools that can parse the ftrace ring buffer binary format, 742 the trace_pipe_raw file can be used to extract the data 743 from the ring buffer directly. With the use of the splice() 744 system call, the buffer data can be quickly transferred to 745 a file or to the network where a server is collecting the 746 data. 747 748 Like trace_pipe, this is a consuming reader, where multiple 749 reads will always produce different data. 750 751 per_cpu/cpu0/snapshot: 752 753 This is similar to the main "snapshot" file, but will only 754 snapshot the current CPU (if supported). It only displays 755 the content of the snapshot for a given CPU, and if 756 written to, only clears this CPU buffer. 757 758 per_cpu/cpu0/snapshot_raw: 759 760 Similar to the trace_pipe_raw, but will read the binary format 761 from the snapshot buffer for the given CPU. 762 763 per_cpu/cpu0/stats: 764 765 This displays certain stats about the ring buffer: 766 767 entries: 768 The number of events that are still in the buffer. 769 770 overrun: 771 The number of lost events due to overwriting when 772 the buffer was full. 773 774 commit overrun: 775 Should always be zero. 776 This gets set if so many events happened within a nested 777 event (ring buffer is re-entrant), that it fills the 778 buffer and starts dropping events. 779 780 bytes: 781 Bytes actually read (not overwritten). 782 783 oldest event ts: 784 The oldest timestamp in the buffer 785 786 now ts: 787 The current timestamp 788 789 dropped events: 790 Events lost due to overwrite option being off. 791 792 read events: 793 The number of events read. 794 795The Tracers 796----------- 797 798Here is the list of current tracers that may be configured. 799 800 "function" 801 802 Function call tracer to trace all kernel functions. 803 804 "function_graph" 805 806 Similar to the function tracer except that the 807 function tracer probes the functions on their entry 808 whereas the function graph tracer traces on both entry 809 and exit of the functions. It then provides the ability 810 to draw a graph of function calls similar to C code 811 source. 812 813 Note that the function graph calculates the timings of when the 814 function starts and returns internally and for each instance. If 815 there are two instances that run function graph tracer and traces 816 the same functions, the length of the timings may be slightly off as 817 each read the timestamp separately and not at the same time. 818 819 "blk" 820 821 The block tracer. The tracer used by the blktrace user 822 application. 823 824 "hwlat" 825 826 The Hardware Latency tracer is used to detect if the hardware 827 produces any latency. See "Hardware Latency Detector" section 828 below. 829 830 "irqsoff" 831 832 Traces the areas that disable interrupts and saves 833 the trace with the longest max latency. 834 See tracing_max_latency. When a new max is recorded, 835 it replaces the old trace. It is best to view this 836 trace with the latency-format option enabled, which 837 happens automatically when the tracer is selected. 838 839 "preemptoff" 840 841 Similar to irqsoff but traces and records the amount of 842 time for which preemption is disabled. 843 844 "preemptirqsoff" 845 846 Similar to irqsoff and preemptoff, but traces and 847 records the largest time for which irqs and/or preemption 848 is disabled. 849 850 "wakeup" 851 852 Traces and records the max latency that it takes for 853 the highest priority task to get scheduled after 854 it has been woken up. 855 Traces all tasks as an average developer would expect. 856 857 "wakeup_rt" 858 859 Traces and records the max latency that it takes for just 860 RT tasks (as the current "wakeup" does). This is useful 861 for those interested in wake up timings of RT tasks. 862 863 "wakeup_dl" 864 865 Traces and records the max latency that it takes for 866 a SCHED_DEADLINE task to be woken (as the "wakeup" and 867 "wakeup_rt" does). 868 869 "mmiotrace" 870 871 A special tracer that is used to trace binary module. 872 It will trace all the calls that a module makes to the 873 hardware. Everything it writes and reads from the I/O 874 as well. 875 876 "branch" 877 878 This tracer can be configured when tracing likely/unlikely 879 calls within the kernel. It will trace when a likely and 880 unlikely branch is hit and if it was correct in its prediction 881 of being correct. 882 883 "nop" 884 885 This is the "trace nothing" tracer. To remove all 886 tracers from tracing simply echo "nop" into 887 current_tracer. 888 889Error conditions 890---------------- 891 892 For most ftrace commands, failure modes are obvious and communicated 893 using standard return codes. 894 895 For other more involved commands, extended error information may be 896 available via the tracing/error_log file. For the commands that 897 support it, reading the tracing/error_log file after an error will 898 display more detailed information about what went wrong, if 899 information is available. The tracing/error_log file is a circular 900 error log displaying a small number (currently, 8) of ftrace errors 901 for the last (8) failed commands. 902 903 The extended error information and usage takes the form shown in 904 this example:: 905 906 # echo xxx > /sys/kernel/tracing/events/sched/sched_wakeup/trigger 907 echo: write error: Invalid argument 908 909 # cat /sys/kernel/tracing/error_log 910 [ 5348.887237] location: error: Couldn't yyy: zzz 911 Command: xxx 912 ^ 913 [ 7517.023364] location: error: Bad rrr: sss 914 Command: ppp qqq 915 ^ 916 917 To clear the error log, echo the empty string into it:: 918 919 # echo > /sys/kernel/tracing/error_log 920 921Examples of using the tracer 922---------------------------- 923 924Here are typical examples of using the tracers when controlling 925them only with the tracefs interface (without using any 926user-land utilities). 927 928Output format: 929-------------- 930 931Here is an example of the output format of the file "trace":: 932 933 # tracer: function 934 # 935 # entries-in-buffer/entries-written: 140080/250280 #P:4 936 # 937 # _-----=> irqs-off 938 # / _----=> need-resched 939 # | / _---=> hardirq/softirq 940 # || / _--=> preempt-depth 941 # ||| / delay 942 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 943 # | | | |||| | | 944 bash-1977 [000] .... 17284.993652: sys_close <-system_call_fastpath 945 bash-1977 [000] .... 17284.993653: __close_fd <-sys_close 946 bash-1977 [000] .... 17284.993653: _raw_spin_lock <-__close_fd 947 sshd-1974 [003] .... 17284.993653: __srcu_read_unlock <-fsnotify 948 bash-1977 [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock 949 bash-1977 [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd 950 bash-1977 [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock 951 bash-1977 [000] .... 17284.993657: filp_close <-__close_fd 952 bash-1977 [000] .... 17284.993657: dnotify_flush <-filp_close 953 sshd-1974 [003] .... 17284.993658: sys_select <-system_call_fastpath 954 .... 955 956A header is printed with the tracer name that is represented by 957the trace. In this case the tracer is "function". Then it shows the 958number of events in the buffer as well as the total number of entries 959that were written. The difference is the number of entries that were 960lost due to the buffer filling up (250280 - 140080 = 110200 events 961lost). 962 963The header explains the content of the events. Task name "bash", the task 964PID "1977", the CPU that it was running on "000", the latency format 965(explained below), the timestamp in <secs>.<usecs> format, the 966function name that was traced "sys_close" and the parent function that 967called this function "system_call_fastpath". The timestamp is the time 968at which the function was entered. 969 970Latency trace format 971-------------------- 972 973When the latency-format option is enabled or when one of the latency 974tracers is set, the trace file gives somewhat more information to see 975why a latency happened. Here is a typical trace:: 976 977 # tracer: irqsoff 978 # 979 # irqsoff latency trace v1.1.5 on 3.8.0-test+ 980 # -------------------------------------------------------------------- 981 # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 982 # ----------------- 983 # | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0) 984 # ----------------- 985 # => started at: __lock_task_sighand 986 # => ended at: _raw_spin_unlock_irqrestore 987 # 988 # 989 # _------=> CPU# 990 # / _-----=> irqs-off 991 # | / _----=> need-resched 992 # || / _---=> hardirq/softirq 993 # ||| / _--=> preempt-depth 994 # |||| / delay 995 # cmd pid ||||| time | caller 996 # \ / ||||| \ | / 997 ps-6143 2d... 0us!: trace_hardirqs_off <-__lock_task_sighand 998 ps-6143 2d..1 259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore 999 ps-6143 2d..1 263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore 1000 ps-6143 2d..1 306us : <stack trace> 1001 => trace_hardirqs_on_caller 1002 => trace_hardirqs_on 1003 => _raw_spin_unlock_irqrestore 1004 => do_task_stat 1005 => proc_tgid_stat 1006 => proc_single_show 1007 => seq_read 1008 => vfs_read 1009 => sys_read 1010 => system_call_fastpath 1011 1012 1013This shows that the current tracer is "irqsoff" tracing the time 1014for which interrupts were disabled. It gives the trace version (which 1015never changes) and the version of the kernel upon which this was executed on 1016(3.8). Then it displays the max latency in microseconds (259 us). The number 1017of trace entries displayed and the total number (both are four: #4/4). 1018VP, KP, SP, and HP are always zero and are reserved for later use. 1019#P is the number of online CPUs (#P:4). 1020 1021The task is the process that was running when the latency 1022occurred. (ps pid: 6143). 1023 1024The start and stop (the functions in which the interrupts were 1025disabled and enabled respectively) that caused the latencies: 1026 1027 - __lock_task_sighand is where the interrupts were disabled. 1028 - _raw_spin_unlock_irqrestore is where they were enabled again. 1029 1030The next lines after the header are the trace itself. The header 1031explains which is which. 1032 1033 cmd: The name of the process in the trace. 1034 1035 pid: The PID of that process. 1036 1037 CPU#: The CPU which the process was running on. 1038 1039 irqs-off: 'd' interrupts are disabled. '.' otherwise. 1040 1041 need-resched: 1042 - 'B' all, TIF_NEED_RESCHED, PREEMPT_NEED_RESCHED and TIF_RESCHED_LAZY is set, 1043 - 'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set, 1044 - 'n' only TIF_NEED_RESCHED is set, 1045 - 'p' only PREEMPT_NEED_RESCHED is set, 1046 - 'L' both PREEMPT_NEED_RESCHED and TIF_RESCHED_LAZY is set, 1047 - 'b' both TIF_NEED_RESCHED and TIF_RESCHED_LAZY is set, 1048 - 'l' only TIF_RESCHED_LAZY is set 1049 - '.' otherwise. 1050 1051 hardirq/softirq: 1052 - 'Z' - NMI occurred inside a hardirq 1053 - 'z' - NMI is running 1054 - 'H' - hard irq occurred inside a softirq. 1055 - 'h' - hard irq is running 1056 - 's' - soft irq is running 1057 - '.' - normal context. 1058 1059 preempt-depth: The level of preempt_disabled 1060 1061The above is mostly meaningful for kernel developers. 1062 1063 time: 1064 When the latency-format option is enabled, the trace file 1065 output includes a timestamp relative to the start of the 1066 trace. This differs from the output when latency-format 1067 is disabled, which includes an absolute timestamp. 1068 1069 delay: 1070 This is just to help catch your eye a bit better. And 1071 needs to be fixed to be only relative to the same CPU. 1072 The marks are determined by the difference between this 1073 current trace and the next trace. 1074 1075 - '$' - greater than 1 second 1076 - '@' - greater than 100 millisecond 1077 - '*' - greater than 10 millisecond 1078 - '#' - greater than 1000 microsecond 1079 - '!' - greater than 100 microsecond 1080 - '+' - greater than 10 microsecond 1081 - ' ' - less than or equal to 10 microsecond. 1082 1083 The rest is the same as the 'trace' file. 1084 1085 Note, the latency tracers will usually end with a back trace 1086 to easily find where the latency occurred. 1087 1088trace_options 1089------------- 1090 1091The trace_options file (or the options directory) is used to control 1092what gets printed in the trace output, or manipulate the tracers. 1093To see what is available, simply cat the file:: 1094 1095 cat trace_options 1096 print-parent 1097 nosym-offset 1098 nosym-addr 1099 noverbose 1100 noraw 1101 nohex 1102 nobin 1103 noblock 1104 nofields 1105 trace_printk 1106 annotate 1107 nouserstacktrace 1108 nosym-userobj 1109 noprintk-msg-only 1110 context-info 1111 nolatency-format 1112 record-cmd 1113 norecord-tgid 1114 overwrite 1115 nodisable_on_free 1116 irq-info 1117 markers 1118 noevent-fork 1119 function-trace 1120 nofunction-fork 1121 nodisplay-graph 1122 nostacktrace 1123 nobranch 1124 1125To disable one of the options, echo in the option prepended with 1126"no":: 1127 1128 echo noprint-parent > trace_options 1129 1130To enable an option, leave off the "no":: 1131 1132 echo sym-offset > trace_options 1133 1134Here are the available options: 1135 1136 print-parent 1137 On function traces, display the calling (parent) 1138 function as well as the function being traced. 1139 :: 1140 1141 print-parent: 1142 bash-4000 [01] 1477.606694: simple_strtoul <-kstrtoul 1143 1144 noprint-parent: 1145 bash-4000 [01] 1477.606694: simple_strtoul 1146 1147 1148 sym-offset 1149 Display not only the function name, but also the 1150 offset in the function. For example, instead of 1151 seeing just "ktime_get", you will see 1152 "ktime_get+0xb/0x20". 1153 :: 1154 1155 sym-offset: 1156 bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0 1157 1158 sym-addr 1159 This will also display the function address as well 1160 as the function name. 1161 :: 1162 1163 sym-addr: 1164 bash-4000 [01] 1477.606694: simple_strtoul <c0339346> 1165 1166 verbose 1167 This deals with the trace file when the 1168 latency-format option is enabled. 1169 :: 1170 1171 bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \ 1172 (+0.000ms): simple_strtoul (kstrtoul) 1173 1174 raw 1175 This will display raw numbers. This option is best for 1176 use with user applications that can translate the raw 1177 numbers better than having it done in the kernel. 1178 1179 hex 1180 Similar to raw, but the numbers will be in a hexadecimal format. 1181 1182 bin 1183 This will print out the formats in raw binary. 1184 1185 block 1186 When set, reading trace_pipe will not block when polled. 1187 1188 fields 1189 Print the fields as described by their types. This is a better 1190 option than using hex, bin or raw, as it gives a better parsing 1191 of the content of the event. 1192 1193 trace_printk 1194 Can disable trace_printk() from writing into the buffer. 1195 1196 trace_printk_dest 1197 Set to have trace_printk() and similar internal tracing functions 1198 write into this instance. Note, only one trace instance can have 1199 this set. By setting this flag, it clears the trace_printk_dest flag 1200 of the instance that had it set previously. By default, the top 1201 level trace has this set, and will get it set again if another 1202 instance has it set then clears it. 1203 1204 This flag cannot be cleared by the top level instance, as it is the 1205 default instance. The only way the top level instance has this flag 1206 cleared, is by it being set in another instance. 1207 1208 copy_trace_marker 1209 If there are applications that hard code writing into the top level 1210 trace_marker file (/sys/kernel/tracing/trace_marker or trace_marker_raw), 1211 and the tooling would like it to go into an instance, this option can 1212 be used. Create an instance and set this option, and then all writes 1213 into the top level trace_marker file will also be redirected into this 1214 instance. 1215 1216 Note, by default this option is set for the top level instance. If it 1217 is disabled, then writes to the trace_marker or trace_marker_raw files 1218 will not be written into the top level file. If no instance has this 1219 option set, then a write will error with the errno of ENODEV. 1220 1221 annotate 1222 It is sometimes confusing when the CPU buffers are full 1223 and one CPU buffer had a lot of events recently, thus 1224 a shorter time frame, were another CPU may have only had 1225 a few events, which lets it have older events. When 1226 the trace is reported, it shows the oldest events first, 1227 and it may look like only one CPU ran (the one with the 1228 oldest events). When the annotate option is set, it will 1229 display when a new CPU buffer started:: 1230 1231 <idle>-0 [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on 1232 <idle>-0 [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on 1233 <idle>-0 [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore 1234 ##### CPU 2 buffer started #### 1235 <idle>-0 [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle 1236 <idle>-0 [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog 1237 <idle>-0 [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock 1238 1239 userstacktrace 1240 This option changes the trace. It records a 1241 stacktrace of the current user space thread after 1242 each trace event. 1243 1244 sym-userobj 1245 when user stacktrace are enabled, look up which 1246 object the address belongs to, and print a 1247 relative address. This is especially useful when 1248 ASLR is on, otherwise you don't get a chance to 1249 resolve the address to object/file/line after 1250 the app is no longer running 1251 1252 The lookup is performed when you read 1253 trace,trace_pipe. Example:: 1254 1255 a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0 1256 x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6] 1257 1258 1259 printk-msg-only 1260 When set, trace_printk()s will only show the format 1261 and not their parameters (if trace_bprintk() or 1262 trace_bputs() was used to save the trace_printk()). 1263 1264 context-info 1265 Show only the event data. Hides the comm, PID, 1266 timestamp, CPU, and other useful data. 1267 1268 latency-format 1269 This option changes the trace output. When it is enabled, 1270 the trace displays additional information about the 1271 latency, as described in "Latency trace format". 1272 1273 pause-on-trace 1274 When set, opening the trace file for read, will pause 1275 writing to the ring buffer (as if tracing_on was set to zero). 1276 This simulates the original behavior of the trace file. 1277 When the file is closed, tracing will be enabled again. 1278 1279 hash-ptr 1280 When set, "%p" in the event printk format displays the 1281 hashed pointer value instead of real address. 1282 This will be useful if you want to find out which hashed 1283 value is corresponding to the real value in trace log. 1284 1285 record-cmd 1286 When any event or tracer is enabled, a hook is enabled 1287 in the sched_switch trace point to fill comm cache 1288 with mapped pids and comms. But this may cause some 1289 overhead, and if you only care about pids, and not the 1290 name of the task, disabling this option can lower the 1291 impact of tracing. See "saved_cmdlines". 1292 1293 record-tgid 1294 When any event or tracer is enabled, a hook is enabled 1295 in the sched_switch trace point to fill the cache of 1296 mapped Thread Group IDs (TGID) mapping to pids. See 1297 "saved_tgids". 1298 1299 overwrite 1300 This controls what happens when the trace buffer is 1301 full. If "1" (default), the oldest events are 1302 discarded and overwritten. If "0", then the newest 1303 events are discarded. 1304 (see per_cpu/cpu0/stats for overrun and dropped) 1305 1306 disable_on_free 1307 When the free_buffer is closed, tracing will 1308 stop (tracing_on set to 0). 1309 1310 irq-info 1311 Shows the interrupt, preempt count, need resched data. 1312 When disabled, the trace looks like:: 1313 1314 # tracer: function 1315 # 1316 # entries-in-buffer/entries-written: 144405/9452052 #P:4 1317 # 1318 # TASK-PID CPU# TIMESTAMP FUNCTION 1319 # | | | | | 1320 <idle>-0 [002] 23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up 1321 <idle>-0 [002] 23636.756054: activate_task <-ttwu_do_activate.constprop.89 1322 <idle>-0 [002] 23636.756055: enqueue_task <-activate_task 1323 1324 1325 markers 1326 When set, the trace_marker is writable (only by root). 1327 When disabled, the trace_marker will error with EINVAL 1328 on write. 1329 1330 event-fork 1331 When set, tasks with PIDs listed in set_event_pid will have 1332 the PIDs of their children added to set_event_pid when those 1333 tasks fork. Also, when tasks with PIDs in set_event_pid exit, 1334 their PIDs will be removed from the file. 1335 1336 This affects PIDs listed in set_event_notrace_pid as well. 1337 1338 function-trace 1339 The latency tracers will enable function tracing 1340 if this option is enabled (default it is). When 1341 it is disabled, the latency tracers do not trace 1342 functions. This keeps the overhead of the tracer down 1343 when performing latency tests. 1344 1345 function-fork 1346 When set, tasks with PIDs listed in set_ftrace_pid will 1347 have the PIDs of their children added to set_ftrace_pid 1348 when those tasks fork. Also, when tasks with PIDs in 1349 set_ftrace_pid exit, their PIDs will be removed from the 1350 file. 1351 1352 This affects PIDs in set_ftrace_notrace_pid as well. 1353 1354 display-graph 1355 When set, the latency tracers (irqsoff, wakeup, etc) will 1356 use function graph tracing instead of function tracing. 1357 1358 stacktrace 1359 When set, a stack trace is recorded after any trace event 1360 is recorded. 1361 1362 branch 1363 Enable branch tracing with the tracer. This enables branch 1364 tracer along with the currently set tracer. Enabling this 1365 with the "nop" tracer is the same as just enabling the 1366 "branch" tracer. 1367 1368.. tip:: Some tracers have their own options. They only appear in this 1369 file when the tracer is active. They always appear in the 1370 options directory. 1371 1372 1373Here are the per tracer options: 1374 1375Options for function tracer: 1376 1377 func_stack_trace 1378 When set, a stack trace is recorded after every 1379 function that is recorded. NOTE! Limit the functions 1380 that are recorded before enabling this, with 1381 "set_ftrace_filter" otherwise the system performance 1382 will be critically degraded. Remember to disable 1383 this option before clearing the function filter. 1384 1385Options for function_graph tracer: 1386 1387 Since the function_graph tracer has a slightly different output 1388 it has its own options to control what is displayed. 1389 1390 funcgraph-overrun 1391 When set, the "overrun" of the graph stack is 1392 displayed after each function traced. The 1393 overrun, is when the stack depth of the calls 1394 is greater than what is reserved for each task. 1395 Each task has a fixed array of functions to 1396 trace in the call graph. If the depth of the 1397 calls exceeds that, the function is not traced. 1398 The overrun is the number of functions missed 1399 due to exceeding this array. 1400 1401 funcgraph-cpu 1402 When set, the CPU number of the CPU where the trace 1403 occurred is displayed. 1404 1405 funcgraph-overhead 1406 When set, if the function takes longer than 1407 A certain amount, then a delay marker is 1408 displayed. See "delay" above, under the 1409 header description. 1410 1411 funcgraph-proc 1412 Unlike other tracers, the process' command line 1413 is not displayed by default, but instead only 1414 when a task is traced in and out during a context 1415 switch. Enabling this options has the command 1416 of each process displayed at every line. 1417 1418 funcgraph-duration 1419 At the end of each function (the return) 1420 the duration of the amount of time in the 1421 function is displayed in microseconds. 1422 1423 funcgraph-abstime 1424 When set, the timestamp is displayed at each line. 1425 1426 funcgraph-irqs 1427 When disabled, functions that happen inside an 1428 interrupt will not be traced. 1429 1430 funcgraph-tail 1431 When set, the return event will include the function 1432 that it represents. By default this is off, and 1433 only a closing curly bracket "}" is displayed for 1434 the return of a function. 1435 1436 funcgraph-retval 1437 When set, the return value of each traced function 1438 will be printed after an equal sign "=". By default 1439 this is off. 1440 1441 funcgraph-retval-hex 1442 When set, the return value will always be printed 1443 in hexadecimal format. If the option is not set and 1444 the return value is an error code, it will be printed 1445 in signed decimal format; otherwise it will also be 1446 printed in hexadecimal format. By default, this option 1447 is off. 1448 1449 sleep-time 1450 When running function graph tracer, to include 1451 the time a task schedules out in its function. 1452 When enabled, it will account time the task has been 1453 scheduled out as part of the function call. 1454 1455 graph-time 1456 When running function profiler with function graph tracer, 1457 to include the time to call nested functions. When this is 1458 not set, the time reported for the function will only 1459 include the time the function itself executed for, not the 1460 time for functions that it called. 1461 1462Options for blk tracer: 1463 1464 blk_classic 1465 Shows a more minimalistic output. 1466 1467 1468irqsoff 1469------- 1470 1471When interrupts are disabled, the CPU can not react to any other 1472external event (besides NMIs and SMIs). This prevents the timer 1473interrupt from triggering or the mouse interrupt from letting 1474the kernel know of a new mouse event. The result is a latency 1475with the reaction time. 1476 1477The irqsoff tracer tracks the time for which interrupts are 1478disabled. When a new maximum latency is hit, the tracer saves 1479the trace leading up to that latency point so that every time a 1480new maximum is reached, the old saved trace is discarded and the 1481new trace is saved. 1482 1483To reset the maximum, echo 0 into tracing_max_latency. Here is 1484an example:: 1485 1486 # echo 0 > options/function-trace 1487 # echo irqsoff > current_tracer 1488 # echo 1 > tracing_on 1489 # echo 0 > tracing_max_latency 1490 # ls -ltr 1491 [...] 1492 # echo 0 > tracing_on 1493 # cat trace 1494 # tracer: irqsoff 1495 # 1496 # irqsoff latency trace v1.1.5 on 3.8.0-test+ 1497 # -------------------------------------------------------------------- 1498 # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1499 # ----------------- 1500 # | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0) 1501 # ----------------- 1502 # => started at: run_timer_softirq 1503 # => ended at: run_timer_softirq 1504 # 1505 # 1506 # _------=> CPU# 1507 # / _-----=> irqs-off 1508 # | / _----=> need-resched 1509 # || / _---=> hardirq/softirq 1510 # ||| / _--=> preempt-depth 1511 # |||| / delay 1512 # cmd pid ||||| time | caller 1513 # \ / ||||| \ | / 1514 <idle>-0 0d.s2 0us+: _raw_spin_lock_irq <-run_timer_softirq 1515 <idle>-0 0dNs3 17us : _raw_spin_unlock_irq <-run_timer_softirq 1516 <idle>-0 0dNs3 17us+: trace_hardirqs_on <-run_timer_softirq 1517 <idle>-0 0dNs3 25us : <stack trace> 1518 => _raw_spin_unlock_irq 1519 => run_timer_softirq 1520 => __do_softirq 1521 => call_softirq 1522 => do_softirq 1523 => irq_exit 1524 => smp_apic_timer_interrupt 1525 => apic_timer_interrupt 1526 => rcu_idle_exit 1527 => cpu_idle 1528 => rest_init 1529 => start_kernel 1530 => x86_64_start_reservations 1531 => x86_64_start_kernel 1532 1533Here we see that we had a latency of 16 microseconds (which is 1534very good). The _raw_spin_lock_irq in run_timer_softirq disabled 1535interrupts. The difference between the 16 and the displayed 1536timestamp 25us occurred because the clock was incremented 1537between the time of recording the max latency and the time of 1538recording the function that had that latency. 1539 1540Note the above example had function-trace not set. If we set 1541function-trace, we get a much larger output:: 1542 1543 with echo 1 > options/function-trace 1544 1545 # tracer: irqsoff 1546 # 1547 # irqsoff latency trace v1.1.5 on 3.8.0-test+ 1548 # -------------------------------------------------------------------- 1549 # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1550 # ----------------- 1551 # | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0) 1552 # ----------------- 1553 # => started at: ata_scsi_queuecmd 1554 # => ended at: ata_scsi_queuecmd 1555 # 1556 # 1557 # _------=> CPU# 1558 # / _-----=> irqs-off 1559 # | / _----=> need-resched 1560 # || / _---=> hardirq/softirq 1561 # ||| / _--=> preempt-depth 1562 # |||| / delay 1563 # cmd pid ||||| time | caller 1564 # \ / ||||| \ | / 1565 bash-2042 3d... 0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd 1566 bash-2042 3d... 0us : add_preempt_count <-_raw_spin_lock_irqsave 1567 bash-2042 3d..1 1us : ata_scsi_find_dev <-ata_scsi_queuecmd 1568 bash-2042 3d..1 1us : __ata_scsi_find_dev <-ata_scsi_find_dev 1569 bash-2042 3d..1 2us : ata_find_dev.part.14 <-__ata_scsi_find_dev 1570 bash-2042 3d..1 2us : ata_qc_new_init <-__ata_scsi_queuecmd 1571 bash-2042 3d..1 3us : ata_sg_init <-__ata_scsi_queuecmd 1572 bash-2042 3d..1 4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd 1573 bash-2042 3d..1 4us : ata_build_rw_tf <-ata_scsi_rw_xlat 1574 [...] 1575 bash-2042 3d..1 67us : delay_tsc <-__delay 1576 bash-2042 3d..1 67us : add_preempt_count <-delay_tsc 1577 bash-2042 3d..2 67us : sub_preempt_count <-delay_tsc 1578 bash-2042 3d..1 67us : add_preempt_count <-delay_tsc 1579 bash-2042 3d..2 68us : sub_preempt_count <-delay_tsc 1580 bash-2042 3d..1 68us+: ata_bmdma_start <-ata_bmdma_qc_issue 1581 bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd 1582 bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd 1583 bash-2042 3d..1 72us+: trace_hardirqs_on <-ata_scsi_queuecmd 1584 bash-2042 3d..1 120us : <stack trace> 1585 => _raw_spin_unlock_irqrestore 1586 => ata_scsi_queuecmd 1587 => scsi_dispatch_cmd 1588 => scsi_request_fn 1589 => __blk_run_queue_uncond 1590 => __blk_run_queue 1591 => blk_queue_bio 1592 => submit_bio_noacct 1593 => submit_bio 1594 => submit_bh 1595 => __ext3_get_inode_loc 1596 => ext3_iget 1597 => ext3_lookup 1598 => lookup_real 1599 => __lookup_hash 1600 => walk_component 1601 => lookup_last 1602 => path_lookupat 1603 => filename_lookup 1604 => user_path_at_empty 1605 => user_path_at 1606 => vfs_fstatat 1607 => vfs_stat 1608 => sys_newstat 1609 => system_call_fastpath 1610 1611 1612Here we traced a 71 microsecond latency. But we also see all the 1613functions that were called during that time. Note that by 1614enabling function tracing, we incur an added overhead. This 1615overhead may extend the latency times. But nevertheless, this 1616trace has provided some very helpful debugging information. 1617 1618If we prefer function graph output instead of function, we can set 1619display-graph option:: 1620 1621 with echo 1 > options/display-graph 1622 1623 # tracer: irqsoff 1624 # 1625 # irqsoff latency trace v1.1.5 on 4.20.0-rc6+ 1626 # -------------------------------------------------------------------- 1627 # latency: 3751 us, #274/274, CPU#0 | (M:desktop VP:0, KP:0, SP:0 HP:0 #P:4) 1628 # ----------------- 1629 # | task: bash-1507 (uid:0 nice:0 policy:0 rt_prio:0) 1630 # ----------------- 1631 # => started at: free_debug_processing 1632 # => ended at: return_to_handler 1633 # 1634 # 1635 # _-----=> irqs-off 1636 # / _----=> need-resched 1637 # | / _---=> hardirq/softirq 1638 # || / _--=> preempt-depth 1639 # ||| / 1640 # REL TIME CPU TASK/PID |||| DURATION FUNCTION CALLS 1641 # | | | | |||| | | | | | | 1642 0 us | 0) bash-1507 | d... | 0.000 us | _raw_spin_lock_irqsave(); 1643 0 us | 0) bash-1507 | d..1 | 0.378 us | do_raw_spin_trylock(); 1644 1 us | 0) bash-1507 | d..2 | | set_track() { 1645 2 us | 0) bash-1507 | d..2 | | save_stack_trace() { 1646 2 us | 0) bash-1507 | d..2 | | __save_stack_trace() { 1647 3 us | 0) bash-1507 | d..2 | | __unwind_start() { 1648 3 us | 0) bash-1507 | d..2 | | get_stack_info() { 1649 3 us | 0) bash-1507 | d..2 | 0.351 us | in_task_stack(); 1650 4 us | 0) bash-1507 | d..2 | 1.107 us | } 1651 [...] 1652 3750 us | 0) bash-1507 | d..1 | 0.516 us | do_raw_spin_unlock(); 1653 3750 us | 0) bash-1507 | d..1 | 0.000 us | _raw_spin_unlock_irqrestore(); 1654 3764 us | 0) bash-1507 | d..1 | 0.000 us | tracer_hardirqs_on(); 1655 bash-1507 0d..1 3792us : <stack trace> 1656 => free_debug_processing 1657 => __slab_free 1658 => kmem_cache_free 1659 => vm_area_free 1660 => remove_vma 1661 => exit_mmap 1662 => mmput 1663 => begin_new_exec 1664 => load_elf_binary 1665 => search_binary_handler 1666 => __do_execve_file.isra.32 1667 => __x64_sys_execve 1668 => do_syscall_64 1669 => entry_SYSCALL_64_after_hwframe 1670 1671preemptoff 1672---------- 1673 1674When preemption is disabled, we may be able to receive 1675interrupts but the task cannot be preempted and a higher 1676priority task must wait for preemption to be enabled again 1677before it can preempt a lower priority task. 1678 1679The preemptoff tracer traces the places that disable preemption. 1680Like the irqsoff tracer, it records the maximum latency for 1681which preemption was disabled. The control of preemptoff tracer 1682is much like the irqsoff tracer. 1683:: 1684 1685 # echo 0 > options/function-trace 1686 # echo preemptoff > current_tracer 1687 # echo 1 > tracing_on 1688 # echo 0 > tracing_max_latency 1689 # ls -ltr 1690 [...] 1691 # echo 0 > tracing_on 1692 # cat trace 1693 # tracer: preemptoff 1694 # 1695 # preemptoff latency trace v1.1.5 on 3.8.0-test+ 1696 # -------------------------------------------------------------------- 1697 # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1698 # ----------------- 1699 # | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0) 1700 # ----------------- 1701 # => started at: do_IRQ 1702 # => ended at: do_IRQ 1703 # 1704 # 1705 # _------=> CPU# 1706 # / _-----=> irqs-off 1707 # | / _----=> need-resched 1708 # || / _---=> hardirq/softirq 1709 # ||| / _--=> preempt-depth 1710 # |||| / delay 1711 # cmd pid ||||| time | caller 1712 # \ / ||||| \ | / 1713 sshd-1991 1d.h. 0us+: irq_enter <-do_IRQ 1714 sshd-1991 1d..1 46us : irq_exit <-do_IRQ 1715 sshd-1991 1d..1 47us+: trace_preempt_on <-do_IRQ 1716 sshd-1991 1d..1 52us : <stack trace> 1717 => sub_preempt_count 1718 => irq_exit 1719 => do_IRQ 1720 => ret_from_intr 1721 1722 1723This has some more changes. Preemption was disabled when an 1724interrupt came in (notice the 'h'), and was enabled on exit. 1725But we also see that interrupts have been disabled when entering 1726the preempt off section and leaving it (the 'd'). We do not know if 1727interrupts were enabled in the mean time or shortly after this 1728was over. 1729:: 1730 1731 # tracer: preemptoff 1732 # 1733 # preemptoff latency trace v1.1.5 on 3.8.0-test+ 1734 # -------------------------------------------------------------------- 1735 # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1736 # ----------------- 1737 # | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0) 1738 # ----------------- 1739 # => started at: wake_up_new_task 1740 # => ended at: task_rq_unlock 1741 # 1742 # 1743 # _------=> CPU# 1744 # / _-----=> irqs-off 1745 # | / _----=> need-resched 1746 # || / _---=> hardirq/softirq 1747 # ||| / _--=> preempt-depth 1748 # |||| / delay 1749 # cmd pid ||||| time | caller 1750 # \ / ||||| \ | / 1751 bash-1994 1d..1 0us : _raw_spin_lock_irqsave <-wake_up_new_task 1752 bash-1994 1d..1 0us : select_task_rq_fair <-select_task_rq 1753 bash-1994 1d..1 1us : __rcu_read_lock <-select_task_rq_fair 1754 bash-1994 1d..1 1us : source_load <-select_task_rq_fair 1755 bash-1994 1d..1 1us : source_load <-select_task_rq_fair 1756 [...] 1757 bash-1994 1d..1 12us : irq_enter <-smp_apic_timer_interrupt 1758 bash-1994 1d..1 12us : rcu_irq_enter <-irq_enter 1759 bash-1994 1d..1 13us : add_preempt_count <-irq_enter 1760 bash-1994 1d.h1 13us : exit_idle <-smp_apic_timer_interrupt 1761 bash-1994 1d.h1 13us : hrtimer_interrupt <-smp_apic_timer_interrupt 1762 bash-1994 1d.h1 13us : _raw_spin_lock <-hrtimer_interrupt 1763 bash-1994 1d.h1 14us : add_preempt_count <-_raw_spin_lock 1764 bash-1994 1d.h2 14us : ktime_get_update_offsets <-hrtimer_interrupt 1765 [...] 1766 bash-1994 1d.h1 35us : lapic_next_event <-clockevents_program_event 1767 bash-1994 1d.h1 35us : irq_exit <-smp_apic_timer_interrupt 1768 bash-1994 1d.h1 36us : sub_preempt_count <-irq_exit 1769 bash-1994 1d..2 36us : do_softirq <-irq_exit 1770 bash-1994 1d..2 36us : __do_softirq <-call_softirq 1771 bash-1994 1d..2 36us : __local_bh_disable <-__do_softirq 1772 bash-1994 1d.s2 37us : add_preempt_count <-_raw_spin_lock_irq 1773 bash-1994 1d.s3 38us : _raw_spin_unlock <-run_timer_softirq 1774 bash-1994 1d.s3 39us : sub_preempt_count <-_raw_spin_unlock 1775 bash-1994 1d.s2 39us : call_timer_fn <-run_timer_softirq 1776 [...] 1777 bash-1994 1dNs2 81us : cpu_needs_another_gp <-rcu_process_callbacks 1778 bash-1994 1dNs2 82us : __local_bh_enable <-__do_softirq 1779 bash-1994 1dNs2 82us : sub_preempt_count <-__local_bh_enable 1780 bash-1994 1dN.2 82us : idle_cpu <-irq_exit 1781 bash-1994 1dN.2 83us : rcu_irq_exit <-irq_exit 1782 bash-1994 1dN.2 83us : sub_preempt_count <-irq_exit 1783 bash-1994 1.N.1 84us : _raw_spin_unlock_irqrestore <-task_rq_unlock 1784 bash-1994 1.N.1 84us+: trace_preempt_on <-task_rq_unlock 1785 bash-1994 1.N.1 104us : <stack trace> 1786 => sub_preempt_count 1787 => _raw_spin_unlock_irqrestore 1788 => task_rq_unlock 1789 => wake_up_new_task 1790 => do_fork 1791 => sys_clone 1792 => stub_clone 1793 1794 1795The above is an example of the preemptoff trace with 1796function-trace set. Here we see that interrupts were not disabled 1797the entire time. The irq_enter code lets us know that we entered 1798an interrupt 'h'. Before that, the functions being traced still 1799show that it is not in an interrupt, but we can see from the 1800functions themselves that this is not the case. 1801 1802preemptirqsoff 1803-------------- 1804 1805Knowing the locations that have interrupts disabled or 1806preemption disabled for the longest times is helpful. But 1807sometimes we would like to know when either preemption and/or 1808interrupts are disabled. 1809 1810Consider the following code:: 1811 1812 local_irq_disable(); 1813 call_function_with_irqs_off(); 1814 preempt_disable(); 1815 call_function_with_irqs_and_preemption_off(); 1816 local_irq_enable(); 1817 call_function_with_preemption_off(); 1818 preempt_enable(); 1819 1820The irqsoff tracer will record the total length of 1821call_function_with_irqs_off() and 1822call_function_with_irqs_and_preemption_off(). 1823 1824The preemptoff tracer will record the total length of 1825call_function_with_irqs_and_preemption_off() and 1826call_function_with_preemption_off(). 1827 1828But neither will trace the time that interrupts and/or 1829preemption is disabled. This total time is the time that we can 1830not schedule. To record this time, use the preemptirqsoff 1831tracer. 1832 1833Again, using this trace is much like the irqsoff and preemptoff 1834tracers. 1835:: 1836 1837 # echo 0 > options/function-trace 1838 # echo preemptirqsoff > current_tracer 1839 # echo 1 > tracing_on 1840 # echo 0 > tracing_max_latency 1841 # ls -ltr 1842 [...] 1843 # echo 0 > tracing_on 1844 # cat trace 1845 # tracer: preemptirqsoff 1846 # 1847 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+ 1848 # -------------------------------------------------------------------- 1849 # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1850 # ----------------- 1851 # | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0) 1852 # ----------------- 1853 # => started at: ata_scsi_queuecmd 1854 # => ended at: ata_scsi_queuecmd 1855 # 1856 # 1857 # _------=> CPU# 1858 # / _-----=> irqs-off 1859 # | / _----=> need-resched 1860 # || / _---=> hardirq/softirq 1861 # ||| / _--=> preempt-depth 1862 # |||| / delay 1863 # cmd pid ||||| time | caller 1864 # \ / ||||| \ | / 1865 ls-2230 3d... 0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd 1866 ls-2230 3...1 100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd 1867 ls-2230 3...1 101us+: trace_preempt_on <-ata_scsi_queuecmd 1868 ls-2230 3...1 111us : <stack trace> 1869 => sub_preempt_count 1870 => _raw_spin_unlock_irqrestore 1871 => ata_scsi_queuecmd 1872 => scsi_dispatch_cmd 1873 => scsi_request_fn 1874 => __blk_run_queue_uncond 1875 => __blk_run_queue 1876 => blk_queue_bio 1877 => submit_bio_noacct 1878 => submit_bio 1879 => submit_bh 1880 => ext3_bread 1881 => ext3_dir_bread 1882 => htree_dirblock_to_tree 1883 => ext3_htree_fill_tree 1884 => ext3_readdir 1885 => vfs_readdir 1886 => sys_getdents 1887 => system_call_fastpath 1888 1889 1890The trace_hardirqs_off_thunk is called from assembly on x86 when 1891interrupts are disabled in the assembly code. Without the 1892function tracing, we do not know if interrupts were enabled 1893within the preemption points. We do see that it started with 1894preemption enabled. 1895 1896Here is a trace with function-trace set:: 1897 1898 # tracer: preemptirqsoff 1899 # 1900 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+ 1901 # -------------------------------------------------------------------- 1902 # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1903 # ----------------- 1904 # | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0) 1905 # ----------------- 1906 # => started at: schedule 1907 # => ended at: mutex_unlock 1908 # 1909 # 1910 # _------=> CPU# 1911 # / _-----=> irqs-off 1912 # | / _----=> need-resched 1913 # || / _---=> hardirq/softirq 1914 # ||| / _--=> preempt-depth 1915 # |||| / delay 1916 # cmd pid ||||| time | caller 1917 # \ / ||||| \ | / 1918 kworker/-59 3...1 0us : __schedule <-schedule 1919 kworker/-59 3d..1 0us : rcu_preempt_qs <-rcu_note_context_switch 1920 kworker/-59 3d..1 1us : add_preempt_count <-_raw_spin_lock_irq 1921 kworker/-59 3d..2 1us : deactivate_task <-__schedule 1922 kworker/-59 3d..2 1us : dequeue_task <-deactivate_task 1923 kworker/-59 3d..2 2us : update_rq_clock <-dequeue_task 1924 kworker/-59 3d..2 2us : dequeue_task_fair <-dequeue_task 1925 kworker/-59 3d..2 2us : update_curr <-dequeue_task_fair 1926 kworker/-59 3d..2 2us : update_min_vruntime <-update_curr 1927 kworker/-59 3d..2 3us : cpuacct_charge <-update_curr 1928 kworker/-59 3d..2 3us : __rcu_read_lock <-cpuacct_charge 1929 kworker/-59 3d..2 3us : __rcu_read_unlock <-cpuacct_charge 1930 kworker/-59 3d..2 3us : update_cfs_rq_blocked_load <-dequeue_task_fair 1931 kworker/-59 3d..2 4us : clear_buddies <-dequeue_task_fair 1932 kworker/-59 3d..2 4us : account_entity_dequeue <-dequeue_task_fair 1933 kworker/-59 3d..2 4us : update_min_vruntime <-dequeue_task_fair 1934 kworker/-59 3d..2 4us : update_cfs_shares <-dequeue_task_fair 1935 kworker/-59 3d..2 5us : hrtick_update <-dequeue_task_fair 1936 kworker/-59 3d..2 5us : wq_worker_sleeping <-__schedule 1937 kworker/-59 3d..2 5us : kthread_data <-wq_worker_sleeping 1938 kworker/-59 3d..2 5us : put_prev_task_fair <-__schedule 1939 kworker/-59 3d..2 6us : pick_next_task_fair <-pick_next_task 1940 kworker/-59 3d..2 6us : clear_buddies <-pick_next_task_fair 1941 kworker/-59 3d..2 6us : set_next_entity <-pick_next_task_fair 1942 kworker/-59 3d..2 6us : update_stats_wait_end <-set_next_entity 1943 ls-2269 3d..2 7us : finish_task_switch <-__schedule 1944 ls-2269 3d..2 7us : _raw_spin_unlock_irq <-finish_task_switch 1945 ls-2269 3d..2 8us : do_IRQ <-ret_from_intr 1946 ls-2269 3d..2 8us : irq_enter <-do_IRQ 1947 ls-2269 3d..2 8us : rcu_irq_enter <-irq_enter 1948 ls-2269 3d..2 9us : add_preempt_count <-irq_enter 1949 ls-2269 3d.h2 9us : exit_idle <-do_IRQ 1950 [...] 1951 ls-2269 3d.h3 20us : sub_preempt_count <-_raw_spin_unlock 1952 ls-2269 3d.h2 20us : irq_exit <-do_IRQ 1953 ls-2269 3d.h2 21us : sub_preempt_count <-irq_exit 1954 ls-2269 3d..3 21us : do_softirq <-irq_exit 1955 ls-2269 3d..3 21us : __do_softirq <-call_softirq 1956 ls-2269 3d..3 21us+: __local_bh_disable <-__do_softirq 1957 ls-2269 3d.s4 29us : sub_preempt_count <-_local_bh_enable_ip 1958 ls-2269 3d.s5 29us : sub_preempt_count <-_local_bh_enable_ip 1959 ls-2269 3d.s5 31us : do_IRQ <-ret_from_intr 1960 ls-2269 3d.s5 31us : irq_enter <-do_IRQ 1961 ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter 1962 [...] 1963 ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter 1964 ls-2269 3d.s5 32us : add_preempt_count <-irq_enter 1965 ls-2269 3d.H5 32us : exit_idle <-do_IRQ 1966 ls-2269 3d.H5 32us : handle_irq <-do_IRQ 1967 ls-2269 3d.H5 32us : irq_to_desc <-handle_irq 1968 ls-2269 3d.H5 33us : handle_fasteoi_irq <-handle_irq 1969 [...] 1970 ls-2269 3d.s5 158us : _raw_spin_unlock_irqrestore <-rtl8139_poll 1971 ls-2269 3d.s3 158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action 1972 ls-2269 3d.s3 159us : __local_bh_enable <-__do_softirq 1973 ls-2269 3d.s3 159us : sub_preempt_count <-__local_bh_enable 1974 ls-2269 3d..3 159us : idle_cpu <-irq_exit 1975 ls-2269 3d..3 159us : rcu_irq_exit <-irq_exit 1976 ls-2269 3d..3 160us : sub_preempt_count <-irq_exit 1977 ls-2269 3d... 161us : __mutex_unlock_slowpath <-mutex_unlock 1978 ls-2269 3d... 162us+: trace_hardirqs_on <-mutex_unlock 1979 ls-2269 3d... 186us : <stack trace> 1980 => __mutex_unlock_slowpath 1981 => mutex_unlock 1982 => process_output 1983 => n_tty_write 1984 => tty_write 1985 => vfs_write 1986 => sys_write 1987 => system_call_fastpath 1988 1989This is an interesting trace. It started with kworker running and 1990scheduling out and ls taking over. But as soon as ls released the 1991rq lock and enabled interrupts (but not preemption) an interrupt 1992triggered. When the interrupt finished, it started running softirqs. 1993But while the softirq was running, another interrupt triggered. 1994When an interrupt is running inside a softirq, the annotation is 'H'. 1995 1996 1997wakeup 1998------ 1999 2000One common case that people are interested in tracing is the 2001time it takes for a task that is woken to actually wake up. 2002Now for non Real-Time tasks, this can be arbitrary. But tracing 2003it nonetheless can be interesting. 2004 2005Without function tracing:: 2006 2007 # echo 0 > options/function-trace 2008 # echo wakeup > current_tracer 2009 # echo 1 > tracing_on 2010 # echo 0 > tracing_max_latency 2011 # chrt -f 5 sleep 1 2012 # echo 0 > tracing_on 2013 # cat trace 2014 # tracer: wakeup 2015 # 2016 # wakeup latency trace v1.1.5 on 3.8.0-test+ 2017 # -------------------------------------------------------------------- 2018 # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 2019 # ----------------- 2020 # | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0) 2021 # ----------------- 2022 # 2023 # _------=> CPU# 2024 # / _-----=> irqs-off 2025 # | / _----=> need-resched 2026 # || / _---=> hardirq/softirq 2027 # ||| / _--=> preempt-depth 2028 # |||| / delay 2029 # cmd pid ||||| time | caller 2030 # \ / ||||| \ | / 2031 <idle>-0 3dNs7 0us : 0:120:R + [003] 312:100:R kworker/3:1H 2032 <idle>-0 3dNs7 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up 2033 <idle>-0 3d..3 15us : __schedule <-schedule 2034 <idle>-0 3d..3 15us : 0:120:R ==> [003] 312:100:R kworker/3:1H 2035 2036The tracer only traces the highest priority task in the system 2037to avoid tracing the normal circumstances. Here we see that 2038the kworker with a nice priority of -20 (not very nice), took 2039just 15 microseconds from the time it woke up, to the time it 2040ran. 2041 2042Non Real-Time tasks are not that interesting. A more interesting 2043trace is to concentrate only on Real-Time tasks. 2044 2045wakeup_rt 2046--------- 2047 2048In a Real-Time environment it is very important to know the 2049wakeup time it takes for the highest priority task that is woken 2050up to the time that it executes. This is also known as "schedule 2051latency". I stress the point that this is about RT tasks. It is 2052also important to know the scheduling latency of non-RT tasks, 2053but the average schedule latency is better for non-RT tasks. 2054Tools like LatencyTop are more appropriate for such 2055measurements. 2056 2057Real-Time environments are interested in the worst case latency. 2058That is the longest latency it takes for something to happen, 2059and not the average. We can have a very fast scheduler that may 2060only have a large latency once in a while, but that would not 2061work well with Real-Time tasks. The wakeup_rt tracer was designed 2062to record the worst case wakeups of RT tasks. Non-RT tasks are 2063not recorded because the tracer only records one worst case and 2064tracing non-RT tasks that are unpredictable will overwrite the 2065worst case latency of RT tasks (just run the normal wakeup 2066tracer for a while to see that effect). 2067 2068Since this tracer only deals with RT tasks, we will run this 2069slightly differently than we did with the previous tracers. 2070Instead of performing an 'ls', we will run 'sleep 1' under 2071'chrt' which changes the priority of the task. 2072:: 2073 2074 # echo 0 > options/function-trace 2075 # echo wakeup_rt > current_tracer 2076 # echo 1 > tracing_on 2077 # echo 0 > tracing_max_latency 2078 # chrt -f 5 sleep 1 2079 # echo 0 > tracing_on 2080 # cat trace 2081 # tracer: wakeup 2082 # 2083 # tracer: wakeup_rt 2084 # 2085 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+ 2086 # -------------------------------------------------------------------- 2087 # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 2088 # ----------------- 2089 # | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5) 2090 # ----------------- 2091 # 2092 # _------=> CPU# 2093 # / _-----=> irqs-off 2094 # | / _----=> need-resched 2095 # || / _---=> hardirq/softirq 2096 # ||| / _--=> preempt-depth 2097 # |||| / delay 2098 # cmd pid ||||| time | caller 2099 # \ / ||||| \ | / 2100 <idle>-0 3d.h4 0us : 0:120:R + [003] 2389: 94:R sleep 2101 <idle>-0 3d.h4 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up 2102 <idle>-0 3d..3 5us : __schedule <-schedule 2103 <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep 2104 2105 2106Running this on an idle system, we see that it only took 5 microseconds 2107to perform the task switch. Note, since the trace point in the schedule 2108is before the actual "switch", we stop the tracing when the recorded task 2109is about to schedule in. This may change if we add a new marker at the 2110end of the scheduler. 2111 2112Notice that the recorded task is 'sleep' with the PID of 2389 2113and it has an rt_prio of 5. This priority is user-space priority 2114and not the internal kernel priority. The policy is 1 for 2115SCHED_FIFO and 2 for SCHED_RR. 2116 2117Note, that the trace data shows the internal priority (99 - rtprio). 2118:: 2119 2120 <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep 2121 2122The 0:120:R means idle was running with a nice priority of 0 (120 - 120) 2123and in the running state 'R'. The sleep task was scheduled in with 21242389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94) 2125and it too is in the running state. 2126 2127Doing the same with chrt -r 5 and function-trace set. 2128:: 2129 2130 echo 1 > options/function-trace 2131 2132 # tracer: wakeup_rt 2133 # 2134 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+ 2135 # -------------------------------------------------------------------- 2136 # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 2137 # ----------------- 2138 # | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5) 2139 # ----------------- 2140 # 2141 # _------=> CPU# 2142 # / _-----=> irqs-off 2143 # | / _----=> need-resched 2144 # || / _---=> hardirq/softirq 2145 # ||| / _--=> preempt-depth 2146 # |||| / delay 2147 # cmd pid ||||| time | caller 2148 # \ / ||||| \ | / 2149 <idle>-0 3d.h4 1us+: 0:120:R + [003] 2448: 94:R sleep 2150 <idle>-0 3d.h4 2us : ttwu_do_activate.constprop.87 <-try_to_wake_up 2151 <idle>-0 3d.h3 3us : check_preempt_curr <-ttwu_do_wakeup 2152 <idle>-0 3d.h3 3us : resched_curr <-check_preempt_curr 2153 <idle>-0 3dNh3 4us : task_woken_rt <-ttwu_do_wakeup 2154 <idle>-0 3dNh3 4us : _raw_spin_unlock <-try_to_wake_up 2155 <idle>-0 3dNh3 4us : sub_preempt_count <-_raw_spin_unlock 2156 <idle>-0 3dNh2 5us : ttwu_stat <-try_to_wake_up 2157 <idle>-0 3dNh2 5us : _raw_spin_unlock_irqrestore <-try_to_wake_up 2158 <idle>-0 3dNh2 6us : sub_preempt_count <-_raw_spin_unlock_irqrestore 2159 <idle>-0 3dNh1 6us : _raw_spin_lock <-__run_hrtimer 2160 <idle>-0 3dNh1 6us : add_preempt_count <-_raw_spin_lock 2161 <idle>-0 3dNh2 7us : _raw_spin_unlock <-hrtimer_interrupt 2162 <idle>-0 3dNh2 7us : sub_preempt_count <-_raw_spin_unlock 2163 <idle>-0 3dNh1 7us : tick_program_event <-hrtimer_interrupt 2164 <idle>-0 3dNh1 7us : clockevents_program_event <-tick_program_event 2165 <idle>-0 3dNh1 8us : ktime_get <-clockevents_program_event 2166 <idle>-0 3dNh1 8us : lapic_next_event <-clockevents_program_event 2167 <idle>-0 3dNh1 8us : irq_exit <-smp_apic_timer_interrupt 2168 <idle>-0 3dNh1 9us : sub_preempt_count <-irq_exit 2169 <idle>-0 3dN.2 9us : idle_cpu <-irq_exit 2170 <idle>-0 3dN.2 9us : rcu_irq_exit <-irq_exit 2171 <idle>-0 3dN.2 10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit 2172 <idle>-0 3dN.2 10us : sub_preempt_count <-irq_exit 2173 <idle>-0 3.N.1 11us : rcu_idle_exit <-cpu_idle 2174 <idle>-0 3dN.1 11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit 2175 <idle>-0 3.N.1 11us : tick_nohz_idle_exit <-cpu_idle 2176 <idle>-0 3dN.1 12us : menu_hrtimer_cancel <-tick_nohz_idle_exit 2177 <idle>-0 3dN.1 12us : ktime_get <-tick_nohz_idle_exit 2178 <idle>-0 3dN.1 12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit 2179 <idle>-0 3dN.1 13us : cpu_load_update_nohz <-tick_nohz_idle_exit 2180 <idle>-0 3dN.1 13us : _raw_spin_lock <-cpu_load_update_nohz 2181 <idle>-0 3dN.1 13us : add_preempt_count <-_raw_spin_lock 2182 <idle>-0 3dN.2 13us : __cpu_load_update <-cpu_load_update_nohz 2183 <idle>-0 3dN.2 14us : sched_avg_update <-__cpu_load_update 2184 <idle>-0 3dN.2 14us : _raw_spin_unlock <-cpu_load_update_nohz 2185 <idle>-0 3dN.2 14us : sub_preempt_count <-_raw_spin_unlock 2186 <idle>-0 3dN.1 15us : calc_load_nohz_stop <-tick_nohz_idle_exit 2187 <idle>-0 3dN.1 15us : touch_softlockup_watchdog <-tick_nohz_idle_exit 2188 <idle>-0 3dN.1 15us : hrtimer_cancel <-tick_nohz_idle_exit 2189 <idle>-0 3dN.1 15us : hrtimer_try_to_cancel <-hrtimer_cancel 2190 <idle>-0 3dN.1 16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel 2191 <idle>-0 3dN.1 16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18 2192 <idle>-0 3dN.1 16us : add_preempt_count <-_raw_spin_lock_irqsave 2193 <idle>-0 3dN.2 17us : __remove_hrtimer <-remove_hrtimer.part.16 2194 <idle>-0 3dN.2 17us : hrtimer_force_reprogram <-__remove_hrtimer 2195 <idle>-0 3dN.2 17us : tick_program_event <-hrtimer_force_reprogram 2196 <idle>-0 3dN.2 18us : clockevents_program_event <-tick_program_event 2197 <idle>-0 3dN.2 18us : ktime_get <-clockevents_program_event 2198 <idle>-0 3dN.2 18us : lapic_next_event <-clockevents_program_event 2199 <idle>-0 3dN.2 19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel 2200 <idle>-0 3dN.2 19us : sub_preempt_count <-_raw_spin_unlock_irqrestore 2201 <idle>-0 3dN.1 19us : hrtimer_forward <-tick_nohz_idle_exit 2202 <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward 2203 <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward 2204 <idle>-0 3dN.1 20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11 2205 <idle>-0 3dN.1 20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns 2206 <idle>-0 3dN.1 21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns 2207 <idle>-0 3dN.1 21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18 2208 <idle>-0 3dN.1 21us : add_preempt_count <-_raw_spin_lock_irqsave 2209 <idle>-0 3dN.2 22us : ktime_add_safe <-__hrtimer_start_range_ns 2210 <idle>-0 3dN.2 22us : enqueue_hrtimer <-__hrtimer_start_range_ns 2211 <idle>-0 3dN.2 22us : tick_program_event <-__hrtimer_start_range_ns 2212 <idle>-0 3dN.2 23us : clockevents_program_event <-tick_program_event 2213 <idle>-0 3dN.2 23us : ktime_get <-clockevents_program_event 2214 <idle>-0 3dN.2 23us : lapic_next_event <-clockevents_program_event 2215 <idle>-0 3dN.2 24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns 2216 <idle>-0 3dN.2 24us : sub_preempt_count <-_raw_spin_unlock_irqrestore 2217 <idle>-0 3dN.1 24us : account_idle_ticks <-tick_nohz_idle_exit 2218 <idle>-0 3dN.1 24us : account_idle_time <-account_idle_ticks 2219 <idle>-0 3.N.1 25us : sub_preempt_count <-cpu_idle 2220 <idle>-0 3.N.. 25us : schedule <-cpu_idle 2221 <idle>-0 3.N.. 25us : __schedule <-preempt_schedule 2222 <idle>-0 3.N.. 26us : add_preempt_count <-__schedule 2223 <idle>-0 3.N.1 26us : rcu_note_context_switch <-__schedule 2224 <idle>-0 3.N.1 26us : rcu_sched_qs <-rcu_note_context_switch 2225 <idle>-0 3dN.1 27us : rcu_preempt_qs <-rcu_note_context_switch 2226 <idle>-0 3.N.1 27us : _raw_spin_lock_irq <-__schedule 2227 <idle>-0 3dN.1 27us : add_preempt_count <-_raw_spin_lock_irq 2228 <idle>-0 3dN.2 28us : put_prev_task_idle <-__schedule 2229 <idle>-0 3dN.2 28us : pick_next_task_stop <-pick_next_task 2230 <idle>-0 3dN.2 28us : pick_next_task_rt <-pick_next_task 2231 <idle>-0 3dN.2 29us : dequeue_pushable_task <-pick_next_task_rt 2232 <idle>-0 3d..3 29us : __schedule <-preempt_schedule 2233 <idle>-0 3d..3 30us : 0:120:R ==> [003] 2448: 94:R sleep 2234 2235This isn't that big of a trace, even with function tracing enabled, 2236so I included the entire trace. 2237 2238The interrupt went off while when the system was idle. Somewhere 2239before task_woken_rt() was called, the NEED_RESCHED flag was set, 2240this is indicated by the first occurrence of the 'N' flag. 2241 2242Latency tracing and events 2243-------------------------- 2244As function tracing can induce a much larger latency, but without 2245seeing what happens within the latency it is hard to know what 2246caused it. There is a middle ground, and that is with enabling 2247events. 2248:: 2249 2250 # echo 0 > options/function-trace 2251 # echo wakeup_rt > current_tracer 2252 # echo 1 > events/enable 2253 # echo 1 > tracing_on 2254 # echo 0 > tracing_max_latency 2255 # chrt -f 5 sleep 1 2256 # echo 0 > tracing_on 2257 # cat trace 2258 # tracer: wakeup_rt 2259 # 2260 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+ 2261 # -------------------------------------------------------------------- 2262 # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 2263 # ----------------- 2264 # | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5) 2265 # ----------------- 2266 # 2267 # _------=> CPU# 2268 # / _-----=> irqs-off 2269 # | / _----=> need-resched 2270 # || / _---=> hardirq/softirq 2271 # ||| / _--=> preempt-depth 2272 # |||| / delay 2273 # cmd pid ||||| time | caller 2274 # \ / ||||| \ | / 2275 <idle>-0 2d.h4 0us : 0:120:R + [002] 5882: 94:R sleep 2276 <idle>-0 2d.h4 0us : ttwu_do_activate.constprop.87 <-try_to_wake_up 2277 <idle>-0 2d.h4 1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002 2278 <idle>-0 2dNh2 1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8 2279 <idle>-0 2.N.2 2us : power_end: cpu_id=2 2280 <idle>-0 2.N.2 3us : cpu_idle: state=4294967295 cpu_id=2 2281 <idle>-0 2dN.3 4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0 2282 <idle>-0 2dN.3 4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000 2283 <idle>-0 2.N.2 5us : rcu_utilization: Start context switch 2284 <idle>-0 2.N.2 5us : rcu_utilization: End context switch 2285 <idle>-0 2d..3 6us : __schedule <-schedule 2286 <idle>-0 2d..3 6us : 0:120:R ==> [002] 5882: 94:R sleep 2287 2288 2289Hardware Latency Detector 2290------------------------- 2291 2292The hardware latency detector is executed by enabling the "hwlat" tracer. 2293 2294NOTE, this tracer will affect the performance of the system as it will 2295periodically make a CPU constantly busy with interrupts disabled. 2296:: 2297 2298 # echo hwlat > current_tracer 2299 # sleep 100 2300 # cat trace 2301 # tracer: hwlat 2302 # 2303 # entries-in-buffer/entries-written: 13/13 #P:8 2304 # 2305 # _-----=> irqs-off 2306 # / _----=> need-resched 2307 # | / _---=> hardirq/softirq 2308 # || / _--=> preempt-depth 2309 # ||| / delay 2310 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 2311 # | | | |||| | | 2312 <...>-1729 [001] d... 678.473449: #1 inner/outer(us): 11/12 ts:1581527483.343962693 count:6 2313 <...>-1729 [004] d... 689.556542: #2 inner/outer(us): 16/9 ts:1581527494.889008092 count:1 2314 <...>-1729 [005] d... 714.756290: #3 inner/outer(us): 16/16 ts:1581527519.678961629 count:5 2315 <...>-1729 [001] d... 718.788247: #4 inner/outer(us): 9/17 ts:1581527523.889012713 count:1 2316 <...>-1729 [002] d... 719.796341: #5 inner/outer(us): 13/9 ts:1581527524.912872606 count:1 2317 <...>-1729 [006] d... 844.787091: #6 inner/outer(us): 9/12 ts:1581527649.889048502 count:2 2318 <...>-1729 [003] d... 849.827033: #7 inner/outer(us): 18/9 ts:1581527654.889013793 count:1 2319 <...>-1729 [007] d... 853.859002: #8 inner/outer(us): 9/12 ts:1581527658.889065736 count:1 2320 <...>-1729 [001] d... 855.874978: #9 inner/outer(us): 9/11 ts:1581527660.861991877 count:1 2321 <...>-1729 [001] d... 863.938932: #10 inner/outer(us): 9/11 ts:1581527668.970010500 count:1 nmi-total:7 nmi-count:1 2322 <...>-1729 [007] d... 878.050780: #11 inner/outer(us): 9/12 ts:1581527683.385002600 count:1 nmi-total:5 nmi-count:1 2323 <...>-1729 [007] d... 886.114702: #12 inner/outer(us): 9/12 ts:1581527691.385001600 count:1 2324 2325 2326The above output is somewhat the same in the header. All events will have 2327interrupts disabled 'd'. Under the FUNCTION title there is: 2328 2329 #1 2330 This is the count of events recorded that were greater than the 2331 tracing_threshold (See below). 2332 2333 inner/outer(us): 11/11 2334 2335 This shows two numbers as "inner latency" and "outer latency". The test 2336 runs in a loop checking a timestamp twice. The latency detected within 2337 the two timestamps is the "inner latency" and the latency detected 2338 after the previous timestamp and the next timestamp in the loop is 2339 the "outer latency". 2340 2341 ts:1581527483.343962693 2342 2343 The absolute timestamp that the first latency was recorded in the window. 2344 2345 count:6 2346 2347 The number of times a latency was detected during the window. 2348 2349 nmi-total:7 nmi-count:1 2350 2351 On architectures that support it, if an NMI comes in during the 2352 test, the time spent in NMI is reported in "nmi-total" (in 2353 microseconds). 2354 2355 All architectures that have NMIs will show the "nmi-count" if an 2356 NMI comes in during the test. 2357 2358hwlat files: 2359 2360 tracing_threshold 2361 This gets automatically set to "10" to represent 10 2362 microseconds. This is the threshold of latency that 2363 needs to be detected before the trace will be recorded. 2364 2365 Note, when hwlat tracer is finished (another tracer is 2366 written into "current_tracer"), the original value for 2367 tracing_threshold is placed back into this file. 2368 2369 hwlat_detector/width 2370 The length of time the test runs with interrupts disabled. 2371 2372 hwlat_detector/window 2373 The length of time of the window which the test 2374 runs. That is, the test will run for "width" 2375 microseconds per "window" microseconds 2376 2377 tracing_cpumask 2378 When the test is started. A kernel thread is created that 2379 runs the test. This thread will alternate between CPUs 2380 listed in the tracing_cpumask between each period 2381 (one "window"). To limit the test to specific CPUs 2382 set the mask in this file to only the CPUs that the test 2383 should run on. 2384 2385function 2386-------- 2387 2388This tracer is the function tracer. Enabling the function tracer 2389can be done from the debug file system. Make sure the 2390ftrace_enabled is set; otherwise this tracer is a nop. 2391See the "ftrace_enabled" section below. 2392:: 2393 2394 # sysctl kernel.ftrace_enabled=1 2395 # echo function > current_tracer 2396 # echo 1 > tracing_on 2397 # usleep 1 2398 # echo 0 > tracing_on 2399 # cat trace 2400 # tracer: function 2401 # 2402 # entries-in-buffer/entries-written: 24799/24799 #P:4 2403 # 2404 # _-----=> irqs-off 2405 # / _----=> need-resched 2406 # | / _---=> hardirq/softirq 2407 # || / _--=> preempt-depth 2408 # ||| / delay 2409 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 2410 # | | | |||| | | 2411 bash-1994 [002] .... 3082.063030: mutex_unlock <-rb_simple_write 2412 bash-1994 [002] .... 3082.063031: __mutex_unlock_slowpath <-mutex_unlock 2413 bash-1994 [002] .... 3082.063031: __fsnotify_parent <-fsnotify_modify 2414 bash-1994 [002] .... 3082.063032: fsnotify <-fsnotify_modify 2415 bash-1994 [002] .... 3082.063032: __srcu_read_lock <-fsnotify 2416 bash-1994 [002] .... 3082.063032: add_preempt_count <-__srcu_read_lock 2417 bash-1994 [002] ...1 3082.063032: sub_preempt_count <-__srcu_read_lock 2418 bash-1994 [002] .... 3082.063033: __srcu_read_unlock <-fsnotify 2419 [...] 2420 2421 2422Note: function tracer uses ring buffers to store the above 2423entries. The newest data may overwrite the oldest data. 2424Sometimes using echo to stop the trace is not sufficient because 2425the tracing could have overwritten the data that you wanted to 2426record. For this reason, it is sometimes better to disable 2427tracing directly from a program. This allows you to stop the 2428tracing at the point that you hit the part that you are 2429interested in. To disable the tracing directly from a C program, 2430something like following code snippet can be used:: 2431 2432 int trace_fd; 2433 [...] 2434 int main(int argc, char *argv[]) { 2435 [...] 2436 trace_fd = open(tracing_file("tracing_on"), O_WRONLY); 2437 [...] 2438 if (condition_hit()) { 2439 write(trace_fd, "0", 1); 2440 } 2441 [...] 2442 } 2443 2444 2445Single thread tracing 2446--------------------- 2447 2448By writing into set_ftrace_pid you can trace a 2449single thread. For example:: 2450 2451 # cat set_ftrace_pid 2452 no pid 2453 # echo 3111 > set_ftrace_pid 2454 # cat set_ftrace_pid 2455 3111 2456 # echo function > current_tracer 2457 # cat trace | head 2458 # tracer: function 2459 # 2460 # TASK-PID CPU# TIMESTAMP FUNCTION 2461 # | | | | | 2462 yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return 2463 yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range 2464 yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel 2465 yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel 2466 yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll 2467 yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll 2468 # echo > set_ftrace_pid 2469 # cat trace |head 2470 # tracer: function 2471 # 2472 # TASK-PID CPU# TIMESTAMP FUNCTION 2473 # | | | | | 2474 ##### CPU 3 buffer started #### 2475 yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait 2476 yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry 2477 yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry 2478 yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit 2479 yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit 2480 2481If you want to trace a function when executing, you could use 2482something like this simple program. 2483:: 2484 2485 #include <stdio.h> 2486 #include <stdlib.h> 2487 #include <sys/types.h> 2488 #include <sys/stat.h> 2489 #include <fcntl.h> 2490 #include <unistd.h> 2491 #include <string.h> 2492 2493 #define _STR(x) #x 2494 #define STR(x) _STR(x) 2495 #define MAX_PATH 256 2496 2497 const char *find_tracefs(void) 2498 { 2499 static char tracefs[MAX_PATH+1]; 2500 static int tracefs_found; 2501 char type[100]; 2502 FILE *fp; 2503 2504 if (tracefs_found) 2505 return tracefs; 2506 2507 if ((fp = fopen("/proc/mounts","r")) == NULL) { 2508 perror("/proc/mounts"); 2509 return NULL; 2510 } 2511 2512 while (fscanf(fp, "%*s %" 2513 STR(MAX_PATH) 2514 "s %99s %*s %*d %*d\n", 2515 tracefs, type) == 2) { 2516 if (strcmp(type, "tracefs") == 0) 2517 break; 2518 } 2519 fclose(fp); 2520 2521 if (strcmp(type, "tracefs") != 0) { 2522 fprintf(stderr, "tracefs not mounted"); 2523 return NULL; 2524 } 2525 2526 strcat(tracefs, "/tracing/"); 2527 tracefs_found = 1; 2528 2529 return tracefs; 2530 } 2531 2532 const char *tracing_file(const char *file_name) 2533 { 2534 static char trace_file[MAX_PATH+1]; 2535 snprintf(trace_file, MAX_PATH, "%s/%s", find_tracefs(), file_name); 2536 return trace_file; 2537 } 2538 2539 int main (int argc, char **argv) 2540 { 2541 if (argc < 1) 2542 exit(-1); 2543 2544 if (fork() > 0) { 2545 int fd, ffd; 2546 char line[64]; 2547 int s; 2548 2549 ffd = open(tracing_file("current_tracer"), O_WRONLY); 2550 if (ffd < 0) 2551 exit(-1); 2552 write(ffd, "nop", 3); 2553 2554 fd = open(tracing_file("set_ftrace_pid"), O_WRONLY); 2555 s = sprintf(line, "%d\n", getpid()); 2556 write(fd, line, s); 2557 2558 write(ffd, "function", 8); 2559 2560 close(fd); 2561 close(ffd); 2562 2563 execvp(argv[1], argv+1); 2564 } 2565 2566 return 0; 2567 } 2568 2569Or this simple script! 2570:: 2571 2572 #!/bin/bash 2573 2574 tracefs=`sed -ne 's/^tracefs \(.*\) tracefs.*/\1/p' /proc/mounts` 2575 echo 0 > $tracefs/tracing_on 2576 echo $$ > $tracefs/set_ftrace_pid 2577 echo function > $tracefs/current_tracer 2578 echo 1 > $tracefs/tracing_on 2579 exec "$@" 2580 2581 2582function graph tracer 2583--------------------------- 2584 2585This tracer is similar to the function tracer except that it 2586probes a function on its entry and its exit. This is done by 2587using a dynamically allocated stack of return addresses in each 2588task_struct. On function entry the tracer overwrites the return 2589address of each function traced to set a custom probe. Thus the 2590original return address is stored on the stack of return address 2591in the task_struct. 2592 2593Probing on both ends of a function leads to special features 2594such as: 2595 2596- measure of a function's time execution 2597- having a reliable call stack to draw function calls graph 2598 2599This tracer is useful in several situations: 2600 2601- you want to find the reason of a strange kernel behavior and 2602 need to see what happens in detail on any areas (or specific 2603 ones). 2604 2605- you are experiencing weird latencies but it's difficult to 2606 find its origin. 2607 2608- you want to find quickly which path is taken by a specific 2609 function 2610 2611- you just want to peek inside a working kernel and want to see 2612 what happens there. 2613 2614:: 2615 2616 # tracer: function_graph 2617 # 2618 # CPU DURATION FUNCTION CALLS 2619 # | | | | | | | 2620 2621 0) | sys_open() { 2622 0) | do_sys_open() { 2623 0) | getname() { 2624 0) | kmem_cache_alloc() { 2625 0) 1.382 us | __might_sleep(); 2626 0) 2.478 us | } 2627 0) | strncpy_from_user() { 2628 0) | might_fault() { 2629 0) 1.389 us | __might_sleep(); 2630 0) 2.553 us | } 2631 0) 3.807 us | } 2632 0) 7.876 us | } 2633 0) | alloc_fd() { 2634 0) 0.668 us | _spin_lock(); 2635 0) 0.570 us | expand_files(); 2636 0) 0.586 us | _spin_unlock(); 2637 2638 2639There are several columns that can be dynamically 2640enabled/disabled. You can use every combination of options you 2641want, depending on your needs. 2642 2643- The cpu number on which the function executed is default 2644 enabled. It is sometimes better to only trace one cpu (see 2645 tracing_cpumask file) or you might sometimes see unordered 2646 function calls while cpu tracing switch. 2647 2648 - hide: echo nofuncgraph-cpu > trace_options 2649 - show: echo funcgraph-cpu > trace_options 2650 2651- The duration (function's time of execution) is displayed on 2652 the closing bracket line of a function or on the same line 2653 than the current function in case of a leaf one. It is default 2654 enabled. 2655 2656 - hide: echo nofuncgraph-duration > trace_options 2657 - show: echo funcgraph-duration > trace_options 2658 2659- The overhead field precedes the duration field in case of 2660 reached duration thresholds. 2661 2662 - hide: echo nofuncgraph-overhead > trace_options 2663 - show: echo funcgraph-overhead > trace_options 2664 - depends on: funcgraph-duration 2665 2666 ie:: 2667 2668 3) # 1837.709 us | } /* __switch_to */ 2669 3) | finish_task_switch() { 2670 3) 0.313 us | _raw_spin_unlock_irq(); 2671 3) 3.177 us | } 2672 3) # 1889.063 us | } /* __schedule */ 2673 3) ! 140.417 us | } /* __schedule */ 2674 3) # 2034.948 us | } /* schedule */ 2675 3) * 33998.59 us | } /* schedule_preempt_disabled */ 2676 2677 [...] 2678 2679 1) 0.260 us | msecs_to_jiffies(); 2680 1) 0.313 us | __rcu_read_unlock(); 2681 1) + 61.770 us | } 2682 1) + 64.479 us | } 2683 1) 0.313 us | rcu_bh_qs(); 2684 1) 0.313 us | __local_bh_enable(); 2685 1) ! 217.240 us | } 2686 1) 0.365 us | idle_cpu(); 2687 1) | rcu_irq_exit() { 2688 1) 0.417 us | rcu_eqs_enter_common.isra.47(); 2689 1) 3.125 us | } 2690 1) ! 227.812 us | } 2691 1) ! 457.395 us | } 2692 1) @ 119760.2 us | } 2693 2694 [...] 2695 2696 2) | handle_IPI() { 2697 1) 6.979 us | } 2698 2) 0.417 us | scheduler_ipi(); 2699 1) 9.791 us | } 2700 1) + 12.917 us | } 2701 2) 3.490 us | } 2702 1) + 15.729 us | } 2703 1) + 18.542 us | } 2704 2) $ 3594274 us | } 2705 2706Flags:: 2707 2708 + means that the function exceeded 10 usecs. 2709 ! means that the function exceeded 100 usecs. 2710 # means that the function exceeded 1000 usecs. 2711 * means that the function exceeded 10 msecs. 2712 @ means that the function exceeded 100 msecs. 2713 $ means that the function exceeded 1 sec. 2714 2715 2716- The task/pid field displays the thread cmdline and pid which 2717 executed the function. It is default disabled. 2718 2719 - hide: echo nofuncgraph-proc > trace_options 2720 - show: echo funcgraph-proc > trace_options 2721 2722 ie:: 2723 2724 # tracer: function_graph 2725 # 2726 # CPU TASK/PID DURATION FUNCTION CALLS 2727 # | | | | | | | | | 2728 0) sh-4802 | | d_free() { 2729 0) sh-4802 | | call_rcu() { 2730 0) sh-4802 | | __call_rcu() { 2731 0) sh-4802 | 0.616 us | rcu_process_gp_end(); 2732 0) sh-4802 | 0.586 us | check_for_new_grace_period(); 2733 0) sh-4802 | 2.899 us | } 2734 0) sh-4802 | 4.040 us | } 2735 0) sh-4802 | 5.151 us | } 2736 0) sh-4802 | + 49.370 us | } 2737 2738 2739- The absolute time field is an absolute timestamp given by the 2740 system clock since it started. A snapshot of this time is 2741 given on each entry/exit of functions 2742 2743 - hide: echo nofuncgraph-abstime > trace_options 2744 - show: echo funcgraph-abstime > trace_options 2745 2746 ie:: 2747 2748 # 2749 # TIME CPU DURATION FUNCTION CALLS 2750 # | | | | | | | | 2751 360.774522 | 1) 0.541 us | } 2752 360.774522 | 1) 4.663 us | } 2753 360.774523 | 1) 0.541 us | __wake_up_bit(); 2754 360.774524 | 1) 6.796 us | } 2755 360.774524 | 1) 7.952 us | } 2756 360.774525 | 1) 9.063 us | } 2757 360.774525 | 1) 0.615 us | journal_mark_dirty(); 2758 360.774527 | 1) 0.578 us | __brelse(); 2759 360.774528 | 1) | reiserfs_prepare_for_journal() { 2760 360.774528 | 1) | unlock_buffer() { 2761 360.774529 | 1) | wake_up_bit() { 2762 360.774529 | 1) | bit_waitqueue() { 2763 360.774530 | 1) 0.594 us | __phys_addr(); 2764 2765 2766The function name is always displayed after the closing bracket 2767for a function if the start of that function is not in the 2768trace buffer. 2769 2770Display of the function name after the closing bracket may be 2771enabled for functions whose start is in the trace buffer, 2772allowing easier searching with grep for function durations. 2773It is default disabled. 2774 2775 - hide: echo nofuncgraph-tail > trace_options 2776 - show: echo funcgraph-tail > trace_options 2777 2778 Example with nofuncgraph-tail (default):: 2779 2780 0) | putname() { 2781 0) | kmem_cache_free() { 2782 0) 0.518 us | __phys_addr(); 2783 0) 1.757 us | } 2784 0) 2.861 us | } 2785 2786 Example with funcgraph-tail:: 2787 2788 0) | putname() { 2789 0) | kmem_cache_free() { 2790 0) 0.518 us | __phys_addr(); 2791 0) 1.757 us | } /* kmem_cache_free() */ 2792 0) 2.861 us | } /* putname() */ 2793 2794The return value of each traced function can be displayed after 2795an equal sign "=". When encountering system call failures, it 2796can be very helpful to quickly locate the function that first 2797returns an error code. 2798 2799 - hide: echo nofuncgraph-retval > trace_options 2800 - show: echo funcgraph-retval > trace_options 2801 2802 Example with funcgraph-retval:: 2803 2804 1) | cgroup_migrate() { 2805 1) 0.651 us | cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */ 2806 1) | cgroup_migrate_execute() { 2807 1) | cpu_cgroup_can_attach() { 2808 1) | cgroup_taskset_first() { 2809 1) 0.732 us | cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */ 2810 1) 1.232 us | } /* cgroup_taskset_first = 0xffff93fc8fb20000 */ 2811 1) 0.380 us | sched_rt_can_attach(); /* = 0x0 */ 2812 1) 2.335 us | } /* cpu_cgroup_can_attach = -22 */ 2813 1) 4.369 us | } /* cgroup_migrate_execute = -22 */ 2814 1) 7.143 us | } /* cgroup_migrate = -22 */ 2815 2816The above example shows that the function cpu_cgroup_can_attach 2817returned the error code -22 firstly, then we can read the code 2818of this function to get the root cause. 2819 2820When the option funcgraph-retval-hex is not set, the return value can 2821be displayed in a smart way. Specifically, if it is an error code, 2822it will be printed in signed decimal format, otherwise it will 2823printed in hexadecimal format. 2824 2825 - smart: echo nofuncgraph-retval-hex > trace_options 2826 - hexadecimal: echo funcgraph-retval-hex > trace_options 2827 2828 Example with funcgraph-retval-hex:: 2829 2830 1) | cgroup_migrate() { 2831 1) 0.651 us | cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */ 2832 1) | cgroup_migrate_execute() { 2833 1) | cpu_cgroup_can_attach() { 2834 1) | cgroup_taskset_first() { 2835 1) 0.732 us | cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */ 2836 1) 1.232 us | } /* cgroup_taskset_first = 0xffff93fc8fb20000 */ 2837 1) 0.380 us | sched_rt_can_attach(); /* = 0x0 */ 2838 1) 2.335 us | } /* cpu_cgroup_can_attach = 0xffffffea */ 2839 1) 4.369 us | } /* cgroup_migrate_execute = 0xffffffea */ 2840 1) 7.143 us | } /* cgroup_migrate = 0xffffffea */ 2841 2842At present, there are some limitations when using the funcgraph-retval 2843option, and these limitations will be eliminated in the future: 2844 2845- Even if the function return type is void, a return value will still 2846 be printed, and you can just ignore it. 2847 2848- Even if return values are stored in multiple registers, only the 2849 value contained in the first register will be recorded and printed. 2850 To illustrate, in the x86 architecture, eax and edx are used to store 2851 a 64-bit return value, with the lower 32 bits saved in eax and the 2852 upper 32 bits saved in edx. However, only the value stored in eax 2853 will be recorded and printed. 2854 2855- In certain procedure call standards, such as arm64's AAPCS64, when a 2856 type is smaller than a GPR, it is the responsibility of the consumer 2857 to perform the narrowing, and the upper bits may contain UNKNOWN values. 2858 Therefore, it is advisable to check the code for such cases. For instance, 2859 when using a u8 in a 64-bit GPR, bits [63:8] may contain arbitrary values, 2860 especially when larger types are truncated, whether explicitly or implicitly. 2861 Here are some specific cases to illustrate this point: 2862 2863 **Case One**: 2864 2865 The function narrow_to_u8 is defined as follows:: 2866 2867 u8 narrow_to_u8(u64 val) 2868 { 2869 // implicitly truncated 2870 return val; 2871 } 2872 2873 It may be compiled to:: 2874 2875 narrow_to_u8: 2876 < ... ftrace instrumentation ... > 2877 RET 2878 2879 If you pass 0x123456789abcdef to this function and want to narrow it, 2880 it may be recorded as 0x123456789abcdef instead of 0xef. 2881 2882 **Case Two**: 2883 2884 The function error_if_not_4g_aligned is defined as follows:: 2885 2886 int error_if_not_4g_aligned(u64 val) 2887 { 2888 if (val & GENMASK(31, 0)) 2889 return -EINVAL; 2890 2891 return 0; 2892 } 2893 2894 It could be compiled to:: 2895 2896 error_if_not_4g_aligned: 2897 CBNZ w0, .Lnot_aligned 2898 RET // bits [31:0] are zero, bits 2899 // [63:32] are UNKNOWN 2900 .Lnot_aligned: 2901 MOV x0, #-EINVAL 2902 RET 2903 2904 When passing 0x2_0000_0000 to it, the return value may be recorded as 2905 0x2_0000_0000 instead of 0. 2906 2907You can put some comments on specific functions by using 2908trace_printk() For example, if you want to put a comment inside 2909the __might_sleep() function, you just have to include 2910<linux/ftrace.h> and call trace_printk() inside __might_sleep():: 2911 2912 trace_printk("I'm a comment!\n") 2913 2914will produce:: 2915 2916 1) | __might_sleep() { 2917 1) | /* I'm a comment! */ 2918 1) 1.449 us | } 2919 2920 2921You might find other useful features for this tracer in the 2922following "dynamic ftrace" section such as tracing only specific 2923functions or tasks. 2924 2925dynamic ftrace 2926-------------- 2927 2928If CONFIG_DYNAMIC_FTRACE is set, the system will run with 2929virtually no overhead when function tracing is disabled. The way 2930this works is the mcount function call (placed at the start of 2931every kernel function, produced by the -pg switch in gcc), 2932starts of pointing to a simple return. (Enabling FTRACE will 2933include the -pg switch in the compiling of the kernel.) 2934 2935At compile time every C file object is run through the 2936recordmcount program (located in the scripts directory). This 2937program will parse the ELF headers in the C object to find all 2938the locations in the .text section that call mcount. Starting 2939with gcc version 4.6, the -mfentry has been added for x86, which 2940calls "__fentry__" instead of "mcount". Which is called before 2941the creation of the stack frame. 2942 2943Note, not all sections are traced. They may be prevented by either 2944a notrace, or blocked another way and all inline functions are not 2945traced. Check the "available_filter_functions" file to see what functions 2946can be traced. 2947 2948A section called "__mcount_loc" is created that holds 2949references to all the mcount/fentry call sites in the .text section. 2950The recordmcount program re-links this section back into the 2951original object. The final linking stage of the kernel will add all these 2952references into a single table. 2953 2954On boot up, before SMP is initialized, the dynamic ftrace code 2955scans this table and updates all the locations into nops. It 2956also records the locations, which are added to the 2957available_filter_functions list. Modules are processed as they 2958are loaded and before they are executed. When a module is 2959unloaded, it also removes its functions from the ftrace function 2960list. This is automatic in the module unload code, and the 2961module author does not need to worry about it. 2962 2963When tracing is enabled, the process of modifying the function 2964tracepoints is dependent on architecture. The old method is to use 2965kstop_machine to prevent races with the CPUs executing code being 2966modified (which can cause the CPU to do undesirable things, especially 2967if the modified code crosses cache (or page) boundaries), and the nops are 2968patched back to calls. But this time, they do not call mcount 2969(which is just a function stub). They now call into the ftrace 2970infrastructure. 2971 2972The new method of modifying the function tracepoints is to place 2973a breakpoint at the location to be modified, sync all CPUs, modify 2974the rest of the instruction not covered by the breakpoint. Sync 2975all CPUs again, and then remove the breakpoint with the finished 2976version to the ftrace call site. 2977 2978Some archs do not even need to monkey around with the synchronization, 2979and can just slap the new code on top of the old without any 2980problems with other CPUs executing it at the same time. 2981 2982One special side-effect to the recording of the functions being 2983traced is that we can now selectively choose which functions we 2984wish to trace and which ones we want the mcount calls to remain 2985as nops. 2986 2987Two files are used, one for enabling and one for disabling the 2988tracing of specified functions. They are: 2989 2990 set_ftrace_filter 2991 2992and 2993 2994 set_ftrace_notrace 2995 2996A list of available functions that you can add to these files is 2997listed in: 2998 2999 available_filter_functions 3000 3001:: 3002 3003 # cat available_filter_functions 3004 put_prev_task_idle 3005 kmem_cache_create 3006 pick_next_task_rt 3007 cpus_read_lock 3008 pick_next_task_fair 3009 mutex_lock 3010 [...] 3011 3012If I am only interested in sys_nanosleep and hrtimer_interrupt:: 3013 3014 # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter 3015 # echo function > current_tracer 3016 # echo 1 > tracing_on 3017 # usleep 1 3018 # echo 0 > tracing_on 3019 # cat trace 3020 # tracer: function 3021 # 3022 # entries-in-buffer/entries-written: 5/5 #P:4 3023 # 3024 # _-----=> irqs-off 3025 # / _----=> need-resched 3026 # | / _---=> hardirq/softirq 3027 # || / _--=> preempt-depth 3028 # ||| / delay 3029 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3030 # | | | |||| | | 3031 usleep-2665 [001] .... 4186.475355: sys_nanosleep <-system_call_fastpath 3032 <idle>-0 [001] d.h1 4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt 3033 usleep-2665 [001] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt 3034 <idle>-0 [003] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt 3035 <idle>-0 [002] d.h1 4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt 3036 3037To see which functions are being traced, you can cat the file: 3038:: 3039 3040 # cat set_ftrace_filter 3041 hrtimer_interrupt 3042 sys_nanosleep 3043 3044 3045Perhaps this is not enough. The filters also allow glob(7) matching. 3046 3047 ``<match>*`` 3048 will match functions that begin with <match> 3049 ``*<match>`` 3050 will match functions that end with <match> 3051 ``*<match>*`` 3052 will match functions that have <match> in it 3053 ``<match1>*<match2>`` 3054 will match functions that begin with <match1> and end with <match2> 3055 3056.. note:: 3057 It is better to use quotes to enclose the wild cards, 3058 otherwise the shell may expand the parameters into names 3059 of files in the local directory. 3060 3061:: 3062 3063 # echo 'hrtimer_*' > set_ftrace_filter 3064 3065Produces:: 3066 3067 # tracer: function 3068 # 3069 # entries-in-buffer/entries-written: 897/897 #P:4 3070 # 3071 # _-----=> irqs-off 3072 # / _----=> need-resched 3073 # | / _---=> hardirq/softirq 3074 # || / _--=> preempt-depth 3075 # ||| / delay 3076 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3077 # | | | |||| | | 3078 <idle>-0 [003] dN.1 4228.547803: hrtimer_cancel <-tick_nohz_idle_exit 3079 <idle>-0 [003] dN.1 4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel 3080 <idle>-0 [003] dN.2 4228.547805: hrtimer_force_reprogram <-__remove_hrtimer 3081 <idle>-0 [003] dN.1 4228.547805: hrtimer_forward <-tick_nohz_idle_exit 3082 <idle>-0 [003] dN.1 4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11 3083 <idle>-0 [003] d..1 4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt 3084 <idle>-0 [003] d..1 4228.547859: hrtimer_start <-__tick_nohz_idle_enter 3085 <idle>-0 [003] d..2 4228.547860: hrtimer_force_reprogram <-__rem 3086 3087Notice that we lost the sys_nanosleep. 3088:: 3089 3090 # cat set_ftrace_filter 3091 hrtimer_run_queues 3092 hrtimer_run_pending 3093 hrtimer_setup 3094 hrtimer_cancel 3095 hrtimer_try_to_cancel 3096 hrtimer_forward 3097 hrtimer_start 3098 hrtimer_reprogram 3099 hrtimer_force_reprogram 3100 hrtimer_get_next_event 3101 hrtimer_interrupt 3102 hrtimer_nanosleep 3103 hrtimer_wakeup 3104 hrtimer_get_remaining 3105 hrtimer_get_res 3106 hrtimer_init_sleeper 3107 3108 3109This is because the '>' and '>>' act just like they do in bash. 3110To rewrite the filters, use '>' 3111To append to the filters, use '>>' 3112 3113To clear out a filter so that all functions will be recorded 3114again:: 3115 3116 # echo > set_ftrace_filter 3117 # cat set_ftrace_filter 3118 # 3119 3120Again, now we want to append. 3121 3122:: 3123 3124 # echo sys_nanosleep > set_ftrace_filter 3125 # cat set_ftrace_filter 3126 sys_nanosleep 3127 # echo 'hrtimer_*' >> set_ftrace_filter 3128 # cat set_ftrace_filter 3129 hrtimer_run_queues 3130 hrtimer_run_pending 3131 hrtimer_setup 3132 hrtimer_cancel 3133 hrtimer_try_to_cancel 3134 hrtimer_forward 3135 hrtimer_start 3136 hrtimer_reprogram 3137 hrtimer_force_reprogram 3138 hrtimer_get_next_event 3139 hrtimer_interrupt 3140 sys_nanosleep 3141 hrtimer_nanosleep 3142 hrtimer_wakeup 3143 hrtimer_get_remaining 3144 hrtimer_get_res 3145 hrtimer_init_sleeper 3146 3147 3148The set_ftrace_notrace prevents those functions from being 3149traced. 3150:: 3151 3152 # echo '*preempt*' '*lock*' > set_ftrace_notrace 3153 3154Produces:: 3155 3156 # tracer: function 3157 # 3158 # entries-in-buffer/entries-written: 39608/39608 #P:4 3159 # 3160 # _-----=> irqs-off 3161 # / _----=> need-resched 3162 # | / _---=> hardirq/softirq 3163 # || / _--=> preempt-depth 3164 # ||| / delay 3165 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3166 # | | | |||| | | 3167 bash-1994 [000] .... 4342.324896: file_ra_state_init <-do_dentry_open 3168 bash-1994 [000] .... 4342.324897: open_check_o_direct <-do_last 3169 bash-1994 [000] .... 4342.324897: ima_file_check <-do_last 3170 bash-1994 [000] .... 4342.324898: process_measurement <-ima_file_check 3171 bash-1994 [000] .... 4342.324898: ima_get_action <-process_measurement 3172 bash-1994 [000] .... 4342.324898: ima_match_policy <-ima_get_action 3173 bash-1994 [000] .... 4342.324899: do_truncate <-do_last 3174 bash-1994 [000] .... 4342.324899: setattr_should_drop_suidgid <-do_truncate 3175 bash-1994 [000] .... 4342.324899: notify_change <-do_truncate 3176 bash-1994 [000] .... 4342.324900: current_fs_time <-notify_change 3177 bash-1994 [000] .... 4342.324900: current_kernel_time <-current_fs_time 3178 bash-1994 [000] .... 4342.324900: timespec_trunc <-current_fs_time 3179 3180We can see that there's no more lock or preempt tracing. 3181 3182Selecting function filters via index 3183------------------------------------ 3184 3185Because processing of strings is expensive (the address of the function 3186needs to be looked up before comparing to the string being passed in), 3187an index can be used as well to enable functions. This is useful in the 3188case of setting thousands of specific functions at a time. By passing 3189in a list of numbers, no string processing will occur. Instead, the function 3190at the specific location in the internal array (which corresponds to the 3191functions in the "available_filter_functions" file), is selected. 3192 3193:: 3194 3195 # echo 1 > set_ftrace_filter 3196 3197Will select the first function listed in "available_filter_functions" 3198 3199:: 3200 3201 # head -1 available_filter_functions 3202 trace_initcall_finish_cb 3203 3204 # cat set_ftrace_filter 3205 trace_initcall_finish_cb 3206 3207 # head -50 available_filter_functions | tail -1 3208 x86_pmu_commit_txn 3209 3210 # echo 1 50 > set_ftrace_filter 3211 # cat set_ftrace_filter 3212 trace_initcall_finish_cb 3213 x86_pmu_commit_txn 3214 3215Dynamic ftrace with the function graph tracer 3216--------------------------------------------- 3217 3218Although what has been explained above concerns both the 3219function tracer and the function-graph-tracer, there are some 3220special features only available in the function-graph tracer. 3221 3222If you want to trace only one function and all of its children, 3223you just have to echo its name into set_graph_function:: 3224 3225 echo __do_fault > set_graph_function 3226 3227will produce the following "expanded" trace of the __do_fault() 3228function:: 3229 3230 0) | __do_fault() { 3231 0) | filemap_fault() { 3232 0) | find_lock_page() { 3233 0) 0.804 us | find_get_page(); 3234 0) | __might_sleep() { 3235 0) 1.329 us | } 3236 0) 3.904 us | } 3237 0) 4.979 us | } 3238 0) 0.653 us | _spin_lock(); 3239 0) 0.578 us | page_add_file_rmap(); 3240 0) 0.525 us | native_set_pte_at(); 3241 0) 0.585 us | _spin_unlock(); 3242 0) | unlock_page() { 3243 0) 0.541 us | page_waitqueue(); 3244 0) 0.639 us | __wake_up_bit(); 3245 0) 2.786 us | } 3246 0) + 14.237 us | } 3247 0) | __do_fault() { 3248 0) | filemap_fault() { 3249 0) | find_lock_page() { 3250 0) 0.698 us | find_get_page(); 3251 0) | __might_sleep() { 3252 0) 1.412 us | } 3253 0) 3.950 us | } 3254 0) 5.098 us | } 3255 0) 0.631 us | _spin_lock(); 3256 0) 0.571 us | page_add_file_rmap(); 3257 0) 0.526 us | native_set_pte_at(); 3258 0) 0.586 us | _spin_unlock(); 3259 0) | unlock_page() { 3260 0) 0.533 us | page_waitqueue(); 3261 0) 0.638 us | __wake_up_bit(); 3262 0) 2.793 us | } 3263 0) + 14.012 us | } 3264 3265You can also expand several functions at once:: 3266 3267 echo sys_open > set_graph_function 3268 echo sys_close >> set_graph_function 3269 3270Now if you want to go back to trace all functions you can clear 3271this special filter via:: 3272 3273 echo > set_graph_function 3274 3275 3276ftrace_enabled 3277-------------- 3278 3279Note, the proc sysctl ftrace_enable is a big on/off switch for the 3280function tracer. By default it is enabled (when function tracing is 3281enabled in the kernel). If it is disabled, all function tracing is 3282disabled. This includes not only the function tracers for ftrace, but 3283also for any other uses (perf, kprobes, stack tracing, profiling, etc). It 3284cannot be disabled if there is a callback with FTRACE_OPS_FL_PERMANENT set 3285registered. 3286 3287Please disable this with care. 3288 3289This can be disable (and enabled) with:: 3290 3291 sysctl kernel.ftrace_enabled=0 3292 sysctl kernel.ftrace_enabled=1 3293 3294 or 3295 3296 echo 0 > /proc/sys/kernel/ftrace_enabled 3297 echo 1 > /proc/sys/kernel/ftrace_enabled 3298 3299 3300Filter commands 3301--------------- 3302 3303A few commands are supported by the set_ftrace_filter interface. 3304Trace commands have the following format:: 3305 3306 <function>:<command>:<parameter> 3307 3308The following commands are supported: 3309 3310- mod: 3311 This command enables function filtering per module. The 3312 parameter defines the module. For example, if only the write* 3313 functions in the ext3 module are desired, run: 3314 3315 echo 'write*:mod:ext3' > set_ftrace_filter 3316 3317 This command interacts with the filter in the same way as 3318 filtering based on function names. Thus, adding more functions 3319 in a different module is accomplished by appending (>>) to the 3320 filter file. Remove specific module functions by prepending 3321 '!':: 3322 3323 echo '!writeback*:mod:ext3' >> set_ftrace_filter 3324 3325 Mod command supports module globbing. Disable tracing for all 3326 functions except a specific module:: 3327 3328 echo '!*:mod:!ext3' >> set_ftrace_filter 3329 3330 Disable tracing for all modules, but still trace kernel:: 3331 3332 echo '!*:mod:*' >> set_ftrace_filter 3333 3334 Enable filter only for kernel:: 3335 3336 echo '*write*:mod:!*' >> set_ftrace_filter 3337 3338 Enable filter for module globbing:: 3339 3340 echo '*write*:mod:*snd*' >> set_ftrace_filter 3341 3342- traceon/traceoff: 3343 These commands turn tracing on and off when the specified 3344 functions are hit. The parameter determines how many times the 3345 tracing system is turned on and off. If unspecified, there is 3346 no limit. For example, to disable tracing when a schedule bug 3347 is hit the first 5 times, run:: 3348 3349 echo '__schedule_bug:traceoff:5' > set_ftrace_filter 3350 3351 To always disable tracing when __schedule_bug is hit:: 3352 3353 echo '__schedule_bug:traceoff' > set_ftrace_filter 3354 3355 These commands are cumulative whether or not they are appended 3356 to set_ftrace_filter. To remove a command, prepend it by '!' 3357 and drop the parameter:: 3358 3359 echo '!__schedule_bug:traceoff:0' > set_ftrace_filter 3360 3361 The above removes the traceoff command for __schedule_bug 3362 that have a counter. To remove commands without counters:: 3363 3364 echo '!__schedule_bug:traceoff' > set_ftrace_filter 3365 3366- snapshot: 3367 Will cause a snapshot to be triggered when the function is hit. 3368 :: 3369 3370 echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter 3371 3372 To only snapshot once: 3373 :: 3374 3375 echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter 3376 3377 To remove the above commands:: 3378 3379 echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter 3380 echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter 3381 3382- enable_event/disable_event: 3383 These commands can enable or disable a trace event. Note, because 3384 function tracing callbacks are very sensitive, when these commands 3385 are registered, the trace point is activated, but disabled in 3386 a "soft" mode. That is, the tracepoint will be called, but 3387 just will not be traced. The event tracepoint stays in this mode 3388 as long as there's a command that triggers it. 3389 :: 3390 3391 echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \ 3392 set_ftrace_filter 3393 3394 The format is:: 3395 3396 <function>:enable_event:<system>:<event>[:count] 3397 <function>:disable_event:<system>:<event>[:count] 3398 3399 To remove the events commands:: 3400 3401 echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \ 3402 set_ftrace_filter 3403 echo '!schedule:disable_event:sched:sched_switch' > \ 3404 set_ftrace_filter 3405 3406- dump: 3407 When the function is hit, it will dump the contents of the ftrace 3408 ring buffer to the console. This is useful if you need to debug 3409 something, and want to dump the trace when a certain function 3410 is hit. Perhaps it's a function that is called before a triple 3411 fault happens and does not allow you to get a regular dump. 3412 3413- cpudump: 3414 When the function is hit, it will dump the contents of the ftrace 3415 ring buffer for the current CPU to the console. Unlike the "dump" 3416 command, it only prints out the contents of the ring buffer for the 3417 CPU that executed the function that triggered the dump. 3418 3419- stacktrace: 3420 When the function is hit, a stack trace is recorded. 3421 3422trace_pipe 3423---------- 3424 3425The trace_pipe outputs the same content as the trace file, but 3426the effect on the tracing is different. Every read from 3427trace_pipe is consumed. This means that subsequent reads will be 3428different. The trace is live. 3429:: 3430 3431 # echo function > current_tracer 3432 # cat trace_pipe > /tmp/trace.out & 3433 [1] 4153 3434 # echo 1 > tracing_on 3435 # usleep 1 3436 # echo 0 > tracing_on 3437 # cat trace 3438 # tracer: function 3439 # 3440 # entries-in-buffer/entries-written: 0/0 #P:4 3441 # 3442 # _-----=> irqs-off 3443 # / _----=> need-resched 3444 # | / _---=> hardirq/softirq 3445 # || / _--=> preempt-depth 3446 # ||| / delay 3447 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3448 # | | | |||| | | 3449 3450 # 3451 # cat /tmp/trace.out 3452 bash-1994 [000] .... 5281.568961: mutex_unlock <-rb_simple_write 3453 bash-1994 [000] .... 5281.568963: __mutex_unlock_slowpath <-mutex_unlock 3454 bash-1994 [000] .... 5281.568963: __fsnotify_parent <-fsnotify_modify 3455 bash-1994 [000] .... 5281.568964: fsnotify <-fsnotify_modify 3456 bash-1994 [000] .... 5281.568964: __srcu_read_lock <-fsnotify 3457 bash-1994 [000] .... 5281.568964: add_preempt_count <-__srcu_read_lock 3458 bash-1994 [000] ...1 5281.568965: sub_preempt_count <-__srcu_read_lock 3459 bash-1994 [000] .... 5281.568965: __srcu_read_unlock <-fsnotify 3460 bash-1994 [000] .... 5281.568967: sys_dup2 <-system_call_fastpath 3461 3462 3463Note, reading the trace_pipe file will block until more input is 3464added. This is contrary to the trace file. If any process opened 3465the trace file for reading, it will actually disable tracing and 3466prevent new entries from being added. The trace_pipe file does 3467not have this limitation. 3468 3469trace entries 3470------------- 3471 3472Having too much or not enough data can be troublesome in 3473diagnosing an issue in the kernel. The file buffer_size_kb is 3474used to modify the size of the internal trace buffers. The 3475number listed is the number of entries that can be recorded per 3476CPU. To know the full size, multiply the number of possible CPUs 3477with the number of entries. 3478:: 3479 3480 # cat buffer_size_kb 3481 1408 (units kilobytes) 3482 3483Or simply read buffer_total_size_kb 3484:: 3485 3486 # cat buffer_total_size_kb 3487 5632 3488 3489To modify the buffer, simple echo in a number (in 1024 byte segments). 3490:: 3491 3492 # echo 10000 > buffer_size_kb 3493 # cat buffer_size_kb 3494 10000 (units kilobytes) 3495 3496It will try to allocate as much as possible. If you allocate too 3497much, it can cause Out-Of-Memory to trigger. 3498:: 3499 3500 # echo 1000000000000 > buffer_size_kb 3501 -bash: echo: write error: Cannot allocate memory 3502 # cat buffer_size_kb 3503 85 3504 3505The per_cpu buffers can be changed individually as well: 3506:: 3507 3508 # echo 10000 > per_cpu/cpu0/buffer_size_kb 3509 # echo 100 > per_cpu/cpu1/buffer_size_kb 3510 3511When the per_cpu buffers are not the same, the buffer_size_kb 3512at the top level will just show an X 3513:: 3514 3515 # cat buffer_size_kb 3516 X 3517 3518This is where the buffer_total_size_kb is useful: 3519:: 3520 3521 # cat buffer_total_size_kb 3522 12916 3523 3524Writing to the top level buffer_size_kb will reset all the buffers 3525to be the same again. 3526 3527Snapshot 3528-------- 3529CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature 3530available to all non latency tracers. (Latency tracers which 3531record max latency, such as "irqsoff" or "wakeup", can't use 3532this feature, since those are already using the snapshot 3533mechanism internally.) 3534 3535Snapshot preserves a current trace buffer at a particular point 3536in time without stopping tracing. Ftrace swaps the current 3537buffer with a spare buffer, and tracing continues in the new 3538current (=previous spare) buffer. 3539 3540The following tracefs files in "tracing" are related to this 3541feature: 3542 3543 snapshot: 3544 3545 This is used to take a snapshot and to read the output 3546 of the snapshot. Echo 1 into this file to allocate a 3547 spare buffer and to take a snapshot (swap), then read 3548 the snapshot from this file in the same format as 3549 "trace" (described above in the section "The File 3550 System"). Both reads snapshot and tracing are executable 3551 in parallel. When the spare buffer is allocated, echoing 3552 0 frees it, and echoing else (positive) values clear the 3553 snapshot contents. 3554 More details are shown in the table below. 3555 3556 +--------------+------------+------------+------------+ 3557 |status\\input | 0 | 1 | else | 3558 +==============+============+============+============+ 3559 |not allocated |(do nothing)| alloc+swap |(do nothing)| 3560 +--------------+------------+------------+------------+ 3561 |allocated | free | swap | clear | 3562 +--------------+------------+------------+------------+ 3563 3564Here is an example of using the snapshot feature. 3565:: 3566 3567 # echo 1 > events/sched/enable 3568 # echo 1 > snapshot 3569 # cat snapshot 3570 # tracer: nop 3571 # 3572 # entries-in-buffer/entries-written: 71/71 #P:8 3573 # 3574 # _-----=> irqs-off 3575 # / _----=> need-resched 3576 # | / _---=> hardirq/softirq 3577 # || / _--=> preempt-depth 3578 # ||| / delay 3579 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3580 # | | | |||| | | 3581 <idle>-0 [005] d... 2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120 3582 sleep-2242 [005] d... 2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120 prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120 3583 [...] 3584 <idle>-0 [002] d... 2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120 3585 3586 # cat trace 3587 # tracer: nop 3588 # 3589 # entries-in-buffer/entries-written: 77/77 #P:8 3590 # 3591 # _-----=> irqs-off 3592 # / _----=> need-resched 3593 # | / _---=> hardirq/softirq 3594 # || / _--=> preempt-depth 3595 # ||| / delay 3596 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3597 # | | | |||| | | 3598 <idle>-0 [007] d... 2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120 3599 snapshot-test-2-2229 [002] d... 2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120 3600 [...] 3601 3602 3603If you try to use this snapshot feature when current tracer is 3604one of the latency tracers, you will get the following results. 3605:: 3606 3607 # echo wakeup > current_tracer 3608 # echo 1 > snapshot 3609 bash: echo: write error: Device or resource busy 3610 # cat snapshot 3611 cat: snapshot: Device or resource busy 3612 3613 3614Instances 3615--------- 3616In the tracefs tracing directory, there is a directory called "instances". 3617This directory can have new directories created inside of it using 3618mkdir, and removing directories with rmdir. The directory created 3619with mkdir in this directory will already contain files and other 3620directories after it is created. 3621:: 3622 3623 # mkdir instances/foo 3624 # ls instances/foo 3625 buffer_size_kb buffer_total_size_kb events free_buffer per_cpu 3626 set_event snapshot trace trace_clock trace_marker trace_options 3627 trace_pipe tracing_on 3628 3629As you can see, the new directory looks similar to the tracing directory 3630itself. In fact, it is very similar, except that the buffer and 3631events are agnostic from the main directory, or from any other 3632instances that are created. 3633 3634The files in the new directory work just like the files with the 3635same name in the tracing directory except the buffer that is used 3636is a separate and new buffer. The files affect that buffer but do not 3637affect the main buffer with the exception of trace_options. Currently, 3638the trace_options affect all instances and the top level buffer 3639the same, but this may change in future releases. That is, options 3640may become specific to the instance they reside in. 3641 3642Notice that none of the function tracer files are there, nor is 3643current_tracer and available_tracers. This is because the buffers 3644can currently only have events enabled for them. 3645:: 3646 3647 # mkdir instances/foo 3648 # mkdir instances/bar 3649 # mkdir instances/zoot 3650 # echo 100000 > buffer_size_kb 3651 # echo 1000 > instances/foo/buffer_size_kb 3652 # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb 3653 # echo function > current_trace 3654 # echo 1 > instances/foo/events/sched/sched_wakeup/enable 3655 # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable 3656 # echo 1 > instances/foo/events/sched/sched_switch/enable 3657 # echo 1 > instances/bar/events/irq/enable 3658 # echo 1 > instances/zoot/events/syscalls/enable 3659 # cat trace_pipe 3660 CPU:2 [LOST 11745 EVENTS] 3661 bash-2044 [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist 3662 bash-2044 [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave 3663 bash-2044 [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist 3664 bash-2044 [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist 3665 bash-2044 [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock 3666 bash-2044 [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype 3667 bash-2044 [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist 3668 bash-2044 [002] d... 10594.481034: zone_statistics <-get_page_from_freelist 3669 bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics 3670 bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics 3671 bash-2044 [002] .... 10594.481035: arch_dup_task_struct <-copy_process 3672 [...] 3673 3674 # cat instances/foo/trace_pipe 3675 bash-1998 [000] d..4 136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000 3676 bash-1998 [000] dN.4 136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000 3677 <idle>-0 [003] d.h3 136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003 3678 <idle>-0 [003] d..3 136.676909: sched_switch: prev_comm=swapper/3 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_preempt next_pid=9 next_prio=120 3679 rcu_preempt-9 [003] d..3 136.676916: sched_switch: prev_comm=rcu_preempt prev_pid=9 prev_prio=120 prev_state=S ==> next_comm=swapper/3 next_pid=0 next_prio=120 3680 bash-1998 [000] d..4 136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000 3681 bash-1998 [000] dN.4 136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000 3682 bash-1998 [000] d..3 136.677018: sched_switch: prev_comm=bash prev_pid=1998 prev_prio=120 prev_state=R+ ==> next_comm=kworker/0:1 next_pid=59 next_prio=120 3683 kworker/0:1-59 [000] d..4 136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001 3684 kworker/0:1-59 [000] d..3 136.677025: sched_switch: prev_comm=kworker/0:1 prev_pid=59 prev_prio=120 prev_state=S ==> next_comm=bash next_pid=1998 next_prio=120 3685 [...] 3686 3687 # cat instances/bar/trace_pipe 3688 migration/1-14 [001] d.h3 138.732674: softirq_raise: vec=3 [action=NET_RX] 3689 <idle>-0 [001] dNh3 138.732725: softirq_raise: vec=3 [action=NET_RX] 3690 bash-1998 [000] d.h1 138.733101: softirq_raise: vec=1 [action=TIMER] 3691 bash-1998 [000] d.h1 138.733102: softirq_raise: vec=9 [action=RCU] 3692 bash-1998 [000] ..s2 138.733105: softirq_entry: vec=1 [action=TIMER] 3693 bash-1998 [000] ..s2 138.733106: softirq_exit: vec=1 [action=TIMER] 3694 bash-1998 [000] ..s2 138.733106: softirq_entry: vec=9 [action=RCU] 3695 bash-1998 [000] ..s2 138.733109: softirq_exit: vec=9 [action=RCU] 3696 sshd-1995 [001] d.h1 138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4 3697 sshd-1995 [001] d.h1 138.733280: irq_handler_exit: irq=21 ret=unhandled 3698 sshd-1995 [001] d.h1 138.733281: irq_handler_entry: irq=21 name=eth0 3699 sshd-1995 [001] d.h1 138.733283: irq_handler_exit: irq=21 ret=handled 3700 [...] 3701 3702 # cat instances/zoot/trace 3703 # tracer: nop 3704 # 3705 # entries-in-buffer/entries-written: 18996/18996 #P:4 3706 # 3707 # _-----=> irqs-off 3708 # / _----=> need-resched 3709 # | / _---=> hardirq/softirq 3710 # || / _--=> preempt-depth 3711 # ||| / delay 3712 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3713 # | | | |||| | | 3714 bash-1998 [000] d... 140.733501: sys_write -> 0x2 3715 bash-1998 [000] d... 140.733504: sys_dup2(oldfd: a, newfd: 1) 3716 bash-1998 [000] d... 140.733506: sys_dup2 -> 0x1 3717 bash-1998 [000] d... 140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0) 3718 bash-1998 [000] d... 140.733509: sys_fcntl -> 0x1 3719 bash-1998 [000] d... 140.733510: sys_close(fd: a) 3720 bash-1998 [000] d... 140.733510: sys_close -> 0x0 3721 bash-1998 [000] d... 140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8) 3722 bash-1998 [000] d... 140.733515: sys_rt_sigprocmask -> 0x0 3723 bash-1998 [000] d... 140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8) 3724 bash-1998 [000] d... 140.733516: sys_rt_sigaction -> 0x0 3725 3726You can see that the trace of the top most trace buffer shows only 3727the function tracing. The foo instance displays wakeups and task 3728switches. 3729 3730To remove the instances, simply delete their directories: 3731:: 3732 3733 # rmdir instances/foo 3734 # rmdir instances/bar 3735 # rmdir instances/zoot 3736 3737Note, if a process has a trace file open in one of the instance 3738directories, the rmdir will fail with EBUSY. 3739 3740 3741Stack trace 3742----------- 3743Since the kernel has a fixed sized stack, it is important not to 3744waste it in functions. A kernel developer must be conscious of 3745what they allocate on the stack. If they add too much, the system 3746can be in danger of a stack overflow, and corruption will occur, 3747usually leading to a system panic. 3748 3749There are some tools that check this, usually with interrupts 3750periodically checking usage. But if you can perform a check 3751at every function call that will become very useful. As ftrace provides 3752a function tracer, it makes it convenient to check the stack size 3753at every function call. This is enabled via the stack tracer. 3754 3755CONFIG_STACK_TRACER enables the ftrace stack tracing functionality. 3756To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled. 3757:: 3758 3759 # echo 1 > /proc/sys/kernel/stack_tracer_enabled 3760 3761You can also enable it from the kernel command line to trace 3762the stack size of the kernel during boot up, by adding "stacktrace" 3763to the kernel command line parameter. 3764 3765After running it for a few minutes, the output looks like: 3766:: 3767 3768 # cat stack_max_size 3769 2928 3770 3771 # cat stack_trace 3772 Depth Size Location (18 entries) 3773 ----- ---- -------- 3774 0) 2928 224 update_sd_lb_stats+0xbc/0x4ac 3775 1) 2704 160 find_busiest_group+0x31/0x1f1 3776 2) 2544 256 load_balance+0xd9/0x662 3777 3) 2288 80 idle_balance+0xbb/0x130 3778 4) 2208 128 __schedule+0x26e/0x5b9 3779 5) 2080 16 schedule+0x64/0x66 3780 6) 2064 128 schedule_timeout+0x34/0xe0 3781 7) 1936 112 wait_for_common+0x97/0xf1 3782 8) 1824 16 wait_for_completion+0x1d/0x1f 3783 9) 1808 128 flush_work+0xfe/0x119 3784 10) 1680 16 tty_flush_to_ldisc+0x1e/0x20 3785 11) 1664 48 input_available_p+0x1d/0x5c 3786 12) 1616 48 n_tty_poll+0x6d/0x134 3787 13) 1568 64 tty_poll+0x64/0x7f 3788 14) 1504 880 do_select+0x31e/0x511 3789 15) 624 400 core_sys_select+0x177/0x216 3790 16) 224 96 sys_select+0x91/0xb9 3791 17) 128 128 system_call_fastpath+0x16/0x1b 3792 3793Note, if -mfentry is being used by gcc, functions get traced before 3794they set up the stack frame. This means that leaf level functions 3795are not tested by the stack tracer when -mfentry is used. 3796 3797Currently, -mfentry is used by gcc 4.6.0 and above on x86 only. 3798 3799More 3800---- 3801More details can be found in the source code, in the `kernel/trace/*.c` files. 3802