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 "blk" 814 815 The block tracer. The tracer used by the blktrace user 816 application. 817 818 "hwlat" 819 820 The Hardware Latency tracer is used to detect if the hardware 821 produces any latency. See "Hardware Latency Detector" section 822 below. 823 824 "irqsoff" 825 826 Traces the areas that disable interrupts and saves 827 the trace with the longest max latency. 828 See tracing_max_latency. When a new max is recorded, 829 it replaces the old trace. It is best to view this 830 trace with the latency-format option enabled, which 831 happens automatically when the tracer is selected. 832 833 "preemptoff" 834 835 Similar to irqsoff but traces and records the amount of 836 time for which preemption is disabled. 837 838 "preemptirqsoff" 839 840 Similar to irqsoff and preemptoff, but traces and 841 records the largest time for which irqs and/or preemption 842 is disabled. 843 844 "wakeup" 845 846 Traces and records the max latency that it takes for 847 the highest priority task to get scheduled after 848 it has been woken up. 849 Traces all tasks as an average developer would expect. 850 851 "wakeup_rt" 852 853 Traces and records the max latency that it takes for just 854 RT tasks (as the current "wakeup" does). This is useful 855 for those interested in wake up timings of RT tasks. 856 857 "wakeup_dl" 858 859 Traces and records the max latency that it takes for 860 a SCHED_DEADLINE task to be woken (as the "wakeup" and 861 "wakeup_rt" does). 862 863 "mmiotrace" 864 865 A special tracer that is used to trace binary module. 866 It will trace all the calls that a module makes to the 867 hardware. Everything it writes and reads from the I/O 868 as well. 869 870 "branch" 871 872 This tracer can be configured when tracing likely/unlikely 873 calls within the kernel. It will trace when a likely and 874 unlikely branch is hit and if it was correct in its prediction 875 of being correct. 876 877 "nop" 878 879 This is the "trace nothing" tracer. To remove all 880 tracers from tracing simply echo "nop" into 881 current_tracer. 882 883Error conditions 884---------------- 885 886 For most ftrace commands, failure modes are obvious and communicated 887 using standard return codes. 888 889 For other more involved commands, extended error information may be 890 available via the tracing/error_log file. For the commands that 891 support it, reading the tracing/error_log file after an error will 892 display more detailed information about what went wrong, if 893 information is available. The tracing/error_log file is a circular 894 error log displaying a small number (currently, 8) of ftrace errors 895 for the last (8) failed commands. 896 897 The extended error information and usage takes the form shown in 898 this example:: 899 900 # echo xxx > /sys/kernel/tracing/events/sched/sched_wakeup/trigger 901 echo: write error: Invalid argument 902 903 # cat /sys/kernel/tracing/error_log 904 [ 5348.887237] location: error: Couldn't yyy: zzz 905 Command: xxx 906 ^ 907 [ 7517.023364] location: error: Bad rrr: sss 908 Command: ppp qqq 909 ^ 910 911 To clear the error log, echo the empty string into it:: 912 913 # echo > /sys/kernel/tracing/error_log 914 915Examples of using the tracer 916---------------------------- 917 918Here are typical examples of using the tracers when controlling 919them only with the tracefs interface (without using any 920user-land utilities). 921 922Output format: 923-------------- 924 925Here is an example of the output format of the file "trace":: 926 927 # tracer: function 928 # 929 # entries-in-buffer/entries-written: 140080/250280 #P:4 930 # 931 # _-----=> irqs-off 932 # / _----=> need-resched 933 # | / _---=> hardirq/softirq 934 # || / _--=> preempt-depth 935 # ||| / delay 936 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 937 # | | | |||| | | 938 bash-1977 [000] .... 17284.993652: sys_close <-system_call_fastpath 939 bash-1977 [000] .... 17284.993653: __close_fd <-sys_close 940 bash-1977 [000] .... 17284.993653: _raw_spin_lock <-__close_fd 941 sshd-1974 [003] .... 17284.993653: __srcu_read_unlock <-fsnotify 942 bash-1977 [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock 943 bash-1977 [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd 944 bash-1977 [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock 945 bash-1977 [000] .... 17284.993657: filp_close <-__close_fd 946 bash-1977 [000] .... 17284.993657: dnotify_flush <-filp_close 947 sshd-1974 [003] .... 17284.993658: sys_select <-system_call_fastpath 948 .... 949 950A header is printed with the tracer name that is represented by 951the trace. In this case the tracer is "function". Then it shows the 952number of events in the buffer as well as the total number of entries 953that were written. The difference is the number of entries that were 954lost due to the buffer filling up (250280 - 140080 = 110200 events 955lost). 956 957The header explains the content of the events. Task name "bash", the task 958PID "1977", the CPU that it was running on "000", the latency format 959(explained below), the timestamp in <secs>.<usecs> format, the 960function name that was traced "sys_close" and the parent function that 961called this function "system_call_fastpath". The timestamp is the time 962at which the function was entered. 963 964Latency trace format 965-------------------- 966 967When the latency-format option is enabled or when one of the latency 968tracers is set, the trace file gives somewhat more information to see 969why a latency happened. Here is a typical trace:: 970 971 # tracer: irqsoff 972 # 973 # irqsoff latency trace v1.1.5 on 3.8.0-test+ 974 # -------------------------------------------------------------------- 975 # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 976 # ----------------- 977 # | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0) 978 # ----------------- 979 # => started at: __lock_task_sighand 980 # => ended at: _raw_spin_unlock_irqrestore 981 # 982 # 983 # _------=> CPU# 984 # / _-----=> irqs-off 985 # | / _----=> need-resched 986 # || / _---=> hardirq/softirq 987 # ||| / _--=> preempt-depth 988 # |||| / delay 989 # cmd pid ||||| time | caller 990 # \ / ||||| \ | / 991 ps-6143 2d... 0us!: trace_hardirqs_off <-__lock_task_sighand 992 ps-6143 2d..1 259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore 993 ps-6143 2d..1 263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore 994 ps-6143 2d..1 306us : <stack trace> 995 => trace_hardirqs_on_caller 996 => trace_hardirqs_on 997 => _raw_spin_unlock_irqrestore 998 => do_task_stat 999 => proc_tgid_stat 1000 => proc_single_show 1001 => seq_read 1002 => vfs_read 1003 => sys_read 1004 => system_call_fastpath 1005 1006 1007This shows that the current tracer is "irqsoff" tracing the time 1008for which interrupts were disabled. It gives the trace version (which 1009never changes) and the version of the kernel upon which this was executed on 1010(3.8). Then it displays the max latency in microseconds (259 us). The number 1011of trace entries displayed and the total number (both are four: #4/4). 1012VP, KP, SP, and HP are always zero and are reserved for later use. 1013#P is the number of online CPUs (#P:4). 1014 1015The task is the process that was running when the latency 1016occurred. (ps pid: 6143). 1017 1018The start and stop (the functions in which the interrupts were 1019disabled and enabled respectively) that caused the latencies: 1020 1021 - __lock_task_sighand is where the interrupts were disabled. 1022 - _raw_spin_unlock_irqrestore is where they were enabled again. 1023 1024The next lines after the header are the trace itself. The header 1025explains which is which. 1026 1027 cmd: The name of the process in the trace. 1028 1029 pid: The PID of that process. 1030 1031 CPU#: The CPU which the process was running on. 1032 1033 irqs-off: 'd' interrupts are disabled. '.' otherwise. 1034 1035 need-resched: 1036 - 'B' all, TIF_NEED_RESCHED, PREEMPT_NEED_RESCHED and TIF_RESCHED_LAZY is set, 1037 - 'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set, 1038 - 'n' only TIF_NEED_RESCHED is set, 1039 - 'p' only PREEMPT_NEED_RESCHED is set, 1040 - 'L' both PREEMPT_NEED_RESCHED and TIF_RESCHED_LAZY is set, 1041 - 'b' both TIF_NEED_RESCHED and TIF_RESCHED_LAZY is set, 1042 - 'l' only TIF_RESCHED_LAZY is set 1043 - '.' otherwise. 1044 1045 hardirq/softirq: 1046 - 'Z' - NMI occurred inside a hardirq 1047 - 'z' - NMI is running 1048 - 'H' - hard irq occurred inside a softirq. 1049 - 'h' - hard irq is running 1050 - 's' - soft irq is running 1051 - '.' - normal context. 1052 1053 preempt-depth: The level of preempt_disabled 1054 1055The above is mostly meaningful for kernel developers. 1056 1057 time: 1058 When the latency-format option is enabled, the trace file 1059 output includes a timestamp relative to the start of the 1060 trace. This differs from the output when latency-format 1061 is disabled, which includes an absolute timestamp. 1062 1063 delay: 1064 This is just to help catch your eye a bit better. And 1065 needs to be fixed to be only relative to the same CPU. 1066 The marks are determined by the difference between this 1067 current trace and the next trace. 1068 1069 - '$' - greater than 1 second 1070 - '@' - greater than 100 millisecond 1071 - '*' - greater than 10 millisecond 1072 - '#' - greater than 1000 microsecond 1073 - '!' - greater than 100 microsecond 1074 - '+' - greater than 10 microsecond 1075 - ' ' - less than or equal to 10 microsecond. 1076 1077 The rest is the same as the 'trace' file. 1078 1079 Note, the latency tracers will usually end with a back trace 1080 to easily find where the latency occurred. 1081 1082trace_options 1083------------- 1084 1085The trace_options file (or the options directory) is used to control 1086what gets printed in the trace output, or manipulate the tracers. 1087To see what is available, simply cat the file:: 1088 1089 cat trace_options 1090 print-parent 1091 nosym-offset 1092 nosym-addr 1093 noverbose 1094 noraw 1095 nohex 1096 nobin 1097 noblock 1098 nofields 1099 trace_printk 1100 annotate 1101 nouserstacktrace 1102 nosym-userobj 1103 noprintk-msg-only 1104 context-info 1105 nolatency-format 1106 record-cmd 1107 norecord-tgid 1108 overwrite 1109 nodisable_on_free 1110 irq-info 1111 markers 1112 noevent-fork 1113 function-trace 1114 nofunction-fork 1115 nodisplay-graph 1116 nostacktrace 1117 nobranch 1118 1119To disable one of the options, echo in the option prepended with 1120"no":: 1121 1122 echo noprint-parent > trace_options 1123 1124To enable an option, leave off the "no":: 1125 1126 echo sym-offset > trace_options 1127 1128Here are the available options: 1129 1130 print-parent 1131 On function traces, display the calling (parent) 1132 function as well as the function being traced. 1133 :: 1134 1135 print-parent: 1136 bash-4000 [01] 1477.606694: simple_strtoul <-kstrtoul 1137 1138 noprint-parent: 1139 bash-4000 [01] 1477.606694: simple_strtoul 1140 1141 1142 sym-offset 1143 Display not only the function name, but also the 1144 offset in the function. For example, instead of 1145 seeing just "ktime_get", you will see 1146 "ktime_get+0xb/0x20". 1147 :: 1148 1149 sym-offset: 1150 bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0 1151 1152 sym-addr 1153 This will also display the function address as well 1154 as the function name. 1155 :: 1156 1157 sym-addr: 1158 bash-4000 [01] 1477.606694: simple_strtoul <c0339346> 1159 1160 verbose 1161 This deals with the trace file when the 1162 latency-format option is enabled. 1163 :: 1164 1165 bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \ 1166 (+0.000ms): simple_strtoul (kstrtoul) 1167 1168 raw 1169 This will display raw numbers. This option is best for 1170 use with user applications that can translate the raw 1171 numbers better than having it done in the kernel. 1172 1173 hex 1174 Similar to raw, but the numbers will be in a hexadecimal format. 1175 1176 bin 1177 This will print out the formats in raw binary. 1178 1179 block 1180 When set, reading trace_pipe will not block when polled. 1181 1182 fields 1183 Print the fields as described by their types. This is a better 1184 option than using hex, bin or raw, as it gives a better parsing 1185 of the content of the event. 1186 1187 trace_printk 1188 Can disable trace_printk() from writing into the buffer. 1189 1190 trace_printk_dest 1191 Set to have trace_printk() and similar internal tracing functions 1192 write into this instance. Note, only one trace instance can have 1193 this set. By setting this flag, it clears the trace_printk_dest flag 1194 of the instance that had it set previously. By default, the top 1195 level trace has this set, and will get it set again if another 1196 instance has it set then clears it. 1197 1198 This flag cannot be cleared by the top level instance, as it is the 1199 default instance. The only way the top level instance has this flag 1200 cleared, is by it being set in another instance. 1201 1202 annotate 1203 It is sometimes confusing when the CPU buffers are full 1204 and one CPU buffer had a lot of events recently, thus 1205 a shorter time frame, were another CPU may have only had 1206 a few events, which lets it have older events. When 1207 the trace is reported, it shows the oldest events first, 1208 and it may look like only one CPU ran (the one with the 1209 oldest events). When the annotate option is set, it will 1210 display when a new CPU buffer started:: 1211 1212 <idle>-0 [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on 1213 <idle>-0 [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on 1214 <idle>-0 [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore 1215 ##### CPU 2 buffer started #### 1216 <idle>-0 [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle 1217 <idle>-0 [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog 1218 <idle>-0 [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock 1219 1220 userstacktrace 1221 This option changes the trace. It records a 1222 stacktrace of the current user space thread after 1223 each trace event. 1224 1225 sym-userobj 1226 when user stacktrace are enabled, look up which 1227 object the address belongs to, and print a 1228 relative address. This is especially useful when 1229 ASLR is on, otherwise you don't get a chance to 1230 resolve the address to object/file/line after 1231 the app is no longer running 1232 1233 The lookup is performed when you read 1234 trace,trace_pipe. Example:: 1235 1236 a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0 1237 x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6] 1238 1239 1240 printk-msg-only 1241 When set, trace_printk()s will only show the format 1242 and not their parameters (if trace_bprintk() or 1243 trace_bputs() was used to save the trace_printk()). 1244 1245 context-info 1246 Show only the event data. Hides the comm, PID, 1247 timestamp, CPU, and other useful data. 1248 1249 latency-format 1250 This option changes the trace output. When it is enabled, 1251 the trace displays additional information about the 1252 latency, as described in "Latency trace format". 1253 1254 pause-on-trace 1255 When set, opening the trace file for read, will pause 1256 writing to the ring buffer (as if tracing_on was set to zero). 1257 This simulates the original behavior of the trace file. 1258 When the file is closed, tracing will be enabled again. 1259 1260 hash-ptr 1261 When set, "%p" in the event printk format displays the 1262 hashed pointer value instead of real address. 1263 This will be useful if you want to find out which hashed 1264 value is corresponding to the real value in trace log. 1265 1266 record-cmd 1267 When any event or tracer is enabled, a hook is enabled 1268 in the sched_switch trace point to fill comm cache 1269 with mapped pids and comms. But this may cause some 1270 overhead, and if you only care about pids, and not the 1271 name of the task, disabling this option can lower the 1272 impact of tracing. See "saved_cmdlines". 1273 1274 record-tgid 1275 When any event or tracer is enabled, a hook is enabled 1276 in the sched_switch trace point to fill the cache of 1277 mapped Thread Group IDs (TGID) mapping to pids. See 1278 "saved_tgids". 1279 1280 overwrite 1281 This controls what happens when the trace buffer is 1282 full. If "1" (default), the oldest events are 1283 discarded and overwritten. If "0", then the newest 1284 events are discarded. 1285 (see per_cpu/cpu0/stats for overrun and dropped) 1286 1287 disable_on_free 1288 When the free_buffer is closed, tracing will 1289 stop (tracing_on set to 0). 1290 1291 irq-info 1292 Shows the interrupt, preempt count, need resched data. 1293 When disabled, the trace looks like:: 1294 1295 # tracer: function 1296 # 1297 # entries-in-buffer/entries-written: 144405/9452052 #P:4 1298 # 1299 # TASK-PID CPU# TIMESTAMP FUNCTION 1300 # | | | | | 1301 <idle>-0 [002] 23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up 1302 <idle>-0 [002] 23636.756054: activate_task <-ttwu_do_activate.constprop.89 1303 <idle>-0 [002] 23636.756055: enqueue_task <-activate_task 1304 1305 1306 markers 1307 When set, the trace_marker is writable (only by root). 1308 When disabled, the trace_marker will error with EINVAL 1309 on write. 1310 1311 event-fork 1312 When set, tasks with PIDs listed in set_event_pid will have 1313 the PIDs of their children added to set_event_pid when those 1314 tasks fork. Also, when tasks with PIDs in set_event_pid exit, 1315 their PIDs will be removed from the file. 1316 1317 This affects PIDs listed in set_event_notrace_pid as well. 1318 1319 function-trace 1320 The latency tracers will enable function tracing 1321 if this option is enabled (default it is). When 1322 it is disabled, the latency tracers do not trace 1323 functions. This keeps the overhead of the tracer down 1324 when performing latency tests. 1325 1326 function-fork 1327 When set, tasks with PIDs listed in set_ftrace_pid will 1328 have the PIDs of their children added to set_ftrace_pid 1329 when those tasks fork. Also, when tasks with PIDs in 1330 set_ftrace_pid exit, their PIDs will be removed from the 1331 file. 1332 1333 This affects PIDs in set_ftrace_notrace_pid as well. 1334 1335 display-graph 1336 When set, the latency tracers (irqsoff, wakeup, etc) will 1337 use function graph tracing instead of function tracing. 1338 1339 stacktrace 1340 When set, a stack trace is recorded after any trace event 1341 is recorded. 1342 1343 branch 1344 Enable branch tracing with the tracer. This enables branch 1345 tracer along with the currently set tracer. Enabling this 1346 with the "nop" tracer is the same as just enabling the 1347 "branch" tracer. 1348 1349.. tip:: Some tracers have their own options. They only appear in this 1350 file when the tracer is active. They always appear in the 1351 options directory. 1352 1353 1354Here are the per tracer options: 1355 1356Options for function tracer: 1357 1358 func_stack_trace 1359 When set, a stack trace is recorded after every 1360 function that is recorded. NOTE! Limit the functions 1361 that are recorded before enabling this, with 1362 "set_ftrace_filter" otherwise the system performance 1363 will be critically degraded. Remember to disable 1364 this option before clearing the function filter. 1365 1366Options for function_graph tracer: 1367 1368 Since the function_graph tracer has a slightly different output 1369 it has its own options to control what is displayed. 1370 1371 funcgraph-overrun 1372 When set, the "overrun" of the graph stack is 1373 displayed after each function traced. The 1374 overrun, is when the stack depth of the calls 1375 is greater than what is reserved for each task. 1376 Each task has a fixed array of functions to 1377 trace in the call graph. If the depth of the 1378 calls exceeds that, the function is not traced. 1379 The overrun is the number of functions missed 1380 due to exceeding this array. 1381 1382 funcgraph-cpu 1383 When set, the CPU number of the CPU where the trace 1384 occurred is displayed. 1385 1386 funcgraph-overhead 1387 When set, if the function takes longer than 1388 A certain amount, then a delay marker is 1389 displayed. See "delay" above, under the 1390 header description. 1391 1392 funcgraph-proc 1393 Unlike other tracers, the process' command line 1394 is not displayed by default, but instead only 1395 when a task is traced in and out during a context 1396 switch. Enabling this options has the command 1397 of each process displayed at every line. 1398 1399 funcgraph-duration 1400 At the end of each function (the return) 1401 the duration of the amount of time in the 1402 function is displayed in microseconds. 1403 1404 funcgraph-abstime 1405 When set, the timestamp is displayed at each line. 1406 1407 funcgraph-irqs 1408 When disabled, functions that happen inside an 1409 interrupt will not be traced. 1410 1411 funcgraph-tail 1412 When set, the return event will include the function 1413 that it represents. By default this is off, and 1414 only a closing curly bracket "}" is displayed for 1415 the return of a function. 1416 1417 funcgraph-retval 1418 When set, the return value of each traced function 1419 will be printed after an equal sign "=". By default 1420 this is off. 1421 1422 funcgraph-retval-hex 1423 When set, the return value will always be printed 1424 in hexadecimal format. If the option is not set and 1425 the return value is an error code, it will be printed 1426 in signed decimal format; otherwise it will also be 1427 printed in hexadecimal format. By default, this option 1428 is off. 1429 1430 sleep-time 1431 When running function graph tracer, to include 1432 the time a task schedules out in its function. 1433 When enabled, it will account time the task has been 1434 scheduled out as part of the function call. 1435 1436 graph-time 1437 When running function profiler with function graph tracer, 1438 to include the time to call nested functions. When this is 1439 not set, the time reported for the function will only 1440 include the time the function itself executed for, not the 1441 time for functions that it called. 1442 1443Options for blk tracer: 1444 1445 blk_classic 1446 Shows a more minimalistic output. 1447 1448 1449irqsoff 1450------- 1451 1452When interrupts are disabled, the CPU can not react to any other 1453external event (besides NMIs and SMIs). This prevents the timer 1454interrupt from triggering or the mouse interrupt from letting 1455the kernel know of a new mouse event. The result is a latency 1456with the reaction time. 1457 1458The irqsoff tracer tracks the time for which interrupts are 1459disabled. When a new maximum latency is hit, the tracer saves 1460the trace leading up to that latency point so that every time a 1461new maximum is reached, the old saved trace is discarded and the 1462new trace is saved. 1463 1464To reset the maximum, echo 0 into tracing_max_latency. Here is 1465an example:: 1466 1467 # echo 0 > options/function-trace 1468 # echo irqsoff > current_tracer 1469 # echo 1 > tracing_on 1470 # echo 0 > tracing_max_latency 1471 # ls -ltr 1472 [...] 1473 # echo 0 > tracing_on 1474 # cat trace 1475 # tracer: irqsoff 1476 # 1477 # irqsoff latency trace v1.1.5 on 3.8.0-test+ 1478 # -------------------------------------------------------------------- 1479 # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1480 # ----------------- 1481 # | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0) 1482 # ----------------- 1483 # => started at: run_timer_softirq 1484 # => ended at: run_timer_softirq 1485 # 1486 # 1487 # _------=> CPU# 1488 # / _-----=> irqs-off 1489 # | / _----=> need-resched 1490 # || / _---=> hardirq/softirq 1491 # ||| / _--=> preempt-depth 1492 # |||| / delay 1493 # cmd pid ||||| time | caller 1494 # \ / ||||| \ | / 1495 <idle>-0 0d.s2 0us+: _raw_spin_lock_irq <-run_timer_softirq 1496 <idle>-0 0dNs3 17us : _raw_spin_unlock_irq <-run_timer_softirq 1497 <idle>-0 0dNs3 17us+: trace_hardirqs_on <-run_timer_softirq 1498 <idle>-0 0dNs3 25us : <stack trace> 1499 => _raw_spin_unlock_irq 1500 => run_timer_softirq 1501 => __do_softirq 1502 => call_softirq 1503 => do_softirq 1504 => irq_exit 1505 => smp_apic_timer_interrupt 1506 => apic_timer_interrupt 1507 => rcu_idle_exit 1508 => cpu_idle 1509 => rest_init 1510 => start_kernel 1511 => x86_64_start_reservations 1512 => x86_64_start_kernel 1513 1514Here we see that we had a latency of 16 microseconds (which is 1515very good). The _raw_spin_lock_irq in run_timer_softirq disabled 1516interrupts. The difference between the 16 and the displayed 1517timestamp 25us occurred because the clock was incremented 1518between the time of recording the max latency and the time of 1519recording the function that had that latency. 1520 1521Note the above example had function-trace not set. If we set 1522function-trace, we get a much larger output:: 1523 1524 with echo 1 > options/function-trace 1525 1526 # tracer: irqsoff 1527 # 1528 # irqsoff latency trace v1.1.5 on 3.8.0-test+ 1529 # -------------------------------------------------------------------- 1530 # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1531 # ----------------- 1532 # | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0) 1533 # ----------------- 1534 # => started at: ata_scsi_queuecmd 1535 # => ended at: ata_scsi_queuecmd 1536 # 1537 # 1538 # _------=> CPU# 1539 # / _-----=> irqs-off 1540 # | / _----=> need-resched 1541 # || / _---=> hardirq/softirq 1542 # ||| / _--=> preempt-depth 1543 # |||| / delay 1544 # cmd pid ||||| time | caller 1545 # \ / ||||| \ | / 1546 bash-2042 3d... 0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd 1547 bash-2042 3d... 0us : add_preempt_count <-_raw_spin_lock_irqsave 1548 bash-2042 3d..1 1us : ata_scsi_find_dev <-ata_scsi_queuecmd 1549 bash-2042 3d..1 1us : __ata_scsi_find_dev <-ata_scsi_find_dev 1550 bash-2042 3d..1 2us : ata_find_dev.part.14 <-__ata_scsi_find_dev 1551 bash-2042 3d..1 2us : ata_qc_new_init <-__ata_scsi_queuecmd 1552 bash-2042 3d..1 3us : ata_sg_init <-__ata_scsi_queuecmd 1553 bash-2042 3d..1 4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd 1554 bash-2042 3d..1 4us : ata_build_rw_tf <-ata_scsi_rw_xlat 1555 [...] 1556 bash-2042 3d..1 67us : delay_tsc <-__delay 1557 bash-2042 3d..1 67us : add_preempt_count <-delay_tsc 1558 bash-2042 3d..2 67us : sub_preempt_count <-delay_tsc 1559 bash-2042 3d..1 67us : add_preempt_count <-delay_tsc 1560 bash-2042 3d..2 68us : sub_preempt_count <-delay_tsc 1561 bash-2042 3d..1 68us+: ata_bmdma_start <-ata_bmdma_qc_issue 1562 bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd 1563 bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd 1564 bash-2042 3d..1 72us+: trace_hardirqs_on <-ata_scsi_queuecmd 1565 bash-2042 3d..1 120us : <stack trace> 1566 => _raw_spin_unlock_irqrestore 1567 => ata_scsi_queuecmd 1568 => scsi_dispatch_cmd 1569 => scsi_request_fn 1570 => __blk_run_queue_uncond 1571 => __blk_run_queue 1572 => blk_queue_bio 1573 => submit_bio_noacct 1574 => submit_bio 1575 => submit_bh 1576 => __ext3_get_inode_loc 1577 => ext3_iget 1578 => ext3_lookup 1579 => lookup_real 1580 => __lookup_hash 1581 => walk_component 1582 => lookup_last 1583 => path_lookupat 1584 => filename_lookup 1585 => user_path_at_empty 1586 => user_path_at 1587 => vfs_fstatat 1588 => vfs_stat 1589 => sys_newstat 1590 => system_call_fastpath 1591 1592 1593Here we traced a 71 microsecond latency. But we also see all the 1594functions that were called during that time. Note that by 1595enabling function tracing, we incur an added overhead. This 1596overhead may extend the latency times. But nevertheless, this 1597trace has provided some very helpful debugging information. 1598 1599If we prefer function graph output instead of function, we can set 1600display-graph option:: 1601 1602 with echo 1 > options/display-graph 1603 1604 # tracer: irqsoff 1605 # 1606 # irqsoff latency trace v1.1.5 on 4.20.0-rc6+ 1607 # -------------------------------------------------------------------- 1608 # latency: 3751 us, #274/274, CPU#0 | (M:desktop VP:0, KP:0, SP:0 HP:0 #P:4) 1609 # ----------------- 1610 # | task: bash-1507 (uid:0 nice:0 policy:0 rt_prio:0) 1611 # ----------------- 1612 # => started at: free_debug_processing 1613 # => ended at: return_to_handler 1614 # 1615 # 1616 # _-----=> irqs-off 1617 # / _----=> need-resched 1618 # | / _---=> hardirq/softirq 1619 # || / _--=> preempt-depth 1620 # ||| / 1621 # REL TIME CPU TASK/PID |||| DURATION FUNCTION CALLS 1622 # | | | | |||| | | | | | | 1623 0 us | 0) bash-1507 | d... | 0.000 us | _raw_spin_lock_irqsave(); 1624 0 us | 0) bash-1507 | d..1 | 0.378 us | do_raw_spin_trylock(); 1625 1 us | 0) bash-1507 | d..2 | | set_track() { 1626 2 us | 0) bash-1507 | d..2 | | save_stack_trace() { 1627 2 us | 0) bash-1507 | d..2 | | __save_stack_trace() { 1628 3 us | 0) bash-1507 | d..2 | | __unwind_start() { 1629 3 us | 0) bash-1507 | d..2 | | get_stack_info() { 1630 3 us | 0) bash-1507 | d..2 | 0.351 us | in_task_stack(); 1631 4 us | 0) bash-1507 | d..2 | 1.107 us | } 1632 [...] 1633 3750 us | 0) bash-1507 | d..1 | 0.516 us | do_raw_spin_unlock(); 1634 3750 us | 0) bash-1507 | d..1 | 0.000 us | _raw_spin_unlock_irqrestore(); 1635 3764 us | 0) bash-1507 | d..1 | 0.000 us | tracer_hardirqs_on(); 1636 bash-1507 0d..1 3792us : <stack trace> 1637 => free_debug_processing 1638 => __slab_free 1639 => kmem_cache_free 1640 => vm_area_free 1641 => remove_vma 1642 => exit_mmap 1643 => mmput 1644 => begin_new_exec 1645 => load_elf_binary 1646 => search_binary_handler 1647 => __do_execve_file.isra.32 1648 => __x64_sys_execve 1649 => do_syscall_64 1650 => entry_SYSCALL_64_after_hwframe 1651 1652preemptoff 1653---------- 1654 1655When preemption is disabled, we may be able to receive 1656interrupts but the task cannot be preempted and a higher 1657priority task must wait for preemption to be enabled again 1658before it can preempt a lower priority task. 1659 1660The preemptoff tracer traces the places that disable preemption. 1661Like the irqsoff tracer, it records the maximum latency for 1662which preemption was disabled. The control of preemptoff tracer 1663is much like the irqsoff tracer. 1664:: 1665 1666 # echo 0 > options/function-trace 1667 # echo preemptoff > current_tracer 1668 # echo 1 > tracing_on 1669 # echo 0 > tracing_max_latency 1670 # ls -ltr 1671 [...] 1672 # echo 0 > tracing_on 1673 # cat trace 1674 # tracer: preemptoff 1675 # 1676 # preemptoff latency trace v1.1.5 on 3.8.0-test+ 1677 # -------------------------------------------------------------------- 1678 # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1679 # ----------------- 1680 # | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0) 1681 # ----------------- 1682 # => started at: do_IRQ 1683 # => ended at: do_IRQ 1684 # 1685 # 1686 # _------=> CPU# 1687 # / _-----=> irqs-off 1688 # | / _----=> need-resched 1689 # || / _---=> hardirq/softirq 1690 # ||| / _--=> preempt-depth 1691 # |||| / delay 1692 # cmd pid ||||| time | caller 1693 # \ / ||||| \ | / 1694 sshd-1991 1d.h. 0us+: irq_enter <-do_IRQ 1695 sshd-1991 1d..1 46us : irq_exit <-do_IRQ 1696 sshd-1991 1d..1 47us+: trace_preempt_on <-do_IRQ 1697 sshd-1991 1d..1 52us : <stack trace> 1698 => sub_preempt_count 1699 => irq_exit 1700 => do_IRQ 1701 => ret_from_intr 1702 1703 1704This has some more changes. Preemption was disabled when an 1705interrupt came in (notice the 'h'), and was enabled on exit. 1706But we also see that interrupts have been disabled when entering 1707the preempt off section and leaving it (the 'd'). We do not know if 1708interrupts were enabled in the mean time or shortly after this 1709was over. 1710:: 1711 1712 # tracer: preemptoff 1713 # 1714 # preemptoff latency trace v1.1.5 on 3.8.0-test+ 1715 # -------------------------------------------------------------------- 1716 # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1717 # ----------------- 1718 # | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0) 1719 # ----------------- 1720 # => started at: wake_up_new_task 1721 # => ended at: task_rq_unlock 1722 # 1723 # 1724 # _------=> CPU# 1725 # / _-----=> irqs-off 1726 # | / _----=> need-resched 1727 # || / _---=> hardirq/softirq 1728 # ||| / _--=> preempt-depth 1729 # |||| / delay 1730 # cmd pid ||||| time | caller 1731 # \ / ||||| \ | / 1732 bash-1994 1d..1 0us : _raw_spin_lock_irqsave <-wake_up_new_task 1733 bash-1994 1d..1 0us : select_task_rq_fair <-select_task_rq 1734 bash-1994 1d..1 1us : __rcu_read_lock <-select_task_rq_fair 1735 bash-1994 1d..1 1us : source_load <-select_task_rq_fair 1736 bash-1994 1d..1 1us : source_load <-select_task_rq_fair 1737 [...] 1738 bash-1994 1d..1 12us : irq_enter <-smp_apic_timer_interrupt 1739 bash-1994 1d..1 12us : rcu_irq_enter <-irq_enter 1740 bash-1994 1d..1 13us : add_preempt_count <-irq_enter 1741 bash-1994 1d.h1 13us : exit_idle <-smp_apic_timer_interrupt 1742 bash-1994 1d.h1 13us : hrtimer_interrupt <-smp_apic_timer_interrupt 1743 bash-1994 1d.h1 13us : _raw_spin_lock <-hrtimer_interrupt 1744 bash-1994 1d.h1 14us : add_preempt_count <-_raw_spin_lock 1745 bash-1994 1d.h2 14us : ktime_get_update_offsets <-hrtimer_interrupt 1746 [...] 1747 bash-1994 1d.h1 35us : lapic_next_event <-clockevents_program_event 1748 bash-1994 1d.h1 35us : irq_exit <-smp_apic_timer_interrupt 1749 bash-1994 1d.h1 36us : sub_preempt_count <-irq_exit 1750 bash-1994 1d..2 36us : do_softirq <-irq_exit 1751 bash-1994 1d..2 36us : __do_softirq <-call_softirq 1752 bash-1994 1d..2 36us : __local_bh_disable <-__do_softirq 1753 bash-1994 1d.s2 37us : add_preempt_count <-_raw_spin_lock_irq 1754 bash-1994 1d.s3 38us : _raw_spin_unlock <-run_timer_softirq 1755 bash-1994 1d.s3 39us : sub_preempt_count <-_raw_spin_unlock 1756 bash-1994 1d.s2 39us : call_timer_fn <-run_timer_softirq 1757 [...] 1758 bash-1994 1dNs2 81us : cpu_needs_another_gp <-rcu_process_callbacks 1759 bash-1994 1dNs2 82us : __local_bh_enable <-__do_softirq 1760 bash-1994 1dNs2 82us : sub_preempt_count <-__local_bh_enable 1761 bash-1994 1dN.2 82us : idle_cpu <-irq_exit 1762 bash-1994 1dN.2 83us : rcu_irq_exit <-irq_exit 1763 bash-1994 1dN.2 83us : sub_preempt_count <-irq_exit 1764 bash-1994 1.N.1 84us : _raw_spin_unlock_irqrestore <-task_rq_unlock 1765 bash-1994 1.N.1 84us+: trace_preempt_on <-task_rq_unlock 1766 bash-1994 1.N.1 104us : <stack trace> 1767 => sub_preempt_count 1768 => _raw_spin_unlock_irqrestore 1769 => task_rq_unlock 1770 => wake_up_new_task 1771 => do_fork 1772 => sys_clone 1773 => stub_clone 1774 1775 1776The above is an example of the preemptoff trace with 1777function-trace set. Here we see that interrupts were not disabled 1778the entire time. The irq_enter code lets us know that we entered 1779an interrupt 'h'. Before that, the functions being traced still 1780show that it is not in an interrupt, but we can see from the 1781functions themselves that this is not the case. 1782 1783preemptirqsoff 1784-------------- 1785 1786Knowing the locations that have interrupts disabled or 1787preemption disabled for the longest times is helpful. But 1788sometimes we would like to know when either preemption and/or 1789interrupts are disabled. 1790 1791Consider the following code:: 1792 1793 local_irq_disable(); 1794 call_function_with_irqs_off(); 1795 preempt_disable(); 1796 call_function_with_irqs_and_preemption_off(); 1797 local_irq_enable(); 1798 call_function_with_preemption_off(); 1799 preempt_enable(); 1800 1801The irqsoff tracer will record the total length of 1802call_function_with_irqs_off() and 1803call_function_with_irqs_and_preemption_off(). 1804 1805The preemptoff tracer will record the total length of 1806call_function_with_irqs_and_preemption_off() and 1807call_function_with_preemption_off(). 1808 1809But neither will trace the time that interrupts and/or 1810preemption is disabled. This total time is the time that we can 1811not schedule. To record this time, use the preemptirqsoff 1812tracer. 1813 1814Again, using this trace is much like the irqsoff and preemptoff 1815tracers. 1816:: 1817 1818 # echo 0 > options/function-trace 1819 # echo preemptirqsoff > current_tracer 1820 # echo 1 > tracing_on 1821 # echo 0 > tracing_max_latency 1822 # ls -ltr 1823 [...] 1824 # echo 0 > tracing_on 1825 # cat trace 1826 # tracer: preemptirqsoff 1827 # 1828 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+ 1829 # -------------------------------------------------------------------- 1830 # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1831 # ----------------- 1832 # | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0) 1833 # ----------------- 1834 # => started at: ata_scsi_queuecmd 1835 # => ended at: ata_scsi_queuecmd 1836 # 1837 # 1838 # _------=> CPU# 1839 # / _-----=> irqs-off 1840 # | / _----=> need-resched 1841 # || / _---=> hardirq/softirq 1842 # ||| / _--=> preempt-depth 1843 # |||| / delay 1844 # cmd pid ||||| time | caller 1845 # \ / ||||| \ | / 1846 ls-2230 3d... 0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd 1847 ls-2230 3...1 100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd 1848 ls-2230 3...1 101us+: trace_preempt_on <-ata_scsi_queuecmd 1849 ls-2230 3...1 111us : <stack trace> 1850 => sub_preempt_count 1851 => _raw_spin_unlock_irqrestore 1852 => ata_scsi_queuecmd 1853 => scsi_dispatch_cmd 1854 => scsi_request_fn 1855 => __blk_run_queue_uncond 1856 => __blk_run_queue 1857 => blk_queue_bio 1858 => submit_bio_noacct 1859 => submit_bio 1860 => submit_bh 1861 => ext3_bread 1862 => ext3_dir_bread 1863 => htree_dirblock_to_tree 1864 => ext3_htree_fill_tree 1865 => ext3_readdir 1866 => vfs_readdir 1867 => sys_getdents 1868 => system_call_fastpath 1869 1870 1871The trace_hardirqs_off_thunk is called from assembly on x86 when 1872interrupts are disabled in the assembly code. Without the 1873function tracing, we do not know if interrupts were enabled 1874within the preemption points. We do see that it started with 1875preemption enabled. 1876 1877Here is a trace with function-trace set:: 1878 1879 # tracer: preemptirqsoff 1880 # 1881 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+ 1882 # -------------------------------------------------------------------- 1883 # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1884 # ----------------- 1885 # | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0) 1886 # ----------------- 1887 # => started at: schedule 1888 # => ended at: mutex_unlock 1889 # 1890 # 1891 # _------=> CPU# 1892 # / _-----=> irqs-off 1893 # | / _----=> need-resched 1894 # || / _---=> hardirq/softirq 1895 # ||| / _--=> preempt-depth 1896 # |||| / delay 1897 # cmd pid ||||| time | caller 1898 # \ / ||||| \ | / 1899 kworker/-59 3...1 0us : __schedule <-schedule 1900 kworker/-59 3d..1 0us : rcu_preempt_qs <-rcu_note_context_switch 1901 kworker/-59 3d..1 1us : add_preempt_count <-_raw_spin_lock_irq 1902 kworker/-59 3d..2 1us : deactivate_task <-__schedule 1903 kworker/-59 3d..2 1us : dequeue_task <-deactivate_task 1904 kworker/-59 3d..2 2us : update_rq_clock <-dequeue_task 1905 kworker/-59 3d..2 2us : dequeue_task_fair <-dequeue_task 1906 kworker/-59 3d..2 2us : update_curr <-dequeue_task_fair 1907 kworker/-59 3d..2 2us : update_min_vruntime <-update_curr 1908 kworker/-59 3d..2 3us : cpuacct_charge <-update_curr 1909 kworker/-59 3d..2 3us : __rcu_read_lock <-cpuacct_charge 1910 kworker/-59 3d..2 3us : __rcu_read_unlock <-cpuacct_charge 1911 kworker/-59 3d..2 3us : update_cfs_rq_blocked_load <-dequeue_task_fair 1912 kworker/-59 3d..2 4us : clear_buddies <-dequeue_task_fair 1913 kworker/-59 3d..2 4us : account_entity_dequeue <-dequeue_task_fair 1914 kworker/-59 3d..2 4us : update_min_vruntime <-dequeue_task_fair 1915 kworker/-59 3d..2 4us : update_cfs_shares <-dequeue_task_fair 1916 kworker/-59 3d..2 5us : hrtick_update <-dequeue_task_fair 1917 kworker/-59 3d..2 5us : wq_worker_sleeping <-__schedule 1918 kworker/-59 3d..2 5us : kthread_data <-wq_worker_sleeping 1919 kworker/-59 3d..2 5us : put_prev_task_fair <-__schedule 1920 kworker/-59 3d..2 6us : pick_next_task_fair <-pick_next_task 1921 kworker/-59 3d..2 6us : clear_buddies <-pick_next_task_fair 1922 kworker/-59 3d..2 6us : set_next_entity <-pick_next_task_fair 1923 kworker/-59 3d..2 6us : update_stats_wait_end <-set_next_entity 1924 ls-2269 3d..2 7us : finish_task_switch <-__schedule 1925 ls-2269 3d..2 7us : _raw_spin_unlock_irq <-finish_task_switch 1926 ls-2269 3d..2 8us : do_IRQ <-ret_from_intr 1927 ls-2269 3d..2 8us : irq_enter <-do_IRQ 1928 ls-2269 3d..2 8us : rcu_irq_enter <-irq_enter 1929 ls-2269 3d..2 9us : add_preempt_count <-irq_enter 1930 ls-2269 3d.h2 9us : exit_idle <-do_IRQ 1931 [...] 1932 ls-2269 3d.h3 20us : sub_preempt_count <-_raw_spin_unlock 1933 ls-2269 3d.h2 20us : irq_exit <-do_IRQ 1934 ls-2269 3d.h2 21us : sub_preempt_count <-irq_exit 1935 ls-2269 3d..3 21us : do_softirq <-irq_exit 1936 ls-2269 3d..3 21us : __do_softirq <-call_softirq 1937 ls-2269 3d..3 21us+: __local_bh_disable <-__do_softirq 1938 ls-2269 3d.s4 29us : sub_preempt_count <-_local_bh_enable_ip 1939 ls-2269 3d.s5 29us : sub_preempt_count <-_local_bh_enable_ip 1940 ls-2269 3d.s5 31us : do_IRQ <-ret_from_intr 1941 ls-2269 3d.s5 31us : irq_enter <-do_IRQ 1942 ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter 1943 [...] 1944 ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter 1945 ls-2269 3d.s5 32us : add_preempt_count <-irq_enter 1946 ls-2269 3d.H5 32us : exit_idle <-do_IRQ 1947 ls-2269 3d.H5 32us : handle_irq <-do_IRQ 1948 ls-2269 3d.H5 32us : irq_to_desc <-handle_irq 1949 ls-2269 3d.H5 33us : handle_fasteoi_irq <-handle_irq 1950 [...] 1951 ls-2269 3d.s5 158us : _raw_spin_unlock_irqrestore <-rtl8139_poll 1952 ls-2269 3d.s3 158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action 1953 ls-2269 3d.s3 159us : __local_bh_enable <-__do_softirq 1954 ls-2269 3d.s3 159us : sub_preempt_count <-__local_bh_enable 1955 ls-2269 3d..3 159us : idle_cpu <-irq_exit 1956 ls-2269 3d..3 159us : rcu_irq_exit <-irq_exit 1957 ls-2269 3d..3 160us : sub_preempt_count <-irq_exit 1958 ls-2269 3d... 161us : __mutex_unlock_slowpath <-mutex_unlock 1959 ls-2269 3d... 162us+: trace_hardirqs_on <-mutex_unlock 1960 ls-2269 3d... 186us : <stack trace> 1961 => __mutex_unlock_slowpath 1962 => mutex_unlock 1963 => process_output 1964 => n_tty_write 1965 => tty_write 1966 => vfs_write 1967 => sys_write 1968 => system_call_fastpath 1969 1970This is an interesting trace. It started with kworker running and 1971scheduling out and ls taking over. But as soon as ls released the 1972rq lock and enabled interrupts (but not preemption) an interrupt 1973triggered. When the interrupt finished, it started running softirqs. 1974But while the softirq was running, another interrupt triggered. 1975When an interrupt is running inside a softirq, the annotation is 'H'. 1976 1977 1978wakeup 1979------ 1980 1981One common case that people are interested in tracing is the 1982time it takes for a task that is woken to actually wake up. 1983Now for non Real-Time tasks, this can be arbitrary. But tracing 1984it nonetheless can be interesting. 1985 1986Without function tracing:: 1987 1988 # echo 0 > options/function-trace 1989 # echo wakeup > current_tracer 1990 # echo 1 > tracing_on 1991 # echo 0 > tracing_max_latency 1992 # chrt -f 5 sleep 1 1993 # echo 0 > tracing_on 1994 # cat trace 1995 # tracer: wakeup 1996 # 1997 # wakeup latency trace v1.1.5 on 3.8.0-test+ 1998 # -------------------------------------------------------------------- 1999 # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 2000 # ----------------- 2001 # | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0) 2002 # ----------------- 2003 # 2004 # _------=> CPU# 2005 # / _-----=> irqs-off 2006 # | / _----=> need-resched 2007 # || / _---=> hardirq/softirq 2008 # ||| / _--=> preempt-depth 2009 # |||| / delay 2010 # cmd pid ||||| time | caller 2011 # \ / ||||| \ | / 2012 <idle>-0 3dNs7 0us : 0:120:R + [003] 312:100:R kworker/3:1H 2013 <idle>-0 3dNs7 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up 2014 <idle>-0 3d..3 15us : __schedule <-schedule 2015 <idle>-0 3d..3 15us : 0:120:R ==> [003] 312:100:R kworker/3:1H 2016 2017The tracer only traces the highest priority task in the system 2018to avoid tracing the normal circumstances. Here we see that 2019the kworker with a nice priority of -20 (not very nice), took 2020just 15 microseconds from the time it woke up, to the time it 2021ran. 2022 2023Non Real-Time tasks are not that interesting. A more interesting 2024trace is to concentrate only on Real-Time tasks. 2025 2026wakeup_rt 2027--------- 2028 2029In a Real-Time environment it is very important to know the 2030wakeup time it takes for the highest priority task that is woken 2031up to the time that it executes. This is also known as "schedule 2032latency". I stress the point that this is about RT tasks. It is 2033also important to know the scheduling latency of non-RT tasks, 2034but the average schedule latency is better for non-RT tasks. 2035Tools like LatencyTop are more appropriate for such 2036measurements. 2037 2038Real-Time environments are interested in the worst case latency. 2039That is the longest latency it takes for something to happen, 2040and not the average. We can have a very fast scheduler that may 2041only have a large latency once in a while, but that would not 2042work well with Real-Time tasks. The wakeup_rt tracer was designed 2043to record the worst case wakeups of RT tasks. Non-RT tasks are 2044not recorded because the tracer only records one worst case and 2045tracing non-RT tasks that are unpredictable will overwrite the 2046worst case latency of RT tasks (just run the normal wakeup 2047tracer for a while to see that effect). 2048 2049Since this tracer only deals with RT tasks, we will run this 2050slightly differently than we did with the previous tracers. 2051Instead of performing an 'ls', we will run 'sleep 1' under 2052'chrt' which changes the priority of the task. 2053:: 2054 2055 # echo 0 > options/function-trace 2056 # echo wakeup_rt > current_tracer 2057 # echo 1 > tracing_on 2058 # echo 0 > tracing_max_latency 2059 # chrt -f 5 sleep 1 2060 # echo 0 > tracing_on 2061 # cat trace 2062 # tracer: wakeup 2063 # 2064 # tracer: wakeup_rt 2065 # 2066 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+ 2067 # -------------------------------------------------------------------- 2068 # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 2069 # ----------------- 2070 # | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5) 2071 # ----------------- 2072 # 2073 # _------=> CPU# 2074 # / _-----=> irqs-off 2075 # | / _----=> need-resched 2076 # || / _---=> hardirq/softirq 2077 # ||| / _--=> preempt-depth 2078 # |||| / delay 2079 # cmd pid ||||| time | caller 2080 # \ / ||||| \ | / 2081 <idle>-0 3d.h4 0us : 0:120:R + [003] 2389: 94:R sleep 2082 <idle>-0 3d.h4 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up 2083 <idle>-0 3d..3 5us : __schedule <-schedule 2084 <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep 2085 2086 2087Running this on an idle system, we see that it only took 5 microseconds 2088to perform the task switch. Note, since the trace point in the schedule 2089is before the actual "switch", we stop the tracing when the recorded task 2090is about to schedule in. This may change if we add a new marker at the 2091end of the scheduler. 2092 2093Notice that the recorded task is 'sleep' with the PID of 2389 2094and it has an rt_prio of 5. This priority is user-space priority 2095and not the internal kernel priority. The policy is 1 for 2096SCHED_FIFO and 2 for SCHED_RR. 2097 2098Note, that the trace data shows the internal priority (99 - rtprio). 2099:: 2100 2101 <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep 2102 2103The 0:120:R means idle was running with a nice priority of 0 (120 - 120) 2104and in the running state 'R'. The sleep task was scheduled in with 21052389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94) 2106and it too is in the running state. 2107 2108Doing the same with chrt -r 5 and function-trace set. 2109:: 2110 2111 echo 1 > options/function-trace 2112 2113 # tracer: wakeup_rt 2114 # 2115 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+ 2116 # -------------------------------------------------------------------- 2117 # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 2118 # ----------------- 2119 # | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5) 2120 # ----------------- 2121 # 2122 # _------=> CPU# 2123 # / _-----=> irqs-off 2124 # | / _----=> need-resched 2125 # || / _---=> hardirq/softirq 2126 # ||| / _--=> preempt-depth 2127 # |||| / delay 2128 # cmd pid ||||| time | caller 2129 # \ / ||||| \ | / 2130 <idle>-0 3d.h4 1us+: 0:120:R + [003] 2448: 94:R sleep 2131 <idle>-0 3d.h4 2us : ttwu_do_activate.constprop.87 <-try_to_wake_up 2132 <idle>-0 3d.h3 3us : check_preempt_curr <-ttwu_do_wakeup 2133 <idle>-0 3d.h3 3us : resched_curr <-check_preempt_curr 2134 <idle>-0 3dNh3 4us : task_woken_rt <-ttwu_do_wakeup 2135 <idle>-0 3dNh3 4us : _raw_spin_unlock <-try_to_wake_up 2136 <idle>-0 3dNh3 4us : sub_preempt_count <-_raw_spin_unlock 2137 <idle>-0 3dNh2 5us : ttwu_stat <-try_to_wake_up 2138 <idle>-0 3dNh2 5us : _raw_spin_unlock_irqrestore <-try_to_wake_up 2139 <idle>-0 3dNh2 6us : sub_preempt_count <-_raw_spin_unlock_irqrestore 2140 <idle>-0 3dNh1 6us : _raw_spin_lock <-__run_hrtimer 2141 <idle>-0 3dNh1 6us : add_preempt_count <-_raw_spin_lock 2142 <idle>-0 3dNh2 7us : _raw_spin_unlock <-hrtimer_interrupt 2143 <idle>-0 3dNh2 7us : sub_preempt_count <-_raw_spin_unlock 2144 <idle>-0 3dNh1 7us : tick_program_event <-hrtimer_interrupt 2145 <idle>-0 3dNh1 7us : clockevents_program_event <-tick_program_event 2146 <idle>-0 3dNh1 8us : ktime_get <-clockevents_program_event 2147 <idle>-0 3dNh1 8us : lapic_next_event <-clockevents_program_event 2148 <idle>-0 3dNh1 8us : irq_exit <-smp_apic_timer_interrupt 2149 <idle>-0 3dNh1 9us : sub_preempt_count <-irq_exit 2150 <idle>-0 3dN.2 9us : idle_cpu <-irq_exit 2151 <idle>-0 3dN.2 9us : rcu_irq_exit <-irq_exit 2152 <idle>-0 3dN.2 10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit 2153 <idle>-0 3dN.2 10us : sub_preempt_count <-irq_exit 2154 <idle>-0 3.N.1 11us : rcu_idle_exit <-cpu_idle 2155 <idle>-0 3dN.1 11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit 2156 <idle>-0 3.N.1 11us : tick_nohz_idle_exit <-cpu_idle 2157 <idle>-0 3dN.1 12us : menu_hrtimer_cancel <-tick_nohz_idle_exit 2158 <idle>-0 3dN.1 12us : ktime_get <-tick_nohz_idle_exit 2159 <idle>-0 3dN.1 12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit 2160 <idle>-0 3dN.1 13us : cpu_load_update_nohz <-tick_nohz_idle_exit 2161 <idle>-0 3dN.1 13us : _raw_spin_lock <-cpu_load_update_nohz 2162 <idle>-0 3dN.1 13us : add_preempt_count <-_raw_spin_lock 2163 <idle>-0 3dN.2 13us : __cpu_load_update <-cpu_load_update_nohz 2164 <idle>-0 3dN.2 14us : sched_avg_update <-__cpu_load_update 2165 <idle>-0 3dN.2 14us : _raw_spin_unlock <-cpu_load_update_nohz 2166 <idle>-0 3dN.2 14us : sub_preempt_count <-_raw_spin_unlock 2167 <idle>-0 3dN.1 15us : calc_load_nohz_stop <-tick_nohz_idle_exit 2168 <idle>-0 3dN.1 15us : touch_softlockup_watchdog <-tick_nohz_idle_exit 2169 <idle>-0 3dN.1 15us : hrtimer_cancel <-tick_nohz_idle_exit 2170 <idle>-0 3dN.1 15us : hrtimer_try_to_cancel <-hrtimer_cancel 2171 <idle>-0 3dN.1 16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel 2172 <idle>-0 3dN.1 16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18 2173 <idle>-0 3dN.1 16us : add_preempt_count <-_raw_spin_lock_irqsave 2174 <idle>-0 3dN.2 17us : __remove_hrtimer <-remove_hrtimer.part.16 2175 <idle>-0 3dN.2 17us : hrtimer_force_reprogram <-__remove_hrtimer 2176 <idle>-0 3dN.2 17us : tick_program_event <-hrtimer_force_reprogram 2177 <idle>-0 3dN.2 18us : clockevents_program_event <-tick_program_event 2178 <idle>-0 3dN.2 18us : ktime_get <-clockevents_program_event 2179 <idle>-0 3dN.2 18us : lapic_next_event <-clockevents_program_event 2180 <idle>-0 3dN.2 19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel 2181 <idle>-0 3dN.2 19us : sub_preempt_count <-_raw_spin_unlock_irqrestore 2182 <idle>-0 3dN.1 19us : hrtimer_forward <-tick_nohz_idle_exit 2183 <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward 2184 <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward 2185 <idle>-0 3dN.1 20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11 2186 <idle>-0 3dN.1 20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns 2187 <idle>-0 3dN.1 21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns 2188 <idle>-0 3dN.1 21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18 2189 <idle>-0 3dN.1 21us : add_preempt_count <-_raw_spin_lock_irqsave 2190 <idle>-0 3dN.2 22us : ktime_add_safe <-__hrtimer_start_range_ns 2191 <idle>-0 3dN.2 22us : enqueue_hrtimer <-__hrtimer_start_range_ns 2192 <idle>-0 3dN.2 22us : tick_program_event <-__hrtimer_start_range_ns 2193 <idle>-0 3dN.2 23us : clockevents_program_event <-tick_program_event 2194 <idle>-0 3dN.2 23us : ktime_get <-clockevents_program_event 2195 <idle>-0 3dN.2 23us : lapic_next_event <-clockevents_program_event 2196 <idle>-0 3dN.2 24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns 2197 <idle>-0 3dN.2 24us : sub_preempt_count <-_raw_spin_unlock_irqrestore 2198 <idle>-0 3dN.1 24us : account_idle_ticks <-tick_nohz_idle_exit 2199 <idle>-0 3dN.1 24us : account_idle_time <-account_idle_ticks 2200 <idle>-0 3.N.1 25us : sub_preempt_count <-cpu_idle 2201 <idle>-0 3.N.. 25us : schedule <-cpu_idle 2202 <idle>-0 3.N.. 25us : __schedule <-preempt_schedule 2203 <idle>-0 3.N.. 26us : add_preempt_count <-__schedule 2204 <idle>-0 3.N.1 26us : rcu_note_context_switch <-__schedule 2205 <idle>-0 3.N.1 26us : rcu_sched_qs <-rcu_note_context_switch 2206 <idle>-0 3dN.1 27us : rcu_preempt_qs <-rcu_note_context_switch 2207 <idle>-0 3.N.1 27us : _raw_spin_lock_irq <-__schedule 2208 <idle>-0 3dN.1 27us : add_preempt_count <-_raw_spin_lock_irq 2209 <idle>-0 3dN.2 28us : put_prev_task_idle <-__schedule 2210 <idle>-0 3dN.2 28us : pick_next_task_stop <-pick_next_task 2211 <idle>-0 3dN.2 28us : pick_next_task_rt <-pick_next_task 2212 <idle>-0 3dN.2 29us : dequeue_pushable_task <-pick_next_task_rt 2213 <idle>-0 3d..3 29us : __schedule <-preempt_schedule 2214 <idle>-0 3d..3 30us : 0:120:R ==> [003] 2448: 94:R sleep 2215 2216This isn't that big of a trace, even with function tracing enabled, 2217so I included the entire trace. 2218 2219The interrupt went off while when the system was idle. Somewhere 2220before task_woken_rt() was called, the NEED_RESCHED flag was set, 2221this is indicated by the first occurrence of the 'N' flag. 2222 2223Latency tracing and events 2224-------------------------- 2225As function tracing can induce a much larger latency, but without 2226seeing what happens within the latency it is hard to know what 2227caused it. There is a middle ground, and that is with enabling 2228events. 2229:: 2230 2231 # echo 0 > options/function-trace 2232 # echo wakeup_rt > current_tracer 2233 # echo 1 > events/enable 2234 # echo 1 > tracing_on 2235 # echo 0 > tracing_max_latency 2236 # chrt -f 5 sleep 1 2237 # echo 0 > tracing_on 2238 # cat trace 2239 # tracer: wakeup_rt 2240 # 2241 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+ 2242 # -------------------------------------------------------------------- 2243 # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 2244 # ----------------- 2245 # | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5) 2246 # ----------------- 2247 # 2248 # _------=> CPU# 2249 # / _-----=> irqs-off 2250 # | / _----=> need-resched 2251 # || / _---=> hardirq/softirq 2252 # ||| / _--=> preempt-depth 2253 # |||| / delay 2254 # cmd pid ||||| time | caller 2255 # \ / ||||| \ | / 2256 <idle>-0 2d.h4 0us : 0:120:R + [002] 5882: 94:R sleep 2257 <idle>-0 2d.h4 0us : ttwu_do_activate.constprop.87 <-try_to_wake_up 2258 <idle>-0 2d.h4 1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002 2259 <idle>-0 2dNh2 1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8 2260 <idle>-0 2.N.2 2us : power_end: cpu_id=2 2261 <idle>-0 2.N.2 3us : cpu_idle: state=4294967295 cpu_id=2 2262 <idle>-0 2dN.3 4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0 2263 <idle>-0 2dN.3 4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000 2264 <idle>-0 2.N.2 5us : rcu_utilization: Start context switch 2265 <idle>-0 2.N.2 5us : rcu_utilization: End context switch 2266 <idle>-0 2d..3 6us : __schedule <-schedule 2267 <idle>-0 2d..3 6us : 0:120:R ==> [002] 5882: 94:R sleep 2268 2269 2270Hardware Latency Detector 2271------------------------- 2272 2273The hardware latency detector is executed by enabling the "hwlat" tracer. 2274 2275NOTE, this tracer will affect the performance of the system as it will 2276periodically make a CPU constantly busy with interrupts disabled. 2277:: 2278 2279 # echo hwlat > current_tracer 2280 # sleep 100 2281 # cat trace 2282 # tracer: hwlat 2283 # 2284 # entries-in-buffer/entries-written: 13/13 #P:8 2285 # 2286 # _-----=> irqs-off 2287 # / _----=> need-resched 2288 # | / _---=> hardirq/softirq 2289 # || / _--=> preempt-depth 2290 # ||| / delay 2291 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 2292 # | | | |||| | | 2293 <...>-1729 [001] d... 678.473449: #1 inner/outer(us): 11/12 ts:1581527483.343962693 count:6 2294 <...>-1729 [004] d... 689.556542: #2 inner/outer(us): 16/9 ts:1581527494.889008092 count:1 2295 <...>-1729 [005] d... 714.756290: #3 inner/outer(us): 16/16 ts:1581527519.678961629 count:5 2296 <...>-1729 [001] d... 718.788247: #4 inner/outer(us): 9/17 ts:1581527523.889012713 count:1 2297 <...>-1729 [002] d... 719.796341: #5 inner/outer(us): 13/9 ts:1581527524.912872606 count:1 2298 <...>-1729 [006] d... 844.787091: #6 inner/outer(us): 9/12 ts:1581527649.889048502 count:2 2299 <...>-1729 [003] d... 849.827033: #7 inner/outer(us): 18/9 ts:1581527654.889013793 count:1 2300 <...>-1729 [007] d... 853.859002: #8 inner/outer(us): 9/12 ts:1581527658.889065736 count:1 2301 <...>-1729 [001] d... 855.874978: #9 inner/outer(us): 9/11 ts:1581527660.861991877 count:1 2302 <...>-1729 [001] d... 863.938932: #10 inner/outer(us): 9/11 ts:1581527668.970010500 count:1 nmi-total:7 nmi-count:1 2303 <...>-1729 [007] d... 878.050780: #11 inner/outer(us): 9/12 ts:1581527683.385002600 count:1 nmi-total:5 nmi-count:1 2304 <...>-1729 [007] d... 886.114702: #12 inner/outer(us): 9/12 ts:1581527691.385001600 count:1 2305 2306 2307The above output is somewhat the same in the header. All events will have 2308interrupts disabled 'd'. Under the FUNCTION title there is: 2309 2310 #1 2311 This is the count of events recorded that were greater than the 2312 tracing_threshold (See below). 2313 2314 inner/outer(us): 11/11 2315 2316 This shows two numbers as "inner latency" and "outer latency". The test 2317 runs in a loop checking a timestamp twice. The latency detected within 2318 the two timestamps is the "inner latency" and the latency detected 2319 after the previous timestamp and the next timestamp in the loop is 2320 the "outer latency". 2321 2322 ts:1581527483.343962693 2323 2324 The absolute timestamp that the first latency was recorded in the window. 2325 2326 count:6 2327 2328 The number of times a latency was detected during the window. 2329 2330 nmi-total:7 nmi-count:1 2331 2332 On architectures that support it, if an NMI comes in during the 2333 test, the time spent in NMI is reported in "nmi-total" (in 2334 microseconds). 2335 2336 All architectures that have NMIs will show the "nmi-count" if an 2337 NMI comes in during the test. 2338 2339hwlat files: 2340 2341 tracing_threshold 2342 This gets automatically set to "10" to represent 10 2343 microseconds. This is the threshold of latency that 2344 needs to be detected before the trace will be recorded. 2345 2346 Note, when hwlat tracer is finished (another tracer is 2347 written into "current_tracer"), the original value for 2348 tracing_threshold is placed back into this file. 2349 2350 hwlat_detector/width 2351 The length of time the test runs with interrupts disabled. 2352 2353 hwlat_detector/window 2354 The length of time of the window which the test 2355 runs. That is, the test will run for "width" 2356 microseconds per "window" microseconds 2357 2358 tracing_cpumask 2359 When the test is started. A kernel thread is created that 2360 runs the test. This thread will alternate between CPUs 2361 listed in the tracing_cpumask between each period 2362 (one "window"). To limit the test to specific CPUs 2363 set the mask in this file to only the CPUs that the test 2364 should run on. 2365 2366function 2367-------- 2368 2369This tracer is the function tracer. Enabling the function tracer 2370can be done from the debug file system. Make sure the 2371ftrace_enabled is set; otherwise this tracer is a nop. 2372See the "ftrace_enabled" section below. 2373:: 2374 2375 # sysctl kernel.ftrace_enabled=1 2376 # echo function > current_tracer 2377 # echo 1 > tracing_on 2378 # usleep 1 2379 # echo 0 > tracing_on 2380 # cat trace 2381 # tracer: function 2382 # 2383 # entries-in-buffer/entries-written: 24799/24799 #P:4 2384 # 2385 # _-----=> irqs-off 2386 # / _----=> need-resched 2387 # | / _---=> hardirq/softirq 2388 # || / _--=> preempt-depth 2389 # ||| / delay 2390 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 2391 # | | | |||| | | 2392 bash-1994 [002] .... 3082.063030: mutex_unlock <-rb_simple_write 2393 bash-1994 [002] .... 3082.063031: __mutex_unlock_slowpath <-mutex_unlock 2394 bash-1994 [002] .... 3082.063031: __fsnotify_parent <-fsnotify_modify 2395 bash-1994 [002] .... 3082.063032: fsnotify <-fsnotify_modify 2396 bash-1994 [002] .... 3082.063032: __srcu_read_lock <-fsnotify 2397 bash-1994 [002] .... 3082.063032: add_preempt_count <-__srcu_read_lock 2398 bash-1994 [002] ...1 3082.063032: sub_preempt_count <-__srcu_read_lock 2399 bash-1994 [002] .... 3082.063033: __srcu_read_unlock <-fsnotify 2400 [...] 2401 2402 2403Note: function tracer uses ring buffers to store the above 2404entries. The newest data may overwrite the oldest data. 2405Sometimes using echo to stop the trace is not sufficient because 2406the tracing could have overwritten the data that you wanted to 2407record. For this reason, it is sometimes better to disable 2408tracing directly from a program. This allows you to stop the 2409tracing at the point that you hit the part that you are 2410interested in. To disable the tracing directly from a C program, 2411something like following code snippet can be used:: 2412 2413 int trace_fd; 2414 [...] 2415 int main(int argc, char *argv[]) { 2416 [...] 2417 trace_fd = open(tracing_file("tracing_on"), O_WRONLY); 2418 [...] 2419 if (condition_hit()) { 2420 write(trace_fd, "0", 1); 2421 } 2422 [...] 2423 } 2424 2425 2426Single thread tracing 2427--------------------- 2428 2429By writing into set_ftrace_pid you can trace a 2430single thread. For example:: 2431 2432 # cat set_ftrace_pid 2433 no pid 2434 # echo 3111 > set_ftrace_pid 2435 # cat set_ftrace_pid 2436 3111 2437 # echo function > current_tracer 2438 # cat trace | head 2439 # tracer: function 2440 # 2441 # TASK-PID CPU# TIMESTAMP FUNCTION 2442 # | | | | | 2443 yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return 2444 yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range 2445 yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel 2446 yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel 2447 yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll 2448 yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll 2449 # echo > set_ftrace_pid 2450 # cat trace |head 2451 # tracer: function 2452 # 2453 # TASK-PID CPU# TIMESTAMP FUNCTION 2454 # | | | | | 2455 ##### CPU 3 buffer started #### 2456 yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait 2457 yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry 2458 yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry 2459 yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit 2460 yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit 2461 2462If you want to trace a function when executing, you could use 2463something like this simple program. 2464:: 2465 2466 #include <stdio.h> 2467 #include <stdlib.h> 2468 #include <sys/types.h> 2469 #include <sys/stat.h> 2470 #include <fcntl.h> 2471 #include <unistd.h> 2472 #include <string.h> 2473 2474 #define _STR(x) #x 2475 #define STR(x) _STR(x) 2476 #define MAX_PATH 256 2477 2478 const char *find_tracefs(void) 2479 { 2480 static char tracefs[MAX_PATH+1]; 2481 static int tracefs_found; 2482 char type[100]; 2483 FILE *fp; 2484 2485 if (tracefs_found) 2486 return tracefs; 2487 2488 if ((fp = fopen("/proc/mounts","r")) == NULL) { 2489 perror("/proc/mounts"); 2490 return NULL; 2491 } 2492 2493 while (fscanf(fp, "%*s %" 2494 STR(MAX_PATH) 2495 "s %99s %*s %*d %*d\n", 2496 tracefs, type) == 2) { 2497 if (strcmp(type, "tracefs") == 0) 2498 break; 2499 } 2500 fclose(fp); 2501 2502 if (strcmp(type, "tracefs") != 0) { 2503 fprintf(stderr, "tracefs not mounted"); 2504 return NULL; 2505 } 2506 2507 strcat(tracefs, "/tracing/"); 2508 tracefs_found = 1; 2509 2510 return tracefs; 2511 } 2512 2513 const char *tracing_file(const char *file_name) 2514 { 2515 static char trace_file[MAX_PATH+1]; 2516 snprintf(trace_file, MAX_PATH, "%s/%s", find_tracefs(), file_name); 2517 return trace_file; 2518 } 2519 2520 int main (int argc, char **argv) 2521 { 2522 if (argc < 1) 2523 exit(-1); 2524 2525 if (fork() > 0) { 2526 int fd, ffd; 2527 char line[64]; 2528 int s; 2529 2530 ffd = open(tracing_file("current_tracer"), O_WRONLY); 2531 if (ffd < 0) 2532 exit(-1); 2533 write(ffd, "nop", 3); 2534 2535 fd = open(tracing_file("set_ftrace_pid"), O_WRONLY); 2536 s = sprintf(line, "%d\n", getpid()); 2537 write(fd, line, s); 2538 2539 write(ffd, "function", 8); 2540 2541 close(fd); 2542 close(ffd); 2543 2544 execvp(argv[1], argv+1); 2545 } 2546 2547 return 0; 2548 } 2549 2550Or this simple script! 2551:: 2552 2553 #!/bin/bash 2554 2555 tracefs=`sed -ne 's/^tracefs \(.*\) tracefs.*/\1/p' /proc/mounts` 2556 echo 0 > $tracefs/tracing_on 2557 echo $$ > $tracefs/set_ftrace_pid 2558 echo function > $tracefs/current_tracer 2559 echo 1 > $tracefs/tracing_on 2560 exec "$@" 2561 2562 2563function graph tracer 2564--------------------------- 2565 2566This tracer is similar to the function tracer except that it 2567probes a function on its entry and its exit. This is done by 2568using a dynamically allocated stack of return addresses in each 2569task_struct. On function entry the tracer overwrites the return 2570address of each function traced to set a custom probe. Thus the 2571original return address is stored on the stack of return address 2572in the task_struct. 2573 2574Probing on both ends of a function leads to special features 2575such as: 2576 2577- measure of a function's time execution 2578- having a reliable call stack to draw function calls graph 2579 2580This tracer is useful in several situations: 2581 2582- you want to find the reason of a strange kernel behavior and 2583 need to see what happens in detail on any areas (or specific 2584 ones). 2585 2586- you are experiencing weird latencies but it's difficult to 2587 find its origin. 2588 2589- you want to find quickly which path is taken by a specific 2590 function 2591 2592- you just want to peek inside a working kernel and want to see 2593 what happens there. 2594 2595:: 2596 2597 # tracer: function_graph 2598 # 2599 # CPU DURATION FUNCTION CALLS 2600 # | | | | | | | 2601 2602 0) | sys_open() { 2603 0) | do_sys_open() { 2604 0) | getname() { 2605 0) | kmem_cache_alloc() { 2606 0) 1.382 us | __might_sleep(); 2607 0) 2.478 us | } 2608 0) | strncpy_from_user() { 2609 0) | might_fault() { 2610 0) 1.389 us | __might_sleep(); 2611 0) 2.553 us | } 2612 0) 3.807 us | } 2613 0) 7.876 us | } 2614 0) | alloc_fd() { 2615 0) 0.668 us | _spin_lock(); 2616 0) 0.570 us | expand_files(); 2617 0) 0.586 us | _spin_unlock(); 2618 2619 2620There are several columns that can be dynamically 2621enabled/disabled. You can use every combination of options you 2622want, depending on your needs. 2623 2624- The cpu number on which the function executed is default 2625 enabled. It is sometimes better to only trace one cpu (see 2626 tracing_cpumask file) or you might sometimes see unordered 2627 function calls while cpu tracing switch. 2628 2629 - hide: echo nofuncgraph-cpu > trace_options 2630 - show: echo funcgraph-cpu > trace_options 2631 2632- The duration (function's time of execution) is displayed on 2633 the closing bracket line of a function or on the same line 2634 than the current function in case of a leaf one. It is default 2635 enabled. 2636 2637 - hide: echo nofuncgraph-duration > trace_options 2638 - show: echo funcgraph-duration > trace_options 2639 2640- The overhead field precedes the duration field in case of 2641 reached duration thresholds. 2642 2643 - hide: echo nofuncgraph-overhead > trace_options 2644 - show: echo funcgraph-overhead > trace_options 2645 - depends on: funcgraph-duration 2646 2647 ie:: 2648 2649 3) # 1837.709 us | } /* __switch_to */ 2650 3) | finish_task_switch() { 2651 3) 0.313 us | _raw_spin_unlock_irq(); 2652 3) 3.177 us | } 2653 3) # 1889.063 us | } /* __schedule */ 2654 3) ! 140.417 us | } /* __schedule */ 2655 3) # 2034.948 us | } /* schedule */ 2656 3) * 33998.59 us | } /* schedule_preempt_disabled */ 2657 2658 [...] 2659 2660 1) 0.260 us | msecs_to_jiffies(); 2661 1) 0.313 us | __rcu_read_unlock(); 2662 1) + 61.770 us | } 2663 1) + 64.479 us | } 2664 1) 0.313 us | rcu_bh_qs(); 2665 1) 0.313 us | __local_bh_enable(); 2666 1) ! 217.240 us | } 2667 1) 0.365 us | idle_cpu(); 2668 1) | rcu_irq_exit() { 2669 1) 0.417 us | rcu_eqs_enter_common.isra.47(); 2670 1) 3.125 us | } 2671 1) ! 227.812 us | } 2672 1) ! 457.395 us | } 2673 1) @ 119760.2 us | } 2674 2675 [...] 2676 2677 2) | handle_IPI() { 2678 1) 6.979 us | } 2679 2) 0.417 us | scheduler_ipi(); 2680 1) 9.791 us | } 2681 1) + 12.917 us | } 2682 2) 3.490 us | } 2683 1) + 15.729 us | } 2684 1) + 18.542 us | } 2685 2) $ 3594274 us | } 2686 2687Flags:: 2688 2689 + means that the function exceeded 10 usecs. 2690 ! means that the function exceeded 100 usecs. 2691 # means that the function exceeded 1000 usecs. 2692 * means that the function exceeded 10 msecs. 2693 @ means that the function exceeded 100 msecs. 2694 $ means that the function exceeded 1 sec. 2695 2696 2697- The task/pid field displays the thread cmdline and pid which 2698 executed the function. It is default disabled. 2699 2700 - hide: echo nofuncgraph-proc > trace_options 2701 - show: echo funcgraph-proc > trace_options 2702 2703 ie:: 2704 2705 # tracer: function_graph 2706 # 2707 # CPU TASK/PID DURATION FUNCTION CALLS 2708 # | | | | | | | | | 2709 0) sh-4802 | | d_free() { 2710 0) sh-4802 | | call_rcu() { 2711 0) sh-4802 | | __call_rcu() { 2712 0) sh-4802 | 0.616 us | rcu_process_gp_end(); 2713 0) sh-4802 | 0.586 us | check_for_new_grace_period(); 2714 0) sh-4802 | 2.899 us | } 2715 0) sh-4802 | 4.040 us | } 2716 0) sh-4802 | 5.151 us | } 2717 0) sh-4802 | + 49.370 us | } 2718 2719 2720- The absolute time field is an absolute timestamp given by the 2721 system clock since it started. A snapshot of this time is 2722 given on each entry/exit of functions 2723 2724 - hide: echo nofuncgraph-abstime > trace_options 2725 - show: echo funcgraph-abstime > trace_options 2726 2727 ie:: 2728 2729 # 2730 # TIME CPU DURATION FUNCTION CALLS 2731 # | | | | | | | | 2732 360.774522 | 1) 0.541 us | } 2733 360.774522 | 1) 4.663 us | } 2734 360.774523 | 1) 0.541 us | __wake_up_bit(); 2735 360.774524 | 1) 6.796 us | } 2736 360.774524 | 1) 7.952 us | } 2737 360.774525 | 1) 9.063 us | } 2738 360.774525 | 1) 0.615 us | journal_mark_dirty(); 2739 360.774527 | 1) 0.578 us | __brelse(); 2740 360.774528 | 1) | reiserfs_prepare_for_journal() { 2741 360.774528 | 1) | unlock_buffer() { 2742 360.774529 | 1) | wake_up_bit() { 2743 360.774529 | 1) | bit_waitqueue() { 2744 360.774530 | 1) 0.594 us | __phys_addr(); 2745 2746 2747The function name is always displayed after the closing bracket 2748for a function if the start of that function is not in the 2749trace buffer. 2750 2751Display of the function name after the closing bracket may be 2752enabled for functions whose start is in the trace buffer, 2753allowing easier searching with grep for function durations. 2754It is default disabled. 2755 2756 - hide: echo nofuncgraph-tail > trace_options 2757 - show: echo funcgraph-tail > trace_options 2758 2759 Example with nofuncgraph-tail (default):: 2760 2761 0) | putname() { 2762 0) | kmem_cache_free() { 2763 0) 0.518 us | __phys_addr(); 2764 0) 1.757 us | } 2765 0) 2.861 us | } 2766 2767 Example with funcgraph-tail:: 2768 2769 0) | putname() { 2770 0) | kmem_cache_free() { 2771 0) 0.518 us | __phys_addr(); 2772 0) 1.757 us | } /* kmem_cache_free() */ 2773 0) 2.861 us | } /* putname() */ 2774 2775The return value of each traced function can be displayed after 2776an equal sign "=". When encountering system call failures, it 2777can be very helpful to quickly locate the function that first 2778returns an error code. 2779 2780 - hide: echo nofuncgraph-retval > trace_options 2781 - show: echo funcgraph-retval > trace_options 2782 2783 Example with funcgraph-retval:: 2784 2785 1) | cgroup_migrate() { 2786 1) 0.651 us | cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */ 2787 1) | cgroup_migrate_execute() { 2788 1) | cpu_cgroup_can_attach() { 2789 1) | cgroup_taskset_first() { 2790 1) 0.732 us | cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */ 2791 1) 1.232 us | } /* cgroup_taskset_first = 0xffff93fc8fb20000 */ 2792 1) 0.380 us | sched_rt_can_attach(); /* = 0x0 */ 2793 1) 2.335 us | } /* cpu_cgroup_can_attach = -22 */ 2794 1) 4.369 us | } /* cgroup_migrate_execute = -22 */ 2795 1) 7.143 us | } /* cgroup_migrate = -22 */ 2796 2797The above example shows that the function cpu_cgroup_can_attach 2798returned the error code -22 firstly, then we can read the code 2799of this function to get the root cause. 2800 2801When the option funcgraph-retval-hex is not set, the return value can 2802be displayed in a smart way. Specifically, if it is an error code, 2803it will be printed in signed decimal format, otherwise it will 2804printed in hexadecimal format. 2805 2806 - smart: echo nofuncgraph-retval-hex > trace_options 2807 - hexadecimal: echo funcgraph-retval-hex > trace_options 2808 2809 Example with funcgraph-retval-hex:: 2810 2811 1) | cgroup_migrate() { 2812 1) 0.651 us | cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */ 2813 1) | cgroup_migrate_execute() { 2814 1) | cpu_cgroup_can_attach() { 2815 1) | cgroup_taskset_first() { 2816 1) 0.732 us | cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */ 2817 1) 1.232 us | } /* cgroup_taskset_first = 0xffff93fc8fb20000 */ 2818 1) 0.380 us | sched_rt_can_attach(); /* = 0x0 */ 2819 1) 2.335 us | } /* cpu_cgroup_can_attach = 0xffffffea */ 2820 1) 4.369 us | } /* cgroup_migrate_execute = 0xffffffea */ 2821 1) 7.143 us | } /* cgroup_migrate = 0xffffffea */ 2822 2823At present, there are some limitations when using the funcgraph-retval 2824option, and these limitations will be eliminated in the future: 2825 2826- Even if the function return type is void, a return value will still 2827 be printed, and you can just ignore it. 2828 2829- Even if return values are stored in multiple registers, only the 2830 value contained in the first register will be recorded and printed. 2831 To illustrate, in the x86 architecture, eax and edx are used to store 2832 a 64-bit return value, with the lower 32 bits saved in eax and the 2833 upper 32 bits saved in edx. However, only the value stored in eax 2834 will be recorded and printed. 2835 2836- In certain procedure call standards, such as arm64's AAPCS64, when a 2837 type is smaller than a GPR, it is the responsibility of the consumer 2838 to perform the narrowing, and the upper bits may contain UNKNOWN values. 2839 Therefore, it is advisable to check the code for such cases. For instance, 2840 when using a u8 in a 64-bit GPR, bits [63:8] may contain arbitrary values, 2841 especially when larger types are truncated, whether explicitly or implicitly. 2842 Here are some specific cases to illustrate this point: 2843 2844 **Case One**: 2845 2846 The function narrow_to_u8 is defined as follows:: 2847 2848 u8 narrow_to_u8(u64 val) 2849 { 2850 // implicitly truncated 2851 return val; 2852 } 2853 2854 It may be compiled to:: 2855 2856 narrow_to_u8: 2857 < ... ftrace instrumentation ... > 2858 RET 2859 2860 If you pass 0x123456789abcdef to this function and want to narrow it, 2861 it may be recorded as 0x123456789abcdef instead of 0xef. 2862 2863 **Case Two**: 2864 2865 The function error_if_not_4g_aligned is defined as follows:: 2866 2867 int error_if_not_4g_aligned(u64 val) 2868 { 2869 if (val & GENMASK(31, 0)) 2870 return -EINVAL; 2871 2872 return 0; 2873 } 2874 2875 It could be compiled to:: 2876 2877 error_if_not_4g_aligned: 2878 CBNZ w0, .Lnot_aligned 2879 RET // bits [31:0] are zero, bits 2880 // [63:32] are UNKNOWN 2881 .Lnot_aligned: 2882 MOV x0, #-EINVAL 2883 RET 2884 2885 When passing 0x2_0000_0000 to it, the return value may be recorded as 2886 0x2_0000_0000 instead of 0. 2887 2888You can put some comments on specific functions by using 2889trace_printk() For example, if you want to put a comment inside 2890the __might_sleep() function, you just have to include 2891<linux/ftrace.h> and call trace_printk() inside __might_sleep():: 2892 2893 trace_printk("I'm a comment!\n") 2894 2895will produce:: 2896 2897 1) | __might_sleep() { 2898 1) | /* I'm a comment! */ 2899 1) 1.449 us | } 2900 2901 2902You might find other useful features for this tracer in the 2903following "dynamic ftrace" section such as tracing only specific 2904functions or tasks. 2905 2906dynamic ftrace 2907-------------- 2908 2909If CONFIG_DYNAMIC_FTRACE is set, the system will run with 2910virtually no overhead when function tracing is disabled. The way 2911this works is the mcount function call (placed at the start of 2912every kernel function, produced by the -pg switch in gcc), 2913starts of pointing to a simple return. (Enabling FTRACE will 2914include the -pg switch in the compiling of the kernel.) 2915 2916At compile time every C file object is run through the 2917recordmcount program (located in the scripts directory). This 2918program will parse the ELF headers in the C object to find all 2919the locations in the .text section that call mcount. Starting 2920with gcc version 4.6, the -mfentry has been added for x86, which 2921calls "__fentry__" instead of "mcount". Which is called before 2922the creation of the stack frame. 2923 2924Note, not all sections are traced. They may be prevented by either 2925a notrace, or blocked another way and all inline functions are not 2926traced. Check the "available_filter_functions" file to see what functions 2927can be traced. 2928 2929A section called "__mcount_loc" is created that holds 2930references to all the mcount/fentry call sites in the .text section. 2931The recordmcount program re-links this section back into the 2932original object. The final linking stage of the kernel will add all these 2933references into a single table. 2934 2935On boot up, before SMP is initialized, the dynamic ftrace code 2936scans this table and updates all the locations into nops. It 2937also records the locations, which are added to the 2938available_filter_functions list. Modules are processed as they 2939are loaded and before they are executed. When a module is 2940unloaded, it also removes its functions from the ftrace function 2941list. This is automatic in the module unload code, and the 2942module author does not need to worry about it. 2943 2944When tracing is enabled, the process of modifying the function 2945tracepoints is dependent on architecture. The old method is to use 2946kstop_machine to prevent races with the CPUs executing code being 2947modified (which can cause the CPU to do undesirable things, especially 2948if the modified code crosses cache (or page) boundaries), and the nops are 2949patched back to calls. But this time, they do not call mcount 2950(which is just a function stub). They now call into the ftrace 2951infrastructure. 2952 2953The new method of modifying the function tracepoints is to place 2954a breakpoint at the location to be modified, sync all CPUs, modify 2955the rest of the instruction not covered by the breakpoint. Sync 2956all CPUs again, and then remove the breakpoint with the finished 2957version to the ftrace call site. 2958 2959Some archs do not even need to monkey around with the synchronization, 2960and can just slap the new code on top of the old without any 2961problems with other CPUs executing it at the same time. 2962 2963One special side-effect to the recording of the functions being 2964traced is that we can now selectively choose which functions we 2965wish to trace and which ones we want the mcount calls to remain 2966as nops. 2967 2968Two files are used, one for enabling and one for disabling the 2969tracing of specified functions. They are: 2970 2971 set_ftrace_filter 2972 2973and 2974 2975 set_ftrace_notrace 2976 2977A list of available functions that you can add to these files is 2978listed in: 2979 2980 available_filter_functions 2981 2982:: 2983 2984 # cat available_filter_functions 2985 put_prev_task_idle 2986 kmem_cache_create 2987 pick_next_task_rt 2988 cpus_read_lock 2989 pick_next_task_fair 2990 mutex_lock 2991 [...] 2992 2993If I am only interested in sys_nanosleep and hrtimer_interrupt:: 2994 2995 # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter 2996 # echo function > current_tracer 2997 # echo 1 > tracing_on 2998 # usleep 1 2999 # echo 0 > tracing_on 3000 # cat trace 3001 # tracer: function 3002 # 3003 # entries-in-buffer/entries-written: 5/5 #P:4 3004 # 3005 # _-----=> irqs-off 3006 # / _----=> need-resched 3007 # | / _---=> hardirq/softirq 3008 # || / _--=> preempt-depth 3009 # ||| / delay 3010 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3011 # | | | |||| | | 3012 usleep-2665 [001] .... 4186.475355: sys_nanosleep <-system_call_fastpath 3013 <idle>-0 [001] d.h1 4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt 3014 usleep-2665 [001] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt 3015 <idle>-0 [003] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt 3016 <idle>-0 [002] d.h1 4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt 3017 3018To see which functions are being traced, you can cat the file: 3019:: 3020 3021 # cat set_ftrace_filter 3022 hrtimer_interrupt 3023 sys_nanosleep 3024 3025 3026Perhaps this is not enough. The filters also allow glob(7) matching. 3027 3028 ``<match>*`` 3029 will match functions that begin with <match> 3030 ``*<match>`` 3031 will match functions that end with <match> 3032 ``*<match>*`` 3033 will match functions that have <match> in it 3034 ``<match1>*<match2>`` 3035 will match functions that begin with <match1> and end with <match2> 3036 3037.. note:: 3038 It is better to use quotes to enclose the wild cards, 3039 otherwise the shell may expand the parameters into names 3040 of files in the local directory. 3041 3042:: 3043 3044 # echo 'hrtimer_*' > set_ftrace_filter 3045 3046Produces:: 3047 3048 # tracer: function 3049 # 3050 # entries-in-buffer/entries-written: 897/897 #P:4 3051 # 3052 # _-----=> irqs-off 3053 # / _----=> need-resched 3054 # | / _---=> hardirq/softirq 3055 # || / _--=> preempt-depth 3056 # ||| / delay 3057 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3058 # | | | |||| | | 3059 <idle>-0 [003] dN.1 4228.547803: hrtimer_cancel <-tick_nohz_idle_exit 3060 <idle>-0 [003] dN.1 4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel 3061 <idle>-0 [003] dN.2 4228.547805: hrtimer_force_reprogram <-__remove_hrtimer 3062 <idle>-0 [003] dN.1 4228.547805: hrtimer_forward <-tick_nohz_idle_exit 3063 <idle>-0 [003] dN.1 4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11 3064 <idle>-0 [003] d..1 4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt 3065 <idle>-0 [003] d..1 4228.547859: hrtimer_start <-__tick_nohz_idle_enter 3066 <idle>-0 [003] d..2 4228.547860: hrtimer_force_reprogram <-__rem 3067 3068Notice that we lost the sys_nanosleep. 3069:: 3070 3071 # cat set_ftrace_filter 3072 hrtimer_run_queues 3073 hrtimer_run_pending 3074 hrtimer_init 3075 hrtimer_cancel 3076 hrtimer_try_to_cancel 3077 hrtimer_forward 3078 hrtimer_start 3079 hrtimer_reprogram 3080 hrtimer_force_reprogram 3081 hrtimer_get_next_event 3082 hrtimer_interrupt 3083 hrtimer_nanosleep 3084 hrtimer_wakeup 3085 hrtimer_get_remaining 3086 hrtimer_get_res 3087 hrtimer_init_sleeper 3088 3089 3090This is because the '>' and '>>' act just like they do in bash. 3091To rewrite the filters, use '>' 3092To append to the filters, use '>>' 3093 3094To clear out a filter so that all functions will be recorded 3095again:: 3096 3097 # echo > set_ftrace_filter 3098 # cat set_ftrace_filter 3099 # 3100 3101Again, now we want to append. 3102 3103:: 3104 3105 # echo sys_nanosleep > set_ftrace_filter 3106 # cat set_ftrace_filter 3107 sys_nanosleep 3108 # echo 'hrtimer_*' >> set_ftrace_filter 3109 # cat set_ftrace_filter 3110 hrtimer_run_queues 3111 hrtimer_run_pending 3112 hrtimer_init 3113 hrtimer_cancel 3114 hrtimer_try_to_cancel 3115 hrtimer_forward 3116 hrtimer_start 3117 hrtimer_reprogram 3118 hrtimer_force_reprogram 3119 hrtimer_get_next_event 3120 hrtimer_interrupt 3121 sys_nanosleep 3122 hrtimer_nanosleep 3123 hrtimer_wakeup 3124 hrtimer_get_remaining 3125 hrtimer_get_res 3126 hrtimer_init_sleeper 3127 3128 3129The set_ftrace_notrace prevents those functions from being 3130traced. 3131:: 3132 3133 # echo '*preempt*' '*lock*' > set_ftrace_notrace 3134 3135Produces:: 3136 3137 # tracer: function 3138 # 3139 # entries-in-buffer/entries-written: 39608/39608 #P:4 3140 # 3141 # _-----=> irqs-off 3142 # / _----=> need-resched 3143 # | / _---=> hardirq/softirq 3144 # || / _--=> preempt-depth 3145 # ||| / delay 3146 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3147 # | | | |||| | | 3148 bash-1994 [000] .... 4342.324896: file_ra_state_init <-do_dentry_open 3149 bash-1994 [000] .... 4342.324897: open_check_o_direct <-do_last 3150 bash-1994 [000] .... 4342.324897: ima_file_check <-do_last 3151 bash-1994 [000] .... 4342.324898: process_measurement <-ima_file_check 3152 bash-1994 [000] .... 4342.324898: ima_get_action <-process_measurement 3153 bash-1994 [000] .... 4342.324898: ima_match_policy <-ima_get_action 3154 bash-1994 [000] .... 4342.324899: do_truncate <-do_last 3155 bash-1994 [000] .... 4342.324899: setattr_should_drop_suidgid <-do_truncate 3156 bash-1994 [000] .... 4342.324899: notify_change <-do_truncate 3157 bash-1994 [000] .... 4342.324900: current_fs_time <-notify_change 3158 bash-1994 [000] .... 4342.324900: current_kernel_time <-current_fs_time 3159 bash-1994 [000] .... 4342.324900: timespec_trunc <-current_fs_time 3160 3161We can see that there's no more lock or preempt tracing. 3162 3163Selecting function filters via index 3164------------------------------------ 3165 3166Because processing of strings is expensive (the address of the function 3167needs to be looked up before comparing to the string being passed in), 3168an index can be used as well to enable functions. This is useful in the 3169case of setting thousands of specific functions at a time. By passing 3170in a list of numbers, no string processing will occur. Instead, the function 3171at the specific location in the internal array (which corresponds to the 3172functions in the "available_filter_functions" file), is selected. 3173 3174:: 3175 3176 # echo 1 > set_ftrace_filter 3177 3178Will select the first function listed in "available_filter_functions" 3179 3180:: 3181 3182 # head -1 available_filter_functions 3183 trace_initcall_finish_cb 3184 3185 # cat set_ftrace_filter 3186 trace_initcall_finish_cb 3187 3188 # head -50 available_filter_functions | tail -1 3189 x86_pmu_commit_txn 3190 3191 # echo 1 50 > set_ftrace_filter 3192 # cat set_ftrace_filter 3193 trace_initcall_finish_cb 3194 x86_pmu_commit_txn 3195 3196Dynamic ftrace with the function graph tracer 3197--------------------------------------------- 3198 3199Although what has been explained above concerns both the 3200function tracer and the function-graph-tracer, there are some 3201special features only available in the function-graph tracer. 3202 3203If you want to trace only one function and all of its children, 3204you just have to echo its name into set_graph_function:: 3205 3206 echo __do_fault > set_graph_function 3207 3208will produce the following "expanded" trace of the __do_fault() 3209function:: 3210 3211 0) | __do_fault() { 3212 0) | filemap_fault() { 3213 0) | find_lock_page() { 3214 0) 0.804 us | find_get_page(); 3215 0) | __might_sleep() { 3216 0) 1.329 us | } 3217 0) 3.904 us | } 3218 0) 4.979 us | } 3219 0) 0.653 us | _spin_lock(); 3220 0) 0.578 us | page_add_file_rmap(); 3221 0) 0.525 us | native_set_pte_at(); 3222 0) 0.585 us | _spin_unlock(); 3223 0) | unlock_page() { 3224 0) 0.541 us | page_waitqueue(); 3225 0) 0.639 us | __wake_up_bit(); 3226 0) 2.786 us | } 3227 0) + 14.237 us | } 3228 0) | __do_fault() { 3229 0) | filemap_fault() { 3230 0) | find_lock_page() { 3231 0) 0.698 us | find_get_page(); 3232 0) | __might_sleep() { 3233 0) 1.412 us | } 3234 0) 3.950 us | } 3235 0) 5.098 us | } 3236 0) 0.631 us | _spin_lock(); 3237 0) 0.571 us | page_add_file_rmap(); 3238 0) 0.526 us | native_set_pte_at(); 3239 0) 0.586 us | _spin_unlock(); 3240 0) | unlock_page() { 3241 0) 0.533 us | page_waitqueue(); 3242 0) 0.638 us | __wake_up_bit(); 3243 0) 2.793 us | } 3244 0) + 14.012 us | } 3245 3246You can also expand several functions at once:: 3247 3248 echo sys_open > set_graph_function 3249 echo sys_close >> set_graph_function 3250 3251Now if you want to go back to trace all functions you can clear 3252this special filter via:: 3253 3254 echo > set_graph_function 3255 3256 3257ftrace_enabled 3258-------------- 3259 3260Note, the proc sysctl ftrace_enable is a big on/off switch for the 3261function tracer. By default it is enabled (when function tracing is 3262enabled in the kernel). If it is disabled, all function tracing is 3263disabled. This includes not only the function tracers for ftrace, but 3264also for any other uses (perf, kprobes, stack tracing, profiling, etc). It 3265cannot be disabled if there is a callback with FTRACE_OPS_FL_PERMANENT set 3266registered. 3267 3268Please disable this with care. 3269 3270This can be disable (and enabled) with:: 3271 3272 sysctl kernel.ftrace_enabled=0 3273 sysctl kernel.ftrace_enabled=1 3274 3275 or 3276 3277 echo 0 > /proc/sys/kernel/ftrace_enabled 3278 echo 1 > /proc/sys/kernel/ftrace_enabled 3279 3280 3281Filter commands 3282--------------- 3283 3284A few commands are supported by the set_ftrace_filter interface. 3285Trace commands have the following format:: 3286 3287 <function>:<command>:<parameter> 3288 3289The following commands are supported: 3290 3291- mod: 3292 This command enables function filtering per module. The 3293 parameter defines the module. For example, if only the write* 3294 functions in the ext3 module are desired, run: 3295 3296 echo 'write*:mod:ext3' > set_ftrace_filter 3297 3298 This command interacts with the filter in the same way as 3299 filtering based on function names. Thus, adding more functions 3300 in a different module is accomplished by appending (>>) to the 3301 filter file. Remove specific module functions by prepending 3302 '!':: 3303 3304 echo '!writeback*:mod:ext3' >> set_ftrace_filter 3305 3306 Mod command supports module globbing. Disable tracing for all 3307 functions except a specific module:: 3308 3309 echo '!*:mod:!ext3' >> set_ftrace_filter 3310 3311 Disable tracing for all modules, but still trace kernel:: 3312 3313 echo '!*:mod:*' >> set_ftrace_filter 3314 3315 Enable filter only for kernel:: 3316 3317 echo '*write*:mod:!*' >> set_ftrace_filter 3318 3319 Enable filter for module globbing:: 3320 3321 echo '*write*:mod:*snd*' >> set_ftrace_filter 3322 3323- traceon/traceoff: 3324 These commands turn tracing on and off when the specified 3325 functions are hit. The parameter determines how many times the 3326 tracing system is turned on and off. If unspecified, there is 3327 no limit. For example, to disable tracing when a schedule bug 3328 is hit the first 5 times, run:: 3329 3330 echo '__schedule_bug:traceoff:5' > set_ftrace_filter 3331 3332 To always disable tracing when __schedule_bug is hit:: 3333 3334 echo '__schedule_bug:traceoff' > set_ftrace_filter 3335 3336 These commands are cumulative whether or not they are appended 3337 to set_ftrace_filter. To remove a command, prepend it by '!' 3338 and drop the parameter:: 3339 3340 echo '!__schedule_bug:traceoff:0' > set_ftrace_filter 3341 3342 The above removes the traceoff command for __schedule_bug 3343 that have a counter. To remove commands without counters:: 3344 3345 echo '!__schedule_bug:traceoff' > set_ftrace_filter 3346 3347- snapshot: 3348 Will cause a snapshot to be triggered when the function is hit. 3349 :: 3350 3351 echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter 3352 3353 To only snapshot once: 3354 :: 3355 3356 echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter 3357 3358 To remove the above commands:: 3359 3360 echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter 3361 echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter 3362 3363- enable_event/disable_event: 3364 These commands can enable or disable a trace event. Note, because 3365 function tracing callbacks are very sensitive, when these commands 3366 are registered, the trace point is activated, but disabled in 3367 a "soft" mode. That is, the tracepoint will be called, but 3368 just will not be traced. The event tracepoint stays in this mode 3369 as long as there's a command that triggers it. 3370 :: 3371 3372 echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \ 3373 set_ftrace_filter 3374 3375 The format is:: 3376 3377 <function>:enable_event:<system>:<event>[:count] 3378 <function>:disable_event:<system>:<event>[:count] 3379 3380 To remove the events commands:: 3381 3382 echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \ 3383 set_ftrace_filter 3384 echo '!schedule:disable_event:sched:sched_switch' > \ 3385 set_ftrace_filter 3386 3387- dump: 3388 When the function is hit, it will dump the contents of the ftrace 3389 ring buffer to the console. This is useful if you need to debug 3390 something, and want to dump the trace when a certain function 3391 is hit. Perhaps it's a function that is called before a triple 3392 fault happens and does not allow you to get a regular dump. 3393 3394- cpudump: 3395 When the function is hit, it will dump the contents of the ftrace 3396 ring buffer for the current CPU to the console. Unlike the "dump" 3397 command, it only prints out the contents of the ring buffer for the 3398 CPU that executed the function that triggered the dump. 3399 3400- stacktrace: 3401 When the function is hit, a stack trace is recorded. 3402 3403trace_pipe 3404---------- 3405 3406The trace_pipe outputs the same content as the trace file, but 3407the effect on the tracing is different. Every read from 3408trace_pipe is consumed. This means that subsequent reads will be 3409different. The trace is live. 3410:: 3411 3412 # echo function > current_tracer 3413 # cat trace_pipe > /tmp/trace.out & 3414 [1] 4153 3415 # echo 1 > tracing_on 3416 # usleep 1 3417 # echo 0 > tracing_on 3418 # cat trace 3419 # tracer: function 3420 # 3421 # entries-in-buffer/entries-written: 0/0 #P:4 3422 # 3423 # _-----=> irqs-off 3424 # / _----=> need-resched 3425 # | / _---=> hardirq/softirq 3426 # || / _--=> preempt-depth 3427 # ||| / delay 3428 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3429 # | | | |||| | | 3430 3431 # 3432 # cat /tmp/trace.out 3433 bash-1994 [000] .... 5281.568961: mutex_unlock <-rb_simple_write 3434 bash-1994 [000] .... 5281.568963: __mutex_unlock_slowpath <-mutex_unlock 3435 bash-1994 [000] .... 5281.568963: __fsnotify_parent <-fsnotify_modify 3436 bash-1994 [000] .... 5281.568964: fsnotify <-fsnotify_modify 3437 bash-1994 [000] .... 5281.568964: __srcu_read_lock <-fsnotify 3438 bash-1994 [000] .... 5281.568964: add_preempt_count <-__srcu_read_lock 3439 bash-1994 [000] ...1 5281.568965: sub_preempt_count <-__srcu_read_lock 3440 bash-1994 [000] .... 5281.568965: __srcu_read_unlock <-fsnotify 3441 bash-1994 [000] .... 5281.568967: sys_dup2 <-system_call_fastpath 3442 3443 3444Note, reading the trace_pipe file will block until more input is 3445added. This is contrary to the trace file. If any process opened 3446the trace file for reading, it will actually disable tracing and 3447prevent new entries from being added. The trace_pipe file does 3448not have this limitation. 3449 3450trace entries 3451------------- 3452 3453Having too much or not enough data can be troublesome in 3454diagnosing an issue in the kernel. The file buffer_size_kb is 3455used to modify the size of the internal trace buffers. The 3456number listed is the number of entries that can be recorded per 3457CPU. To know the full size, multiply the number of possible CPUs 3458with the number of entries. 3459:: 3460 3461 # cat buffer_size_kb 3462 1408 (units kilobytes) 3463 3464Or simply read buffer_total_size_kb 3465:: 3466 3467 # cat buffer_total_size_kb 3468 5632 3469 3470To modify the buffer, simple echo in a number (in 1024 byte segments). 3471:: 3472 3473 # echo 10000 > buffer_size_kb 3474 # cat buffer_size_kb 3475 10000 (units kilobytes) 3476 3477It will try to allocate as much as possible. If you allocate too 3478much, it can cause Out-Of-Memory to trigger. 3479:: 3480 3481 # echo 1000000000000 > buffer_size_kb 3482 -bash: echo: write error: Cannot allocate memory 3483 # cat buffer_size_kb 3484 85 3485 3486The per_cpu buffers can be changed individually as well: 3487:: 3488 3489 # echo 10000 > per_cpu/cpu0/buffer_size_kb 3490 # echo 100 > per_cpu/cpu1/buffer_size_kb 3491 3492When the per_cpu buffers are not the same, the buffer_size_kb 3493at the top level will just show an X 3494:: 3495 3496 # cat buffer_size_kb 3497 X 3498 3499This is where the buffer_total_size_kb is useful: 3500:: 3501 3502 # cat buffer_total_size_kb 3503 12916 3504 3505Writing to the top level buffer_size_kb will reset all the buffers 3506to be the same again. 3507 3508Snapshot 3509-------- 3510CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature 3511available to all non latency tracers. (Latency tracers which 3512record max latency, such as "irqsoff" or "wakeup", can't use 3513this feature, since those are already using the snapshot 3514mechanism internally.) 3515 3516Snapshot preserves a current trace buffer at a particular point 3517in time without stopping tracing. Ftrace swaps the current 3518buffer with a spare buffer, and tracing continues in the new 3519current (=previous spare) buffer. 3520 3521The following tracefs files in "tracing" are related to this 3522feature: 3523 3524 snapshot: 3525 3526 This is used to take a snapshot and to read the output 3527 of the snapshot. Echo 1 into this file to allocate a 3528 spare buffer and to take a snapshot (swap), then read 3529 the snapshot from this file in the same format as 3530 "trace" (described above in the section "The File 3531 System"). Both reads snapshot and tracing are executable 3532 in parallel. When the spare buffer is allocated, echoing 3533 0 frees it, and echoing else (positive) values clear the 3534 snapshot contents. 3535 More details are shown in the table below. 3536 3537 +--------------+------------+------------+------------+ 3538 |status\\input | 0 | 1 | else | 3539 +==============+============+============+============+ 3540 |not allocated |(do nothing)| alloc+swap |(do nothing)| 3541 +--------------+------------+------------+------------+ 3542 |allocated | free | swap | clear | 3543 +--------------+------------+------------+------------+ 3544 3545Here is an example of using the snapshot feature. 3546:: 3547 3548 # echo 1 > events/sched/enable 3549 # echo 1 > snapshot 3550 # cat snapshot 3551 # tracer: nop 3552 # 3553 # entries-in-buffer/entries-written: 71/71 #P:8 3554 # 3555 # _-----=> irqs-off 3556 # / _----=> need-resched 3557 # | / _---=> hardirq/softirq 3558 # || / _--=> preempt-depth 3559 # ||| / delay 3560 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3561 # | | | |||| | | 3562 <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 3563 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 3564 [...] 3565 <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 3566 3567 # cat trace 3568 # tracer: nop 3569 # 3570 # entries-in-buffer/entries-written: 77/77 #P:8 3571 # 3572 # _-----=> irqs-off 3573 # / _----=> need-resched 3574 # | / _---=> hardirq/softirq 3575 # || / _--=> preempt-depth 3576 # ||| / delay 3577 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3578 # | | | |||| | | 3579 <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 3580 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 3581 [...] 3582 3583 3584If you try to use this snapshot feature when current tracer is 3585one of the latency tracers, you will get the following results. 3586:: 3587 3588 # echo wakeup > current_tracer 3589 # echo 1 > snapshot 3590 bash: echo: write error: Device or resource busy 3591 # cat snapshot 3592 cat: snapshot: Device or resource busy 3593 3594 3595Instances 3596--------- 3597In the tracefs tracing directory, there is a directory called "instances". 3598This directory can have new directories created inside of it using 3599mkdir, and removing directories with rmdir. The directory created 3600with mkdir in this directory will already contain files and other 3601directories after it is created. 3602:: 3603 3604 # mkdir instances/foo 3605 # ls instances/foo 3606 buffer_size_kb buffer_total_size_kb events free_buffer per_cpu 3607 set_event snapshot trace trace_clock trace_marker trace_options 3608 trace_pipe tracing_on 3609 3610As you can see, the new directory looks similar to the tracing directory 3611itself. In fact, it is very similar, except that the buffer and 3612events are agnostic from the main directory, or from any other 3613instances that are created. 3614 3615The files in the new directory work just like the files with the 3616same name in the tracing directory except the buffer that is used 3617is a separate and new buffer. The files affect that buffer but do not 3618affect the main buffer with the exception of trace_options. Currently, 3619the trace_options affect all instances and the top level buffer 3620the same, but this may change in future releases. That is, options 3621may become specific to the instance they reside in. 3622 3623Notice that none of the function tracer files are there, nor is 3624current_tracer and available_tracers. This is because the buffers 3625can currently only have events enabled for them. 3626:: 3627 3628 # mkdir instances/foo 3629 # mkdir instances/bar 3630 # mkdir instances/zoot 3631 # echo 100000 > buffer_size_kb 3632 # echo 1000 > instances/foo/buffer_size_kb 3633 # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb 3634 # echo function > current_trace 3635 # echo 1 > instances/foo/events/sched/sched_wakeup/enable 3636 # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable 3637 # echo 1 > instances/foo/events/sched/sched_switch/enable 3638 # echo 1 > instances/bar/events/irq/enable 3639 # echo 1 > instances/zoot/events/syscalls/enable 3640 # cat trace_pipe 3641 CPU:2 [LOST 11745 EVENTS] 3642 bash-2044 [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist 3643 bash-2044 [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave 3644 bash-2044 [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist 3645 bash-2044 [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist 3646 bash-2044 [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock 3647 bash-2044 [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype 3648 bash-2044 [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist 3649 bash-2044 [002] d... 10594.481034: zone_statistics <-get_page_from_freelist 3650 bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics 3651 bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics 3652 bash-2044 [002] .... 10594.481035: arch_dup_task_struct <-copy_process 3653 [...] 3654 3655 # cat instances/foo/trace_pipe 3656 bash-1998 [000] d..4 136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000 3657 bash-1998 [000] dN.4 136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000 3658 <idle>-0 [003] d.h3 136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003 3659 <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 3660 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 3661 bash-1998 [000] d..4 136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000 3662 bash-1998 [000] dN.4 136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000 3663 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 3664 kworker/0:1-59 [000] d..4 136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001 3665 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 3666 [...] 3667 3668 # cat instances/bar/trace_pipe 3669 migration/1-14 [001] d.h3 138.732674: softirq_raise: vec=3 [action=NET_RX] 3670 <idle>-0 [001] dNh3 138.732725: softirq_raise: vec=3 [action=NET_RX] 3671 bash-1998 [000] d.h1 138.733101: softirq_raise: vec=1 [action=TIMER] 3672 bash-1998 [000] d.h1 138.733102: softirq_raise: vec=9 [action=RCU] 3673 bash-1998 [000] ..s2 138.733105: softirq_entry: vec=1 [action=TIMER] 3674 bash-1998 [000] ..s2 138.733106: softirq_exit: vec=1 [action=TIMER] 3675 bash-1998 [000] ..s2 138.733106: softirq_entry: vec=9 [action=RCU] 3676 bash-1998 [000] ..s2 138.733109: softirq_exit: vec=9 [action=RCU] 3677 sshd-1995 [001] d.h1 138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4 3678 sshd-1995 [001] d.h1 138.733280: irq_handler_exit: irq=21 ret=unhandled 3679 sshd-1995 [001] d.h1 138.733281: irq_handler_entry: irq=21 name=eth0 3680 sshd-1995 [001] d.h1 138.733283: irq_handler_exit: irq=21 ret=handled 3681 [...] 3682 3683 # cat instances/zoot/trace 3684 # tracer: nop 3685 # 3686 # entries-in-buffer/entries-written: 18996/18996 #P:4 3687 # 3688 # _-----=> irqs-off 3689 # / _----=> need-resched 3690 # | / _---=> hardirq/softirq 3691 # || / _--=> preempt-depth 3692 # ||| / delay 3693 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3694 # | | | |||| | | 3695 bash-1998 [000] d... 140.733501: sys_write -> 0x2 3696 bash-1998 [000] d... 140.733504: sys_dup2(oldfd: a, newfd: 1) 3697 bash-1998 [000] d... 140.733506: sys_dup2 -> 0x1 3698 bash-1998 [000] d... 140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0) 3699 bash-1998 [000] d... 140.733509: sys_fcntl -> 0x1 3700 bash-1998 [000] d... 140.733510: sys_close(fd: a) 3701 bash-1998 [000] d... 140.733510: sys_close -> 0x0 3702 bash-1998 [000] d... 140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8) 3703 bash-1998 [000] d... 140.733515: sys_rt_sigprocmask -> 0x0 3704 bash-1998 [000] d... 140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8) 3705 bash-1998 [000] d... 140.733516: sys_rt_sigaction -> 0x0 3706 3707You can see that the trace of the top most trace buffer shows only 3708the function tracing. The foo instance displays wakeups and task 3709switches. 3710 3711To remove the instances, simply delete their directories: 3712:: 3713 3714 # rmdir instances/foo 3715 # rmdir instances/bar 3716 # rmdir instances/zoot 3717 3718Note, if a process has a trace file open in one of the instance 3719directories, the rmdir will fail with EBUSY. 3720 3721 3722Stack trace 3723----------- 3724Since the kernel has a fixed sized stack, it is important not to 3725waste it in functions. A kernel developer must be conscious of 3726what they allocate on the stack. If they add too much, the system 3727can be in danger of a stack overflow, and corruption will occur, 3728usually leading to a system panic. 3729 3730There are some tools that check this, usually with interrupts 3731periodically checking usage. But if you can perform a check 3732at every function call that will become very useful. As ftrace provides 3733a function tracer, it makes it convenient to check the stack size 3734at every function call. This is enabled via the stack tracer. 3735 3736CONFIG_STACK_TRACER enables the ftrace stack tracing functionality. 3737To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled. 3738:: 3739 3740 # echo 1 > /proc/sys/kernel/stack_tracer_enabled 3741 3742You can also enable it from the kernel command line to trace 3743the stack size of the kernel during boot up, by adding "stacktrace" 3744to the kernel command line parameter. 3745 3746After running it for a few minutes, the output looks like: 3747:: 3748 3749 # cat stack_max_size 3750 2928 3751 3752 # cat stack_trace 3753 Depth Size Location (18 entries) 3754 ----- ---- -------- 3755 0) 2928 224 update_sd_lb_stats+0xbc/0x4ac 3756 1) 2704 160 find_busiest_group+0x31/0x1f1 3757 2) 2544 256 load_balance+0xd9/0x662 3758 3) 2288 80 idle_balance+0xbb/0x130 3759 4) 2208 128 __schedule+0x26e/0x5b9 3760 5) 2080 16 schedule+0x64/0x66 3761 6) 2064 128 schedule_timeout+0x34/0xe0 3762 7) 1936 112 wait_for_common+0x97/0xf1 3763 8) 1824 16 wait_for_completion+0x1d/0x1f 3764 9) 1808 128 flush_work+0xfe/0x119 3765 10) 1680 16 tty_flush_to_ldisc+0x1e/0x20 3766 11) 1664 48 input_available_p+0x1d/0x5c 3767 12) 1616 48 n_tty_poll+0x6d/0x134 3768 13) 1568 64 tty_poll+0x64/0x7f 3769 14) 1504 880 do_select+0x31e/0x511 3770 15) 624 400 core_sys_select+0x177/0x216 3771 16) 224 96 sys_select+0x91/0xb9 3772 17) 128 128 system_call_fastpath+0x16/0x1b 3773 3774Note, if -mfentry is being used by gcc, functions get traced before 3775they set up the stack frame. This means that leaf level functions 3776are not tested by the stack tracer when -mfentry is used. 3777 3778Currently, -mfentry is used by gcc 4.6.0 and above on x86 only. 3779 3780More 3781---- 3782More details can be found in the source code, in the `kernel/trace/*.c` files. 3783