1############### 2Timerlat tracer 3############### 4 5The timerlat tracer aims to help the preemptive kernel developers to 6find sources of wakeup latencies of real-time threads. Like cyclictest, 7the tracer sets a periodic timer that wakes up a thread. The thread then 8computes a *wakeup latency* value as the difference between the *current 9time* and the *absolute time* that the timer was set to expire. The main 10goal of timerlat is tracing in such a way to help kernel developers. 11 12Usage 13----- 14 15Write the ASCII text "timerlat" into the current_tracer file of the 16tracing system (generally mounted at /sys/kernel/tracing). 17 18For example:: 19 20 [root@f32 ~]# cd /sys/kernel/tracing/ 21 [root@f32 tracing]# echo timerlat > current_tracer 22 23It is possible to follow the trace by reading the trace file:: 24 25 [root@f32 tracing]# cat trace 26 # tracer: timerlat 27 # 28 # _-----=> irqs-off 29 # / _----=> need-resched 30 # | / _---=> hardirq/softirq 31 # || / _--=> preempt-depth 32 # || / 33 # |||| ACTIVATION 34 # TASK-PID CPU# |||| TIMESTAMP ID CONTEXT LATENCY 35 # | | | |||| | | | | 36 <idle>-0 [000] d.h1 54.029328: #1 context irq timer_latency 932 ns 37 <...>-867 [000] .... 54.029339: #1 context thread timer_latency 11700 ns 38 <idle>-0 [001] dNh1 54.029346: #1 context irq timer_latency 2833 ns 39 <...>-868 [001] .... 54.029353: #1 context thread timer_latency 9820 ns 40 <idle>-0 [000] d.h1 54.030328: #2 context irq timer_latency 769 ns 41 <...>-867 [000] .... 54.030330: #2 context thread timer_latency 3070 ns 42 <idle>-0 [001] d.h1 54.030344: #2 context irq timer_latency 935 ns 43 <...>-868 [001] .... 54.030347: #2 context thread timer_latency 4351 ns 44 45 46The tracer creates a per-cpu kernel thread with real-time priority that 47prints two lines at every activation. The first is the *timer latency* 48observed at the *hardirq* context before the activation of the thread. 49The second is the *timer latency* observed by the thread. The ACTIVATION 50ID field serves to relate the *irq* execution to its respective *thread* 51execution. 52 53The *irq*/*thread* splitting is important to clarify in which context 54the unexpected high value is coming from. The *irq* context can be 55delayed by hardware-related actions, such as SMIs, NMIs, IRQs, 56or by thread masking interrupts. Once the timer happens, the delay 57can also be influenced by blocking caused by threads. For example, by 58postponing the scheduler execution via preempt_disable(), scheduler 59execution, or masking interrupts. Threads can also be delayed by the 60interference from other threads and IRQs. 61 62Tracer options 63--------------------- 64 65The timerlat tracer is built on top of osnoise tracer. 66So its configuration is also done in the osnoise/ config 67directory. The timerlat configs are: 68 69 - cpus: CPUs at which a timerlat thread will execute. 70 - timerlat_period_us: the period of the timerlat thread. 71 - stop_tracing_us: stop the system tracing if a 72 timer latency at the *irq* context higher than the configured 73 value happens. Writing 0 disables this option. 74 - stop_tracing_total_us: stop the system tracing if a 75 timer latency at the *thread* context is higher than the configured 76 value happens. Writing 0 disables this option. 77 - print_stack: save the stack of the IRQ occurrence. The stack is printed 78 after the *thread context* event, or at the IRQ handler if *stop_tracing_us* 79 is hit. 80 81timerlat and osnoise 82---------------------------- 83 84The timerlat can also take advantage of the osnoise: traceevents. 85For example:: 86 87 [root@f32 ~]# cd /sys/kernel/tracing/ 88 [root@f32 tracing]# echo timerlat > current_tracer 89 [root@f32 tracing]# echo 1 > events/osnoise/enable 90 [root@f32 tracing]# echo 25 > osnoise/stop_tracing_total_us 91 [root@f32 tracing]# tail -10 trace 92 cc1-87882 [005] d..h... 548.771078: #402268 context irq timer_latency 13585 ns 93 cc1-87882 [005] dNLh1.. 548.771082: irq_noise: local_timer:236 start 548.771077442 duration 7597 ns 94 cc1-87882 [005] dNLh2.. 548.771099: irq_noise: qxl:21 start 548.771085017 duration 7139 ns 95 cc1-87882 [005] d...3.. 548.771102: thread_noise: cc1:87882 start 548.771078243 duration 9909 ns 96 timerlat/5-1035 [005] ....... 548.771104: #402268 context thread timer_latency 39960 ns 97 98In this case, the root cause of the timer latency does not point to a 99single cause but to multiple ones. Firstly, the timer IRQ was delayed 100for 13 us, which may point to a long IRQ disabled section (see IRQ 101stacktrace section). Then the timer interrupt that wakes up the timerlat 102thread took 7597 ns, and the qxl:21 device IRQ took 7139 ns. Finally, 103the cc1 thread noise took 9909 ns of time before the context switch. 104Such pieces of evidence are useful for the developer to use other 105tracing methods to figure out how to debug and optimize the system. 106 107It is worth mentioning that the *duration* values reported 108by the osnoise: events are *net* values. For example, the 109thread_noise does not include the duration of the overhead caused 110by the IRQ execution (which indeed accounted for 12736 ns). But 111the values reported by the timerlat tracer (timerlat_latency) 112are *gross* values. 113 114The art below illustrates a CPU timeline and how the timerlat tracer 115observes it at the top and the osnoise: events at the bottom. Each "-" 116in the timelines means circa 1 us, and the time moves ==>:: 117 118 External timer irq thread 119 clock latency latency 120 event 13585 ns 39960 ns 121 | ^ ^ 122 v | | 123 |-------------| | 124 |-------------+-------------------------| 125 ^ ^ 126 ======================================================================== 127 [tmr irq] [dev irq] 128 [another thread...^ v..^ v.......][timerlat/ thread] <-- CPU timeline 129 ========================================================================= 130 |-------| |-------| 131 |--^ v-------| 132 | | | 133 | | + thread_noise: 9909 ns 134 | +-> irq_noise: 6139 ns 135 +-> irq_noise: 7597 ns 136 137IRQ stacktrace 138--------------------------- 139 140The osnoise/print_stack option is helpful for the cases in which a thread 141noise causes the major factor for the timer latency, because of preempt or 142irq disabled. For example:: 143 144 [root@f32 tracing]# echo 500 > osnoise/stop_tracing_total_us 145 [root@f32 tracing]# echo 500 > osnoise/print_stack 146 [root@f32 tracing]# echo timerlat > current_tracer 147 [root@f32 tracing]# tail -21 per_cpu/cpu7/trace 148 insmod-1026 [007] dN.h1.. 200.201948: irq_noise: local_timer:236 start 200.201939376 duration 7872 ns 149 insmod-1026 [007] d..h1.. 200.202587: #29800 context irq timer_latency 1616 ns 150 insmod-1026 [007] dN.h2.. 200.202598: irq_noise: local_timer:236 start 200.202586162 duration 11855 ns 151 insmod-1026 [007] dN.h3.. 200.202947: irq_noise: local_timer:236 start 200.202939174 duration 7318 ns 152 insmod-1026 [007] d...3.. 200.203444: thread_noise: insmod:1026 start 200.202586933 duration 838681 ns 153 timerlat/7-1001 [007] ....... 200.203445: #29800 context thread timer_latency 859978 ns 154 timerlat/7-1001 [007] ....1.. 200.203446: <stack trace> 155 => timerlat_irq 156 => __hrtimer_run_queues 157 => hrtimer_interrupt 158 => __sysvec_apic_timer_interrupt 159 => asm_call_irq_on_stack 160 => sysvec_apic_timer_interrupt 161 => asm_sysvec_apic_timer_interrupt 162 => delay_tsc 163 => dummy_load_1ms_pd_init 164 => do_one_initcall 165 => do_init_module 166 => __do_sys_finit_module 167 => do_syscall_64 168 => entry_SYSCALL_64_after_hwframe 169 170In this case, it is possible to see that the thread added the highest 171contribution to the *timer latency* and the stack trace, saved during 172the timerlat IRQ handler, points to a function named 173dummy_load_1ms_pd_init, which had the following code (on purpose):: 174 175 static int __init dummy_load_1ms_pd_init(void) 176 { 177 preempt_disable(); 178 mdelay(1); 179 preempt_enable(); 180 return 0; 181 182 } 183 184User-space interface 185--------------------------- 186 187Timerlat allows user-space threads to use timerlat infra-structure to 188measure scheduling latency. This interface is accessible via a per-CPU 189file descriptor inside $tracing_dir/osnoise/per_cpu/cpu$ID/timerlat_fd. 190 191This interface is accessible under the following conditions: 192 193 - timerlat tracer is enable 194 - osnoise workload option is set to NO_OSNOISE_WORKLOAD 195 - The user-space thread is affined to a single processor 196 - The thread opens the file associated with its single processor 197 - Only one thread can access the file at a time 198 199The open() syscall will fail if any of these conditions are not met. 200After opening the file descriptor, the user space can read from it. 201 202The read() system call will run a timerlat code that will arm the 203timer in the future and wait for it as the regular kernel thread does. 204 205When the timer IRQ fires, the timerlat IRQ will execute, report the 206IRQ latency and wake up the thread waiting in the read. The thread will be 207scheduled and report the thread latency via tracer - as for the kernel 208thread. 209 210The difference from the in-kernel timerlat is that, instead of re-arming 211the timer, timerlat will return to the read() system call. At this point, 212the user can run any code. 213 214If the application rereads the file timerlat file descriptor, the tracer 215will report the return from user-space latency, which is the total 216latency. If this is the end of the work, it can be interpreted as the 217response time for the request. 218 219After reporting the total latency, timerlat will restart the cycle, arm 220a timer, and go to sleep for the following activation. 221 222If at any time one of the conditions is broken, e.g., the thread migrates 223while in user space, or the timerlat tracer is disabled, the SIG_KILL 224signal will be sent to the user-space thread. 225 226Here is an basic example of user-space code for timerlat:: 227 228 int main(void) 229 { 230 char buffer[1024]; 231 int timerlat_fd; 232 int retval; 233 long cpu = 0; /* place in CPU 0 */ 234 cpu_set_t set; 235 236 CPU_ZERO(&set); 237 CPU_SET(cpu, &set); 238 239 if (sched_setaffinity(gettid(), sizeof(set), &set) == -1) 240 return 1; 241 242 snprintf(buffer, sizeof(buffer), 243 "/sys/kernel/tracing/osnoise/per_cpu/cpu%ld/timerlat_fd", 244 cpu); 245 246 timerlat_fd = open(buffer, O_RDONLY); 247 if (timerlat_fd < 0) { 248 printf("error opening %s: %s\n", buffer, strerror(errno)); 249 exit(1); 250 } 251 252 for (;;) { 253 retval = read(timerlat_fd, buffer, 1024); 254 if (retval < 0) 255 break; 256 } 257 258 close(timerlat_fd); 259 exit(0); 260 } 261