1 ############### 1 ###############
2 Timerlat tracer 2 Timerlat tracer
3 ############### 3 ###############
4 4
5 The timerlat tracer aims to help the preemptiv 5 The timerlat tracer aims to help the preemptive kernel developers to
6 find sources of wakeup latencies of real-time 6 find sources of wakeup latencies of real-time threads. Like cyclictest,
7 the tracer sets a periodic timer that wakes up 7 the tracer sets a periodic timer that wakes up a thread. The thread then
8 computes a *wakeup latency* value as the diffe 8 computes a *wakeup latency* value as the difference between the *current
9 time* and the *absolute time* that the timer w 9 time* and the *absolute time* that the timer was set to expire. The main
10 goal of timerlat is tracing in such a way to h 10 goal of timerlat is tracing in such a way to help kernel developers.
11 11
12 Usage 12 Usage
13 ----- 13 -----
14 14
15 Write the ASCII text "timerlat" into the curre 15 Write the ASCII text "timerlat" into the current_tracer file of the
16 tracing system (generally mounted at /sys/kern 16 tracing system (generally mounted at /sys/kernel/tracing).
17 17
18 For example:: 18 For example::
19 19
20 [root@f32 ~]# cd /sys/kernel/tracing/ 20 [root@f32 ~]# cd /sys/kernel/tracing/
21 [root@f32 tracing]# echo timerlat > cu 21 [root@f32 tracing]# echo timerlat > current_tracer
22 22
23 It is possible to follow the trace by reading !! 23 It is possible to follow the trace by reading the trace trace file::
24 24
25 [root@f32 tracing]# cat trace 25 [root@f32 tracing]# cat trace
26 # tracer: timerlat 26 # tracer: timerlat
27 # 27 #
28 # _-----=> irqs 28 # _-----=> irqs-off
29 # / _----=> need 29 # / _----=> need-resched
30 # | / _---=> hard 30 # | / _---=> hardirq/softirq
31 # || / _--=> pree 31 # || / _--=> preempt-depth
32 # || / 32 # || /
33 # |||| 33 # |||| ACTIVATION
34 # TASK-PID CPU# |||| TIMESTAM 34 # TASK-PID CPU# |||| TIMESTAMP ID CONTEXT LATENCY
35 # | | | |||| | 35 # | | | |||| | | | |
36 <idle>-0 [000] d.h1 54.0293 36 <idle>-0 [000] d.h1 54.029328: #1 context irq timer_latency 932 ns
37 <...>-867 [000] .... 54.0293 37 <...>-867 [000] .... 54.029339: #1 context thread timer_latency 11700 ns
38 <idle>-0 [001] dNh1 54.0293 38 <idle>-0 [001] dNh1 54.029346: #1 context irq timer_latency 2833 ns
39 <...>-868 [001] .... 54.0293 39 <...>-868 [001] .... 54.029353: #1 context thread timer_latency 9820 ns
40 <idle>-0 [000] d.h1 54.0303 40 <idle>-0 [000] d.h1 54.030328: #2 context irq timer_latency 769 ns
41 <...>-867 [000] .... 54.0303 41 <...>-867 [000] .... 54.030330: #2 context thread timer_latency 3070 ns
42 <idle>-0 [001] d.h1 54.0303 42 <idle>-0 [001] d.h1 54.030344: #2 context irq timer_latency 935 ns
43 <...>-868 [001] .... 54.0303 43 <...>-868 [001] .... 54.030347: #2 context thread timer_latency 4351 ns
44 44
45 45
46 The tracer creates a per-cpu kernel thread wit 46 The tracer creates a per-cpu kernel thread with real-time priority that
47 prints two lines at every activation. The firs 47 prints two lines at every activation. The first is the *timer latency*
48 observed at the *hardirq* context before the a 48 observed at the *hardirq* context before the activation of the thread.
49 The second is the *timer latency* observed by 49 The second is the *timer latency* observed by the thread. The ACTIVATION
50 ID field serves to relate the *irq* execution 50 ID field serves to relate the *irq* execution to its respective *thread*
51 execution. 51 execution.
52 52
53 The *irq*/*thread* splitting is important to c 53 The *irq*/*thread* splitting is important to clarify in which context
54 the unexpected high value is coming from. The 54 the unexpected high value is coming from. The *irq* context can be
55 delayed by hardware-related actions, such as S 55 delayed by hardware-related actions, such as SMIs, NMIs, IRQs,
56 or by thread masking interrupts. Once the time 56 or by thread masking interrupts. Once the timer happens, the delay
57 can also be influenced by blocking caused by t 57 can also be influenced by blocking caused by threads. For example, by
58 postponing the scheduler execution via preempt 58 postponing the scheduler execution via preempt_disable(), scheduler
59 execution, or masking interrupts. Threads can 59 execution, or masking interrupts. Threads can also be delayed by the
60 interference from other threads and IRQs. 60 interference from other threads and IRQs.
61 61
62 Tracer options 62 Tracer options
63 --------------------- 63 ---------------------
64 64
65 The timerlat tracer is built on top of osnoise 65 The timerlat tracer is built on top of osnoise tracer.
66 So its configuration is also done in the osnoi 66 So its configuration is also done in the osnoise/ config
67 directory. The timerlat configs are: 67 directory. The timerlat configs are:
68 68
69 - cpus: CPUs at which a timerlat thread will 69 - cpus: CPUs at which a timerlat thread will execute.
70 - timerlat_period_us: the period of the timer 70 - timerlat_period_us: the period of the timerlat thread.
71 - stop_tracing_us: stop the system tracing if 71 - stop_tracing_us: stop the system tracing if a
72 timer latency at the *irq* context higher t 72 timer latency at the *irq* context higher than the configured
73 value happens. Writing 0 disables this opti 73 value happens. Writing 0 disables this option.
74 - stop_tracing_total_us: stop the system trac 74 - stop_tracing_total_us: stop the system tracing if a
75 timer latency at the *thread* context is hi 75 timer latency at the *thread* context is higher than the configured
76 value happens. Writing 0 disables this opti 76 value happens. Writing 0 disables this option.
77 - print_stack: save the stack of the IRQ occu 77 - print_stack: save the stack of the IRQ occurrence. The stack is printed
78 after the *thread context* event, or at the 78 after the *thread context* event, or at the IRQ handler if *stop_tracing_us*
79 is hit. 79 is hit.
80 80
81 timerlat and osnoise 81 timerlat and osnoise
82 ---------------------------- 82 ----------------------------
83 83
84 The timerlat can also take advantage of the os 84 The timerlat can also take advantage of the osnoise: traceevents.
85 For example:: 85 For example::
86 86
87 [root@f32 ~]# cd /sys/kernel/tracing/ 87 [root@f32 ~]# cd /sys/kernel/tracing/
88 [root@f32 tracing]# echo timerlat > cu 88 [root@f32 tracing]# echo timerlat > current_tracer
89 [root@f32 tracing]# echo 1 > events/os 89 [root@f32 tracing]# echo 1 > events/osnoise/enable
90 [root@f32 tracing]# echo 25 > osnoise/ 90 [root@f32 tracing]# echo 25 > osnoise/stop_tracing_total_us
91 [root@f32 tracing]# tail -10 trace 91 [root@f32 tracing]# tail -10 trace
92 cc1-87882 [005] d..h... 548.7 92 cc1-87882 [005] d..h... 548.771078: #402268 context irq timer_latency 13585 ns
93 cc1-87882 [005] dNLh1.. 548.7 93 cc1-87882 [005] dNLh1.. 548.771082: irq_noise: local_timer:236 start 548.771077442 duration 7597 ns
94 cc1-87882 [005] dNLh2.. 548.7 94 cc1-87882 [005] dNLh2.. 548.771099: irq_noise: qxl:21 start 548.771085017 duration 7139 ns
95 cc1-87882 [005] d...3.. 548.7 95 cc1-87882 [005] d...3.. 548.771102: thread_noise: cc1:87882 start 548.771078243 duration 9909 ns
96 timerlat/5-1035 [005] ....... 548.7 96 timerlat/5-1035 [005] ....... 548.771104: #402268 context thread timer_latency 39960 ns
97 97
98 In this case, the root cause of the timer late 98 In this case, the root cause of the timer latency does not point to a
99 single cause but to multiple ones. Firstly, th 99 single cause but to multiple ones. Firstly, the timer IRQ was delayed
100 for 13 us, which may point to a long IRQ disab 100 for 13 us, which may point to a long IRQ disabled section (see IRQ
101 stacktrace section). Then the timer interrupt 101 stacktrace section). Then the timer interrupt that wakes up the timerlat
102 thread took 7597 ns, and the qxl:21 device IRQ 102 thread took 7597 ns, and the qxl:21 device IRQ took 7139 ns. Finally,
103 the cc1 thread noise took 9909 ns of time befo 103 the cc1 thread noise took 9909 ns of time before the context switch.
104 Such pieces of evidence are useful for the dev 104 Such pieces of evidence are useful for the developer to use other
105 tracing methods to figure out how to debug and 105 tracing methods to figure out how to debug and optimize the system.
106 106
107 It is worth mentioning that the *duration* val 107 It is worth mentioning that the *duration* values reported
108 by the osnoise: events are *net* values. For e 108 by the osnoise: events are *net* values. For example, the
109 thread_noise does not include the duration of 109 thread_noise does not include the duration of the overhead caused
110 by the IRQ execution (which indeed accounted f 110 by the IRQ execution (which indeed accounted for 12736 ns). But
111 the values reported by the timerlat tracer (ti 111 the values reported by the timerlat tracer (timerlat_latency)
112 are *gross* values. 112 are *gross* values.
113 113
114 The art below illustrates a CPU timeline and h 114 The art below illustrates a CPU timeline and how the timerlat tracer
115 observes it at the top and the osnoise: events 115 observes it at the top and the osnoise: events at the bottom. Each "-"
116 in the timelines means circa 1 us, and the tim 116 in the timelines means circa 1 us, and the time moves ==>::
117 117
118 External timer irq 118 External timer irq thread
119 clock latency 119 clock latency latency
120 event 13585 ns 120 event 13585 ns 39960 ns
121 | ^ 121 | ^ ^
122 v | 122 v | |
123 |-------------| 123 |-------------| |
124 |-------------+---------------------- 124 |-------------+-------------------------|
125 ^ 125 ^ ^
126 ============================================ 126 ========================================================================
127 [tmr irq] [dev irq] 127 [tmr irq] [dev irq]
128 [another thread...^ v..^ v...... 128 [another thread...^ v..^ v.......][timerlat/ thread] <-- CPU timeline
129 ============================================ 129 =========================================================================
130 |-------| |-------| 130 |-------| |-------|
131 |--^ v------ 131 |--^ v-------|
132 | | 132 | | |
133 | | 133 | | + thread_noise: 9909 ns
134 | +-> irq 134 | +-> irq_noise: 6139 ns
135 +-> irq_noise: 759 135 +-> irq_noise: 7597 ns
136 136
137 IRQ stacktrace 137 IRQ stacktrace
138 --------------------------- 138 ---------------------------
139 139
140 The osnoise/print_stack option is helpful for 140 The osnoise/print_stack option is helpful for the cases in which a thread
141 noise causes the major factor for the timer la 141 noise causes the major factor for the timer latency, because of preempt or
142 irq disabled. For example:: 142 irq disabled. For example::
143 143
144 [root@f32 tracing]# echo 500 > osnoise 144 [root@f32 tracing]# echo 500 > osnoise/stop_tracing_total_us
145 [root@f32 tracing]# echo 500 > osnoise 145 [root@f32 tracing]# echo 500 > osnoise/print_stack
146 [root@f32 tracing]# echo timerlat > cu 146 [root@f32 tracing]# echo timerlat > current_tracer
147 [root@f32 tracing]# tail -21 per_cpu/c 147 [root@f32 tracing]# tail -21 per_cpu/cpu7/trace
148 insmod-1026 [007] dN.h1.. 200.2 148 insmod-1026 [007] dN.h1.. 200.201948: irq_noise: local_timer:236 start 200.201939376 duration 7872 ns
149 insmod-1026 [007] d..h1.. 200.2 149 insmod-1026 [007] d..h1.. 200.202587: #29800 context irq timer_latency 1616 ns
150 insmod-1026 [007] dN.h2.. 200.2 150 insmod-1026 [007] dN.h2.. 200.202598: irq_noise: local_timer:236 start 200.202586162 duration 11855 ns
151 insmod-1026 [007] dN.h3.. 200.2 151 insmod-1026 [007] dN.h3.. 200.202947: irq_noise: local_timer:236 start 200.202939174 duration 7318 ns
152 insmod-1026 [007] d...3.. 200.2 152 insmod-1026 [007] d...3.. 200.203444: thread_noise: insmod:1026 start 200.202586933 duration 838681 ns
153 timerlat/7-1001 [007] ....... 200.2 153 timerlat/7-1001 [007] ....... 200.203445: #29800 context thread timer_latency 859978 ns
154 timerlat/7-1001 [007] ....1.. 200.2 154 timerlat/7-1001 [007] ....1.. 200.203446: <stack trace>
155 => timerlat_irq 155 => timerlat_irq
156 => __hrtimer_run_queues 156 => __hrtimer_run_queues
157 => hrtimer_interrupt 157 => hrtimer_interrupt
158 => __sysvec_apic_timer_interrupt 158 => __sysvec_apic_timer_interrupt
159 => asm_call_irq_on_stack 159 => asm_call_irq_on_stack
160 => sysvec_apic_timer_interrupt 160 => sysvec_apic_timer_interrupt
161 => asm_sysvec_apic_timer_interrupt 161 => asm_sysvec_apic_timer_interrupt
162 => delay_tsc 162 => delay_tsc
163 => dummy_load_1ms_pd_init 163 => dummy_load_1ms_pd_init
164 => do_one_initcall 164 => do_one_initcall
165 => do_init_module 165 => do_init_module
166 => __do_sys_finit_module 166 => __do_sys_finit_module
167 => do_syscall_64 167 => do_syscall_64
168 => entry_SYSCALL_64_after_hwframe 168 => entry_SYSCALL_64_after_hwframe
169 169
170 In this case, it is possible to see that the t 170 In this case, it is possible to see that the thread added the highest
171 contribution to the *timer latency* and the st 171 contribution to the *timer latency* and the stack trace, saved during
172 the timerlat IRQ handler, points to a function 172 the timerlat IRQ handler, points to a function named
173 dummy_load_1ms_pd_init, which had the followin 173 dummy_load_1ms_pd_init, which had the following code (on purpose)::
174 174
175 static int __init dummy_load_1ms_pd_in 175 static int __init dummy_load_1ms_pd_init(void)
176 { 176 {
177 preempt_disable(); 177 preempt_disable();
178 mdelay(1); 178 mdelay(1);
179 preempt_enable(); 179 preempt_enable();
180 return 0; 180 return 0;
181 181
182 } 182 }
183 <<
184 User-space interface <<
185 --------------------------- <<
186 <<
187 Timerlat allows user-space threads to use time <<
188 measure scheduling latency. This interface is <<
189 file descriptor inside $tracing_dir/osnoise/pe <<
190 <<
191 This interface is accessible under the followi <<
192 <<
193 - timerlat tracer is enable <<
194 - osnoise workload option is set to NO_OSNOIS <<
195 - The user-space thread is affined to a singl <<
196 - The thread opens the file associated with i <<
197 - Only one thread can access the file at a ti <<
198 <<
199 The open() syscall will fail if any of these c <<
200 After opening the file descriptor, the user sp <<
201 <<
202 The read() system call will run a timerlat cod <<
203 timer in the future and wait for it as the reg <<
204 <<
205 When the timer IRQ fires, the timerlat IRQ wil <<
206 IRQ latency and wake up the thread waiting in <<
207 scheduled and report the thread latency via tr <<
208 thread. <<
209 <<
210 The difference from the in-kernel timerlat is <<
211 the timer, timerlat will return to the read() <<
212 the user can run any code. <<
213 <<
214 If the application rereads the file timerlat f <<
215 will report the return from user-space latency <<
216 latency. If this is the end of the work, it ca <<
217 response time for the request. <<
218 <<
219 After reporting the total latency, timerlat wi <<
220 a timer, and go to sleep for the following act <<
221 <<
222 If at any time one of the conditions is broken <<
223 while in user space, or the timerlat tracer is <<
224 signal will be sent to the user-space thread. <<
225 <<
226 Here is an basic example of user-space code fo <<
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 <<
240 return 1; <<
241 <<
242 snprintf(buffer, sizeof(buffer), <<
243 "/sys/kernel/tracing/osnoise/p <<
244 cpu); <<
245 <<
246 timerlat_fd = open(buffer, O_RDONLY); <<
247 if (timerlat_fd < 0) { <<
248 printf("error opening %s: %s\n <<
249 exit(1); <<
250 } <<
251 <<
252 for (;;) { <<
253 retval = read(timerlat_fd, buf <<
254 if (retval < 0) <<
255 break; <<
256 } <<
257 <<
258 close(timerlat_fd); <<
259 exit(0); <<
260 } <<
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