====================
rtla-timerlat-top
====================
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Measures the operating system timer latency
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:Manual section: 1
SYNOPSIS
========
**rtla timerlat top** [*OPTIONS*] ...
DESCRIPTION
===========
.. include:: common_timerlat_description.rst
The **rtla timerlat top** displays a summary of the periodic output
from the *timerlat* tracer. It also provides information for each
operating system noise via the **osnoise:** tracepoints that can be
seem with the option **-T**.
OPTIONS
=======
.. include:: common_timerlat_options.rst
.. include:: common_top_options.rst
.. include:: common_options.rst
EXAMPLE
=======
In the example below, the *timerlat* tracer is set to capture the stack trace at
the IRQ handler, printing it to the buffer if the *Thread* timer latency is
higher than *30 us*. It is also set to stop the session if a *Thread* timer
latency higher than *30 us* is hit. Finally, it is set to save the trace
buffer if the stop condition is hit::
[root@alien ~]# rtla timerlat top -s 30 -T 30 -t
Timer Latency
0 00:00:59 | IRQ Timer Latency (us) | Thread Timer Latency (us)
CPU COUNT | cur min avg max | cur min avg max
0 #58634 | 1 0 1 10 | 11 2 10 23
1 #58634 | 1 0 1 9 | 12 2 9 23
2 #58634 | 0 0 1 11 | 10 2 9 23
3 #58634 | 1 0 1 11 | 11 2 9 24
4 #58634 | 1 0 1 10 | 11 2 9 26
5 #58634 | 1 0 1 8 | 10 2 9 25
6 #58634 | 12 0 1 12 | 30 2 10 30 <--- CPU with spike
7 #58634 | 1 0 1 9 | 11 2 9 23
8 #58633 | 1 0 1 9 | 11 2 9 26
9 #58633 | 1 0 1 9 | 10 2 9 26
10 #58633 | 1 0 1 13 | 11 2 9 28
11 #58633 | 1 0 1 13 | 12 2 9 24
12 #58633 | 1 0 1 8 | 10 2 9 23
13 #58633 | 1 0 1 10 | 10 2 9 22
14 #58633 | 1 0 1 18 | 12 2 9 27
15 #58633 | 1 0 1 10 | 11 2 9 28
16 #58633 | 0 0 1 11 | 7 2 9 26
17 #58633 | 1 0 1 13 | 10 2 9 24
18 #58633 | 1 0 1 9 | 13 2 9 22
19 #58633 | 1 0 1 10 | 11 2 9 23
20 #58633 | 1 0 1 12 | 11 2 9 28
21 #58633 | 1 0 1 14 | 11 2 9 24
22 #58633 | 1 0 1 8 | 11 2 9 22
23 #58633 | 1 0 1 10 | 11 2 9 27
timerlat hit stop tracing
saving trace to timerlat_trace.txt
[root@alien bristot]# tail -60 timerlat_trace.txt
[...]
timerlat/5-79755 [005] ....... 426.271226: #58634 context thread timer_latency 10823 ns
sh-109404 [006] dnLh213 426.271247: #58634 context irq timer_latency 12505 ns
sh-109404 [006] dNLh313 426.271258: irq_noise: local_timer:236 start 426.271245463 duration 12553 ns
sh-109404 [006] d...313 426.271263: thread_noise: sh:109404 start 426.271245853 duration 4769 ns
timerlat/6-79756 [006] ....... 426.271264: #58634 context thread timer_latency 30328 ns
timerlat/6-79756 [006] ....1.. 426.271265: <stack trace>
=> timerlat_irq
=> __hrtimer_run_queues
=> hrtimer_interrupt
=> __sysvec_apic_timer_interrupt
=> sysvec_apic_timer_interrupt
=> asm_sysvec_apic_timer_interrupt
=> _raw_spin_unlock_irqrestore <---- spinlock that disabled interrupt.
=> try_to_wake_up
=> autoremove_wake_function
=> __wake_up_common
=> __wake_up_common_lock
=> ep_poll_callback
=> __wake_up_common
=> __wake_up_common_lock
=> fsnotify_add_event
=> inotify_handle_inode_event
=> fsnotify
=> __fsnotify_parent
=> __fput
=> task_work_run
=> exit_to_user_mode_prepare
=> syscall_exit_to_user_mode
=> do_syscall_64
=> entry_SYSCALL_64_after_hwframe
=> 0x7265000001378c
=> 0x10000cea7
=> 0x25a00000204a
=> 0x12e302d00000000
=> 0x19b51010901b6
=> 0x283ce00726500
=> 0x61ea308872
=> 0x00000fe3
bash-109109 [007] d..h... 426.271265: #58634 context irq timer_latency 1211 ns
timerlat/6-79756 [006] ....... 426.271267: timerlat_main: stop tracing hit on cpu 6
In the trace, it is possible the notice that the *IRQ* timer latency was
already high, accounting *12505 ns*. The IRQ delay was caused by the
*bash-109109* process that disabled IRQs in the wake-up path
(*_try_to_wake_up()* function). The duration of the IRQ handler that woke
up the timerlat thread, informed with the **osnoise:irq_noise** event, was
also high and added more *12553 ns* to the Thread latency. Finally, the
**osnoise:thread_noise** added by the currently running thread (including
the scheduling overhead) added more *4769 ns*. Summing up these values,
the *Thread* timer latency accounted for *30328 ns*.
The primary reason for this high value is the wake-up path that was hit
twice during this case: when the *bash-109109* was waking up a thread
and then when the *timerlat* thread was awakened. This information can
then be used as the starting point of a more fine-grained analysis.
Note that **rtla timerlat** was dispatched without changing *timerlat* tracer
threads' priority. That is generally not needed because these threads hava
priority *FIFO:95* by default, which is a common priority used by real-time
kernel developers to analyze scheduling delays.
SEE ALSO
--------
**rtla-timerlat**\(1), **rtla-timerlat-hist**\(1)
*timerlat* tracer documentation: <https://www.kernel.org/doc/html/latest/trace/timerlat-tracer.html>
AUTHOR
------
Written by Daniel Bristot de Oliveira <bristot@kernel.org>
.. include:: common_appendix.rst