====================
rtla-timerlat-top
====================
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Measures the operating system timer latency
-------------------------------------------

: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
