====================
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
.. include:: common_timerlat_aa.rst
**--aa-only** *us*
Set stop tracing conditions and run without collecting and displaying statistics.
Print the auto-analysis if the system hits the stop tracing condition. This option
is useful to reduce rtla timerlat CPU, enabling the debug without the overhead of
collecting the statistics.
EXAMPLE
=======
In the example below, the timerlat tracer is dispatched in cpus *1-23* in the
automatic trace mode, instructing the tracer to stop if a *40 us* latency or
higher is found::
# timerlat -a 40 -c 1-23 -q
Timer Latency
0 00:00:12 | IRQ Timer Latency (us) | Thread Timer Latency (us)
CPU COUNT | cur min avg max | cur min avg max
1 #12322 | 0 0 1 15 | 10 3 9 31
2 #12322 | 3 0 1 12 | 10 3 9 23
3 #12322 | 1 0 1 21 | 8 2 8 34
4 #12322 | 1 0 1 17 | 10 2 11 33
5 #12322 | 0 0 1 12 | 8 3 8 25
6 #12322 | 1 0 1 14 | 16 3 11 35
7 #12322 | 0 0 1 14 | 9 2 8 29
8 #12322 | 1 0 1 22 | 9 3 9 34
9 #12322 | 0 0 1 14 | 8 2 8 24
10 #12322 | 1 0 0 12 | 9 3 8 24
11 #12322 | 0 0 0 15 | 6 2 7 29
12 #12321 | 1 0 0 13 | 5 3 8 23
13 #12319 | 0 0 1 14 | 9 3 9 26
14 #12321 | 1 0 0 13 | 6 2 8 24
15 #12321 | 1 0 1 15 | 12 3 11 27
16 #12318 | 0 0 1 13 | 7 3 10 24
17 #12319 | 0 0 1 13 | 11 3 9 25
18 #12318 | 0 0 0 12 | 8 2 8 20
19 #12319 | 0 0 1 18 | 10 2 9 28
20 #12317 | 0 0 0 20 | 9 3 8 34
21 #12318 | 0 0 0 13 | 8 3 8 28
22 #12319 | 0 0 1 11 | 8 3 10 22
23 #12320 | 28 0 1 28 | 41 3 11 41
rtla timerlat hit stop tracing
## CPU 23 hit stop tracing, analyzing it ##
IRQ handler delay: 27.49 us (65.52 %)
IRQ latency: 28.13 us
Timerlat IRQ duration: 9.59 us (22.85 %)
Blocking thread: 3.79 us (9.03 %)
objtool:49256 3.79 us
Blocking thread stacktrace
-> timerlat_irq
-> __hrtimer_run_queues
-> hrtimer_interrupt
-> __sysvec_apic_timer_interrupt
-> sysvec_apic_timer_interrupt
-> asm_sysvec_apic_timer_interrupt
-> _raw_spin_unlock_irqrestore
-> cgroup_rstat_flush_locked
-> cgroup_rstat_flush_irqsafe
-> mem_cgroup_flush_stats
-> mem_cgroup_wb_stats
-> balance_dirty_pages
-> balance_dirty_pages_ratelimited_flags
-> btrfs_buffered_write
-> btrfs_do_write_iter
-> vfs_write
-> __x64_sys_pwrite64
-> do_syscall_64
-> entry_SYSCALL_64_after_hwframe
------------------------------------------------------------------------
Thread latency: 41.96 us (100%)
The system has exit from idle latency!
Max timerlat IRQ latency from idle: 17.48 us in cpu 4
Saving trace to timerlat_trace.txt
In this case, the major factor was the delay suffered by the *IRQ handler*
that handles **timerlat** wakeup: *65.52%*. This can be caused by the
current thread masking interrupts, which can be seen in the blocking
thread stacktrace: the current thread (*objtool:49256*) disabled interrupts
via *raw spin lock* operations inside mem cgroup, while doing write
syscall in a btrfs file system.
The raw trace is saved in the **timerlat_trace.txt** file for further analysis.
Note that **rtla timerlat** was dispatched without changing *timerlat* tracer
threads' priority. That is generally not needed because these threads have
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 <[email protected]>
.. include:: common_appendix.rst
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