Re: [RFC PATCH 1/5] tracing/hwlat: Add a cpus file specific for hwlat_detector

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 15 Apr 2021 15:09:50 +0200
Daniel Bristot de Oliveira <bristot@redhat.com> wrote:

> But for the osnoise tracer the cpus file is really useful. For instance, on a 
> system with the CPU 7 isolated:
> 
> ----- %< -----
>  # echo 7 > osnoise/cpus
>  # echo target_cpu == 7 > events/sched/sched_wakeup/filter 
>  # echo stacktrace if target_cpu == 7 > events/sched/sched_wakeup/trigger
>  # echo 1 > events/sched/sched_wakeup/enable
>  # echo osnoise:thread_noise > set_event 
>  # echo osnoise > current_tracer
>  # cat trace 
>     [find...]
>      kworker/0:1-7       [000] d..5  1820.717780: <stack trace>
>  => trace_event_raw_event_sched_wakeup_template
>  => __traceiter_sched_wakeup
>  => ttwu_do_wakeup
>  => try_to_wake_up
>  => __queue_work
>  => queue_delayed_work_on
>  => vmstat_shepherd
>  => process_one_work
>  => worker_thread
>  => kthread
>  => ret_from_fork  
>      kworker/7:1-410     [007] d..3  1820.717790: thread_noise: kworker/7:1:410 start 1820.717786519 duration 3626 ns
>        osnoise/7-1000    [007] ....  1821.582340: 1000000         90  99.99100      15      1      0     12      6      1
> ----- >% -----  
> 
> It was possible to easily find that the '1' thread noise was a kworker,
> dispatched from CPU 0, and that it was dispatched by "vmstat_shepherd".
> 
> Also, the osnoise dir is not added to a new instance... so, it only
> costs "one" file...

Every file counts. ;-)

What you did not articulate well, is that you want the other trace points
to be traced on all CPUs (maybe) when the osnoise threads are on a few (or
vice versa).

OK, for osnoise, I can see how it is useful. But as you said above, for
hwlat tracer, it's not as useful.

-- Steve