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Date:   Thu, 15 Apr 2021 09:49:24 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Daniel Bristot de Oliveira <bristot@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kcarcia@redhat.com,
        Jonathan Corbet <corbet@lwn.net>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        Clark Willaims <williams@redhat.com>,
        John Kacur <jkacur@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>, linux-doc@vger.kernel.org
Subject: Re: [RFC PATCH 1/5] tracing/hwlat: Add a cpus file specific for
 hwlat_detector
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        <e5547e9a-d3d8-2cd1-7cb9-e567c798e78d@redhat.com>
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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