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Date:   Wed, 12 Aug 2020 17:33:54 +0200
From:   Christian König <ckoenig.leichtzumerken@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     christian.koenig@....com, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Shashank.Sharma@....com, daniel.vetter@...ll.ch,
        alexander.deucher@....com
Subject: Re: [PATCH] drm/amdgpu: adjust the pid in the grab_id trace point

Am 12.08.20 um 17:19 schrieb Steven Rostedt:
> On Wed, 12 Aug 2020 16:36:36 +0200
> Christian König <ckoenig.leichtzumerken@...il.com> wrote:
>
>> Am 12.08.20 um 16:17 schrieb Steven Rostedt:
>>> On Fri, Aug 07, 2020 at 03:36:58PM +0200, Christian König wrote:
>>>> Trace something useful instead of the pid of a kernel thread here.
>>>>
>>>> Signed-off-by: Christian König <christian.koenig@....com>
>>>> ---
>>>>    drivers/gpu/drm/amd/amdgpu/amdgpu_trace.h | 1 +
>>>>    1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_trace.h b/drivers/gpu/drm/amd/amdgpu/amdgpu_trace.h
>>>> index 5da20fc166d9..07f99ef69d91 100644
>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_trace.h
>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_trace.h
>>>> @@ -228,6 +228,7 @@ TRACE_EVENT(amdgpu_vm_grab_id,
>>>>    			     ),
>>>>    
>>>>    	    TP_fast_assign(
>>>> +			   __entry->ent.pid = vm->task_info.pid;
>>> If the ent.pid is not the pid you are interested in for this trace event, just
>>> add a "pid" field to the trace event and place it there. Do not modify the
>>> generic pid that is recorded, as we would like that to be consistent for all
>>> trace events.
>> The problem my userspace guys have is that this doesn't work with
>> "trace-cmd -P $pid".
>>
>> But I think I can teach them how filters work :)
> Yep, trace-cmd record -e event -f "pid == $pid"
>
>>> The "ent.pid" turns into "common_pid" in the field, leaving "pid" free to use.
>>> Other trace events (like sched_waking) record a pid field that is not the same
>>> as the pid of the executing task.
>> Yes, we thought about this alternative as well.
>>
>>> The "ent.pid" should always be the pid of the task that executed the event.
>> Why? For the case here we just execute a work item in the background for
>> an userspace process.
>>
>> Tracing the pid of the worker pool which executes it doesn't seem to
>> make to much sense.
> Maybe not for you, but it does for me. All trace events show what
> happened when it happened and who executed it. I like to see what
> worker threads are executing. I may filter on the worker thread, and by
> changing the ent.pid, I wont see what it is doing.

That's enough explanation for me. Going with the separate pid field then.

Thanks,
Christian.

>
> That said, I think I may add a feature to a trace evnt for a special filter
> to say, "test field to the set_event_pid", and if it exists in that
> file to include that event in the filtered trace. This would include
> sched_waking trace events as well.
>
> That way "trace-cmd record -P $pid" will still work for your case.
>
> -- Steve

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