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Date:   Wed, 12 Aug 2020 11:19:20 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Christian =?UTF-8?B?S8O2bmln?= <ckoenig.leichtzumerken@gmail.com>
Cc:     christian.koenig@amd.com, amd-gfx@lists.freedesktop.org,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        Shashank.Sharma@amd.com, daniel.vetter@ffwll.ch,
        alexander.deucher@amd.com
Subject: Re: [PATCH] drm/amdgpu: adjust the pid in the grab_id trace point
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        <20200812141755.GA8136@home.goodmis.org>
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On Wed, 12 Aug 2020 16:36:36 +0200
Christian König <ckoenig.leichtzumerken@gmail.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@amd.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 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