linux-kernel - Re: [RFC PATCH 0/3] Expose gpu counters via perf pmu driver

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Message-ID: <20141030190841.GI23531@worktop.programming.kicks-ass.net>
Date:	Thu, 30 Oct 2014 20:08:41 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Robert Bragg <robert@...bynine.org>
Cc:	linux-kernel@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Chris Wilson <chris@...is-wilson.co.uk>,
	Rob Clark <robdclark@...il.com>,
	Samuel Pitoiset <samuel.pitoiset@...il.com>,
	Ben Skeggs <bskeggs@...hat.com>
Subject: Re: [RFC PATCH 0/3] Expose gpu counters via perf pmu driver

On Wed, Oct 22, 2014 at 04:28:48PM +0100, Robert Bragg wrote:
> Our desired permission model seems consistent with perf's current model
> whereby you would need privileges if you want to profile across all gpu
> contexts but not need special permissions to profile your own context.
> 
> The awkward part is that it doesn't make sense for us to have userspace
> open a perf event with a specific pid as the way to avoid needing root
> permissions because a side effect of doing this is that the events will
> be dynamically added/deleted so as to only monitor while that process is
> scheduled and that's not really meaningful when we're monitoring the
> gpu.

There is precedent in PERF_FLAG_PID_CGROUP to replace the pid argument
with a fd to your object.

And do I take it right that if you're able/allowed/etc.. to open/have
the fd to the GPU/DRM/DRI whatever context you have the right
credentials to also observe these counters?

> Conceptually I suppose we want to be able to open an event that's not
> associated with any cpu or process, but to keep things simple and fit
> with perf's current design, the pmu I have a.t.m expects an event to be
> opened for a specific cpu and unspecified process.

There are no actual scheduling ramifications right? Let me ponder his
for a little while more..
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