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

[Peter Zijlstra <peterz@infradead.org>]

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..