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

* Robert Bragg <robert@sixbynine.org> wrote:

> > I'd strong[ly] suggest thinking about sampling as well, if
> > the hardware exposes sample information: at least for
> > profiling CPU loads the difference is like day and night,
> > compared to aggregated counts and self-profiling.
>
> Here I was thinking of counters or data that can be sampled via
> mmio using a hrtimer. E.g. the current gpu frequency or the
> energy usage. I'm not currently aware of any capability for the
> gpu to say trigger an interrupt after a threshold number of
> events occurs (like clock cycles) so I think we may generally
> be limited to a wall clock time domain for sampling.

In general hrtimer-driven polling gives pretty good profiling
information as well - key is to be able to get a sample of EU
thread execution state.

(Trigger thresholds and so can be useful as well, but are a
second order concern in terms of profiling quality.)

> > It's a very good idea to not expose such limitations to
> > user-space - the GPU driver doing the necessary hrtimer
> > polling to construct a proper count is a much higher quality
> > solution.
>
> That sounds preferable.
>
> I'm open to suggestions for finding another way for userspace
> to initiate a flush besides through read() in case there's a
> concern that might be set a bad precedent. For the i915_oa
> driver it seems ok at the moment since we don't currently
> report a useful counter through read() and for the main use
> case where we want the flushing we expect that most of the time
> there won't be any significant cost involved in flushing since
> we'll be using a very low timer period. Maybe this will bite us
> later though.

You could add an ioctl() as well - we are not religious about
them, there's always things that are special enough to not
warrant a generic syscall.

Anyway, aggregate counts alone are obviously very useful to
analyzing GPU performance, so your initial approach looks
perfectly acceptable to me already.

Thanks,

Ingo


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