Messages in this thread |  | | Date | Sun, 16 Nov 2014 10:27:37 +0100 | From | Ingo Molnar <> | Subject | Re: [RFC PATCH 0/3] Expose gpu counters via perf pmu driver |
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* 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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