[Intel-gfx] [RFC 0/4] GPU/CPU timestamps correlation for relating OA samples with system events

[Robert Bragg]

On Tue, Dec 5, 2017 at 2:16 PM, Lionel Landwerlin <
lionel.g.landwerlin at intel.com> wrote:

> Hey Sagar,
>
> Sorry for the delay looking into this series.
> I've done some userspace/UI work in GPUTop to try to correlate perf
> samples/tracepoints with i915 perf reports.
>
> I wanted to avoid having to add too much logic into the kernel and tried
> to sample both cpu clocks & gpu timestamps from userspace.
> So far that's not working. People more knowledgable than I would have
> realized that the kernel can sneak in work into syscalls.
> So result is that 2 syscalls (one to get the cpu clock, one for the gpu
> timestamp) back to back from the same thread leads to time differences of
> anywhere from a few microseconds to in some cases close to 1millisecond. So
> it's basically unworkable.
> Anyway the UI work won't go to waste :)
>
> I'm thinking to go with your approach.
> From my experiment with gputop, it seems we might want to use a different
> cpu clock source though or make it configurable.
> The perf infrastructure allows you to choose what clock you want to use.
> Since we want to avoid time adjustments on that clock (because we're adding
> deltas), a clock monotonic raw would make most sense.
>

I would guess the most generally useful clock domain to correlate with the
largest number of interesting events would surely be CLOCK_MONOTONIC, not
_MONOTONIC_RAW.

E.g. here's some discussion around why vblank events use CLOCK_MONOTINIC:
https://lists.freedesktop.org/archives/dri-devel/2012-October/028878.html

Br,
- Robert


> I'll look at adding some tests for this too.
>
> Thanks,
>
> -
> Lionel
>
> On 15/11/17 12:13, Sagar Arun Kamble wrote:
>
>> We can compute system time corresponding to GPU timestamp by taking a
>> reference point (CPU monotonic time, GPU timestamp) and then adding
>> delta time computed using timecounter/cyclecounter support in kernel.
>> We have to configure cyclecounter with the GPU timestamp frequency.
>> Earlier approach that was based on cross-timestamp is not needed. It
>> was being used to approximate the frequency based on invalid assumptions
>> (possibly drift was being seen in the time due to precision issue).
>> The precision of time from GPU clocks is already in ns and timecounter
>> takes care of it as verified over variable durations.
>>
>> This series adds base timecounter/cyclecounter changes and changes to
>> get GPU and CPU timestamps in OA samples.
>>
>> Sagar Arun Kamble (1):
>>    drm/i915/perf: Add support to correlate GPU timestamp with system time
>>
>> Sourab Gupta (3):
>>    drm/i915/perf: Add support for collecting 64 bit timestamps with OA
>>      reports
>>    drm/i915/perf: Extract raw GPU timestamps from OA reports
>>    drm/i915/perf: Send system clock monotonic time in perf samples
>>
>>   drivers/gpu/drm/i915/i915_drv.h  |  11 ++++
>>   drivers/gpu/drm/i915/i915_perf.c | 124 ++++++++++++++++++++++++++++++
>> ++++++++-
>>   drivers/gpu/drm/i915/i915_reg.h  |   6 ++
>>   include/uapi/drm/i915_drm.h      |  14 +++++
>>   4 files changed, 154 insertions(+), 1 deletion(-)
>>
>>
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