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

[Lionel Landwerlin]

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'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(-)
>