Re: [Intel-gfx] [PATCH] drm/i915/pmu: Increase the live_engine_busy_stats sample period

[Matthew Brost <matthew.brost@intel.com>]

On Thu, Nov 11, 2021 at 06:52:22PM -0800, Umesh Nerlige Ramappa wrote:
> Irrespective of the backend for request submissions, busyness for an
> engine with an active context is calculated using:
> 
> busyness = total + (current_time - context_switch_in_time)
> 
> In execlists mode of operation, the context switch events are handled
> by the CPU. Context switch in/out time and current_time are captured
> in CPU time domain using ktime_get().
> 
> In GuC mode of submission, context switch events are handled by GuC and
> the times in the above formula are captured in GT clock domain. This
> information is shared with the CPU through shared memory. This results
> in 2 caveats:
> 
> 1) The time taken between start of a batch and the time that CPU is able
> to see the context_switch_in_time in shared memory is dependent on GuC
> and memory bandwidth constraints.
> 
> 2) Determining current_time requires an MMIO read that can take anywhere
> between a few us to a couple ms. A reference CPU time is captured soon
> after reading the MMIO so that the caller can compare the cpu delta
> between 2 busyness samples. The issue here is that the CPU delta and the
> busyness delta can be skewed because of the time taken to read the
> register.
> 
> These 2 factors affect the accuracy of the selftest -
> live_engine_busy_stats. For (1) the selftest waits until busyness stats
> are visible to the CPU. The effects of (2) are more prominent for the
> current busyness sample period of 100 us. Increase the busyness sample
> period from 100 us to 10 ms to overccome (2).
> 
> Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>

Explaination of increased wait period makes sense to me.

With that:
Reviewed-by: Matthew Brost <matthew.brost@intel.com>

> ---
>  drivers/gpu/drm/i915/gt/selftest_engine_pm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/selftest_engine_pm.c b/drivers/gpu/drm/i915/gt/selftest_engine_pm.c
> index 0bfd738dbf3a..96cc565afa78 100644
> --- a/drivers/gpu/drm/i915/gt/selftest_engine_pm.c
> +++ b/drivers/gpu/drm/i915/gt/selftest_engine_pm.c
> @@ -316,7 +316,7 @@ static int live_engine_busy_stats(void *arg)
>  		ENGINE_TRACE(engine, "measuring busy time\n");
>  		preempt_disable();
>  		de = intel_engine_get_busy_time(engine, &t[0]);
> -		udelay(100);
> +		udelay(10000);
>  		de = ktime_sub(intel_engine_get_busy_time(engine, &t[1]), de);
>  		preempt_enable();
>  		dt = ktime_sub(t[1], t[0]);
> -- 
> 2.20.1
>