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@xxxxxxxxx> Explaination of increased wait period makes sense to me. With that: Reviewed-by: Matthew Brost <matthew.brost@xxxxxxxxx> > --- > 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 >
