Re: [Intel-gfx] [PATCH] drm/i915/selftest: Bump up sample period for busy stats selftest

[Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>]

On Thu, Nov 03, 2022 at 12:28:46PM +0000, Tvrtko Ursulin wrote:
>
>On 03/11/2022 00:11, Umesh Nerlige Ramappa wrote:
>>Engine busyness samples around a 10ms period is failing with busyness
>>ranging approx. from 87% to 115%. The expected range is +/- 5% of the
>>sample period.
>>
>>When determining busyness of active engine, the GuC based engine
>>busyness implementation relies on a 64 bit timestamp register read. The
>>latency incurred by this register read causes the failure.
>>
>>On DG1, when the test fails, the observed latencies range from 900us -
>>1.5ms.
>
>Do I read this right - that the latency of a 64 bit timestamp register 
>read is 0.9 - 1.5ms? That would be the read in 
>guc_update_pm_timestamp?

Correct. That is total time taken by intel_uncore_read64_2x32() measured 
with local_clock().

One other thing I missed out in the comments is that enable_dc=0 also 
resolves the issue, but display team confirmed there is no relation to 
display in this case other than that it somehow introduces a latency in 
the reg read.

>
>Regards,
>
>Tvrtko
>
>>One solution tried was to reduce the latency between reg read and
>>CPU timestamp capture, but such optimization does not add value to user
>>since the CPU timestamp obtained here is only used for (1) selftest and
>>(2) i915 rps implementation specific to execlist scheduler. Also, this
>>solution only reduces the frequency of failure and does not eliminate
>>it.

Note that this solution is here - 
https://patchwork.freedesktop.org/patch/509991/?series=110497&rev=1

but I am not intending to use it since it just reduces the frequency of 
failues, but the inherent issue still exists.

Regards,
Umesh

>>
>>In order to make the selftest more robust and account for such
>>latencies, increase the sample period to 100 ms.
>>
>>Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@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 0dcb3ed44a73..87c94314cf67 100644
>>--- a/drivers/gpu/drm/i915/gt/selftest_engine_pm.c
>>+++ b/drivers/gpu/drm/i915/gt/selftest_engine_pm.c
>>@@ -317,7 +317,7 @@ 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]);
>>-		mdelay(10);
>>+		mdelay(100);
>>  		de = ktime_sub(intel_engine_get_busy_time(engine, &t[1]), de);
>>  		preempt_enable();
>>  		dt = ktime_sub(t[1], t[0]);