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@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 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]);
