From d629e1fafbca27951e7be2ff3f2587b27b22f862 Mon Sep 17 00:00:00 2001
From: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Date: Mon, 15 Nov 2021 14:16:40 -0800
Subject: drm/i915/pmu: Increase the live_engine_busy_stats sample period
Git-commit: 5979873ebbb57b72a8f88ba26d3bd405981cd844
Patch-mainline: v5.17-rc1
References: jsc#PED-1166 jsc#PED-1168 jsc#PED-1170 jsc#PED-1218 jsc#PED-1220 jsc#PED-1222 jsc#PED-1223 jsc#PED-1225
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).
v2: Fix checkpatch issues
Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211115221640.30793-1-umesh.nerlige.ramappa@intel.com
Acked-by: Patrik Jakobsson <pjakobsson@suse.de>
---
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..75f6efc9882f 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);
+ mdelay(10);
de = ktime_sub(intel_engine_get_busy_time(engine, &t[1]), de);
preempt_enable();
dt = ktime_sub(t[1], t[0]);
--
2.38.1