[Intel-gfx] [PATCH] drm/i915/selftests: Bump the scheduling threshold for fast heartbeats

[Chris Wilson <chris@chris-wilson.co.uk>]

Since we are system_highpri_wq, we expected the heartbeat to be
scheduled promptly. However, we see delays of over 10ms upsetting our
assertions. Accept this as inevitable and bump the error threshold to
20ms (from 6ms).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/gt/selftest_engine_heartbeat.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/selftest_engine_heartbeat.c b/drivers/gpu/drm/i915/gt/selftest_engine_heartbeat.c
index b88aa35ad75b..807c445f7c70 100644
--- a/drivers/gpu/drm/i915/gt/selftest_engine_heartbeat.c
+++ b/drivers/gpu/drm/i915/gt/selftest_engine_heartbeat.c
@@ -197,6 +197,7 @@ static int cmp_u32(const void *_a, const void *_b)
 
 static int __live_heartbeat_fast(struct intel_engine_cs *engine)
 {
+	const int error_threshold = 20000; /* microseconds */
 	struct intel_context *ce;
 	struct i915_request *rq;
 	ktime_t t0, t1;
@@ -254,12 +255,18 @@ static int __live_heartbeat_fast(struct intel_engine_cs *engine)
 		times[0],
 		times[ARRAY_SIZE(times) - 1]);
 
-	/* Min work delay is 2 * 2 (worst), +1 for scheduling, +1 for slack */
-	if (times[ARRAY_SIZE(times) / 2] > jiffies_to_usecs(6)) {
+	/*
+	 * Ideally, the upper bound on min work delay would be something like
+	 * 2 * 2 (worst), +1 for scheduling, +1 for slack. In practice, we
+	 * are, even with system_wq_highpri, at the mercy of the CPU scheduler
+	 * and may be stuck behind some slow work for many millisecond. Such
+	 * as our very own display workers.
+	 */
+	if (times[ARRAY_SIZE(times) / 2] > error_threshold) {
 		pr_err("%s: Heartbeat delay was %uus, expected less than %dus\n",
 		       engine->name,
 		       times[ARRAY_SIZE(times) / 2],
-		       jiffies_to_usecs(6));
+		       error_threshold);
 		err = -EINVAL;
 	}
 
-- 
2.20.1

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