drm/i915: Make the heartbeat play nice with long pre-emption timeouts

Compute workloads are inherently not pre-emptible for long periods on
current hardware. As a workaround for this, the pre-emption timeout
for compute capable engines was disabled. This is undesirable with GuC
submission as it prevents per engine reset of hung contexts. Hence the
next patch will re-enable the timeout but bumped up by an order of
magnitude.

However, the heartbeat might not respect that. Depending upon current
activity, a pre-emption to the heartbeat pulse might not even be
attempted until the last heartbeat period. Which means that only one
period is granted for the pre-emption to occur. With the aforesaid
bump, the pre-emption timeout could be significantly larger than this
heartbeat period.

So adjust the heartbeat code to take the pre-emption timeout into
account. When it reaches the final (high priority) period, it now
ensures the delay before hitting reset is bigger than the pre-emption
timeout.

v2: Fix for selftests which adjust the heartbeat period manually.
v3: Add FIXME comment about selftests. Add extra FIXME comment and
drm_notices when setting heartbeat to a non-default value (review
feedback from Tvrtko)

Signed-off-by: John Harrison <[e-mail]>
Reviewed-by: Tvrtko Ursulin <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/i915/gt/intel_engine_heartbeat.c

···
 
 static bool next_heartbeat(struct intel_engine_cs *engine)
 {
+	struct i915_request *rq;
 	long delay;
 
 	delay = READ_ONCE(engine->props.heartbeat_interval_ms);
+
+	rq = engine->heartbeat.systole;
+
+	/*
+	 * FIXME: The final period extension is disabled if the period has been
+	 * modified from the default. This is to prevent issues with certain
+	 * selftests which override the value and expect specific behaviour.
+	 * Once the selftests have been updated to either cope with variable
+	 * heartbeat periods (or to override the pre-emption timeout as well,
+	 * or just to add a selftest specific override of the extension), the
+	 * generic override can be removed.
+	 */
+	if (rq && rq->sched.attr.priority >= I915_PRIORITY_BARRIER &&
+	    delay == engine->defaults.heartbeat_interval_ms) {
+		long longer;
+
+		/*
+		 * The final try is at the highest priority possible. Up until now
+		 * a pre-emption might not even have been attempted. So make sure
+		 * this last attempt allows enough time for a pre-emption to occur.
+		 */
+		longer = READ_ONCE(engine->props.preempt_timeout_ms) * 2;
+		longer = intel_clamp_heartbeat_interval_ms(engine, longer);
+		if (longer > delay)
+			delay = longer;
+	}
+
 	if (!delay)
 		return false;
 
···
 
 	if (!delay && !intel_engine_has_preempt_reset(engine))
 		return -ENODEV;
+
+	/* FIXME: Remove together with equally marked hack in next_heartbeat. */
+	if (delay != engine->defaults.heartbeat_interval_ms &&
+	    delay < 2 * engine->props.preempt_timeout_ms) {
+		if (intel_engine_uses_guc(engine))
+			drm_notice(&engine->i915->drm, "%s heartbeat interval adjusted to a non-default value which may downgrade individual engine resets to full GPU resets!\n",
+				   engine->name);
+		else
+			drm_notice(&engine->i915->drm, "%s heartbeat interval adjusted to a non-default value which may cause engine resets to target innocent contexts!\n",
+				   engine->name);
+	}
 
 	intel_engine_pm_get(engine);