drm/i915: Truly bump ready tasks ahead of busywaits

In commit b7404c7ecb ("drm/i915: Bump ready tasks ahead of
busywaits"), I tried cutting a corner in order to not install a signal
for each of our dependencies, and only listened to requests on which we
were intending to busywait. The compromise that was made was that
instead of then being able to promote the request with a full
NOSEMAPHORE like its non-busywaiting brethren, as we had not ensured we
had cleared the semaphore chain, we settled for only using the NEWCLIENT
boost. With an over saturated system with multiple NEWCLIENTS in flight
at any time, this was found to be an inadequate promotion and left us
with a much poorer scheduling order than prior to using semaphores.

The outcome of this patch, is that all requests have NOSEMAPHORE
priority when they have no dependencies and are ready to run and not
busywait, restoring the pre-semaphore ordering on saturated systems.

We can demonstrate the effect of poor scheduling order by oversaturating
the system using gem_wsim on a system with multiple vcs engines
(i.e running the same workloads across more clients than required for
peak throughput, e.g. media_load_balance_17i7.wsim -c4 -b context):

x v5.1 (normalized)
+ tip
* fix
+------------------------------------------------------------------------+
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+------------------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x 120       0.99456       1.00628      0.999985     1.0001545  0.0024387139
+ 120      0.873021       1.00037      0.884134    0.90148752   0.039190862
Difference at 99.5% confidence
	-0.098667 +/- 0.0110762
	-9.86517% +/- 1.10745%
	(Student's t, pooled s = 0.0277657)
% 120      0.990207       1.00165     0.9970265    0.99699748     0.0021024
Difference at 99.5% confidence
	-0.003157 +/- 0.000908245
	-0.315651% +/- 0.0908105%
	(Student's t, pooled s = 0.00227678)

Fixes: b7404c7ecb ("drm/i915: Bump ready tasks ahead of busywaits")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Dmitry Rogozhkin <dmitry.v.rogozhkin@intel.com>
Cc: Dmitry Ermilov <dmitry.ermilov@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190515130052.4475-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_request.c

@@ -575,18 +575,7 @@ semaphore_notify(struct i915_sw_fence *fence, enum i915_sw_fence_notify state)
 
 	switch (state) {
 	case FENCE_COMPLETE:
-		/*
-		 * We only check a small portion of our dependencies
-		 * and so cannot guarantee that there remains no
-		 * semaphore chain across all. Instead of opting
-		 * for the full NOSEMAPHORE boost, we go for the
-		 * smaller (but still preempting) boost of
-		 * NEWCLIENT. This will be enough to boost over
-		 * a busywaiting request (as that cannot be
-		 * NEWCLIENT) without accidentally boosting
-		 * a busywait over real work elsewhere.
-		 */
-		i915_schedule_bump_priority(request, I915_PRIORITY_NEWCLIENT);
+		i915_schedule_bump_priority(request, I915_PRIORITY_NOSEMAPHORE);
 		break;
 
 	case FENCE_FREE:
@@ -852,12 +841,6 @@ emit_semaphore_wait(struct i915_request *to,
 	if (err < 0)
 		return err;
 
-	err = i915_sw_fence_await_dma_fence(&to->semaphore,
-					    &from->fence, 0,
-					    I915_FENCE_GFP);
-	if (err < 0)
-		return err;
-
 	/* Only submit our spinner after the signaler is running! */
 	err = i915_request_await_execution(to, from, gfp);
 	if (err)
@@ -923,8 +906,18 @@ i915_request_await_request(struct i915_request *to, struct i915_request *from)
 						    &from->fence, 0,
 						    I915_FENCE_GFP);
 	}
+	if (ret < 0)
+		return ret;
 
-	return ret < 0 ? ret : 0;
+	if (to->sched.flags & I915_SCHED_HAS_SEMAPHORE_CHAIN) {
+		ret = i915_sw_fence_await_dma_fence(&to->semaphore,
+						    &from->fence, 0,
+						    I915_FENCE_GFP);
+		if (ret < 0)
+			return ret;
+	}
+
+	return 0;
 }
 
 int