drm/i915/execlists: Skip redundant resubmission

If we unwind the active requests, and on resubmission discover that we
intend to preempt the active contexts with themselves, simply skip the
ELSP submission.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191003210100.22250-1-chris@chris-wilson.co.uk

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

@@ -1739,11 +1739,26 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 
 	if (submit) {
 		*port = execlists_schedule_in(last, port - execlists->pending);
-		memset(port + 1, 0, (last_port - port) * sizeof(*port));
 		execlists->switch_priority_hint =
 			switch_prio(engine, *execlists->pending);
+
+		/*
+		 * Skip if we ended up with exactly the same set of requests,
+		 * e.g. trying to timeslice a pair of ordered contexts
+		 */
+		if (!memcmp(execlists->active, execlists->pending,
+			    (port - execlists->pending + 1) * sizeof(*port))) {
+			do
+				execlists_schedule_out(fetch_and_zero(port));
+			while (port-- != execlists->pending);
+
+			goto skip_submit;
+		}
+
+		memset(port + 1, 0, (last_port - port) * sizeof(*port));
 		execlists_submit_ports(engine);
 	} else {
+skip_submit:
 		ring_set_paused(engine, 0);
 	}
 }