[PATCH 02/72] drm/i915/gt: Check for a completed last request once

[Chris Wilson]

Pull the repeated check for the last active request being completed to a
single spot, when deciding whether or not execlist preemption is
required.

In doing so, we remove a tasklet kick if we find the request was
completed but have not yet seen the corresponding CS event. This was
devolving into a busy spin of the tasklet while we waited for the event
as the delivery was not as instantaneous as expected. Under load this
is sufficient to exhaust the tasklet softirq timeslice, and force
ksoftirqd. Quite noticeable overhead for no apparent improvement in
latency.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/gt/intel_lrc.c | 15 ++++-----------
 1 file changed, 4 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index cf11cbac241b..43703efb36d1 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -2141,12 +2141,9 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 	 */
 
 	if ((last = *active)) {
-		if (need_preempt(engine, last, rb)) {
-			if (i915_request_completed(last)) {
-				tasklet_hi_schedule(&execlists->tasklet);
-				return;
-			}
-
+		if (i915_request_completed(last)) {
+			goto check_secondary;
+		} else if (need_preempt(engine, last, rb)) {
 			ENGINE_TRACE(engine,
 				     "preempting last=%llx:%lld, prio=%d, hint=%d\n",
 				     last->fence.context,
@@ -2174,11 +2171,6 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 			last = NULL;
 		} else if (need_timeslice(engine, last, rb) &&
 			   timeslice_expired(execlists, last)) {
-			if (i915_request_completed(last)) {
-				tasklet_hi_schedule(&execlists->tasklet);
-				return;
-			}
-
 			ENGINE_TRACE(engine,
 				     "expired last=%llx:%lld, prio=%d, hint=%d, yield?=%s\n",
 				     last->fence.context,
@@ -2214,6 +2206,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 			 * we hopefully coalesce several updates into a single
 			 * submission.
 			 */
+check_secondary:
 			if (!list_is_last(&last->sched.link,
 					  &engine->active.requests)) {
 				/*
-- 
2.20.1