[Intel-gfx] [PATCH v3] drm/i915: Skip execlists_dequeue() early if the list is empty

[Chris Wilson]

Do an early read of the execlists' queue before we take the spinlock and
start checking. This is safe as the first writer to the execlists queue
will cause the tasklet to be run again after a memory barrier.

v2: Keep guc in sync with execlists queue changes
v3: Explain the mb between the tasklet running on one cpu and the
execlist_first update and schedule from a second cpu.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
Cc: Michał Winiarski <michal.winiarski at intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski at intel.com>
---
 drivers/gpu/drm/i915/i915_guc_submission.c | 12 ++++++++++++
 drivers/gpu/drm/i915/intel_lrc.c           | 12 ++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_guc_submission.c b/drivers/gpu/drm/i915/i915_guc_submission.c
index a3636b31ebc3..832ac9e45801 100644
--- a/drivers/gpu/drm/i915/i915_guc_submission.c
+++ b/drivers/gpu/drm/i915/i915_guc_submission.c
@@ -577,6 +577,18 @@ static bool i915_guc_dequeue(struct intel_engine_cs *engine)
 	struct rb_node *rb;
 	bool submit = false;
 
+	/* After execlist_first is updated, the tasklet will be rescheduled.
+	 *
+	 * If we are currently running (inside the tasklet) and a third
+	 * party queues a request and so updates engine->execlist_first under
+	 * the spinlock (which we have elided), it will atomically set the
+	 * TASKLET_SCHED flag causing the us to be re-executed and pick up
+	 * the change in state (the update to TASKLET_SCHED incurs a memory
+	 * barrier making this cross-cpu checking safe).
+	 */
+	if (!READ_ONCE(engine->execlist_first))
+		return false;
+
 	spin_lock_irqsave(&engine->timeline->lock, flags);
 	rb = engine->execlist_first;
 	while (rb) {
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index becde55b02a3..77168e673e0a 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -403,6 +403,18 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 	struct rb_node *rb;
 	bool submit = false;
 
+	/* After execlist_first is updated, the tasklet will be rescheduled.
+	 *
+	 * If we are currently running (inside the tasklet) and a third
+	 * party queues a request and so updates engine->execlist_first under
+	 * the spinlock (which we have elided), it will atomically set the
+	 * TASKLET_SCHED flag causing the us to be re-executed and pick up
+	 * the change in state (the update to TASKLET_SCHED incurs a memory
+	 * barrier making this cross-cpu checking safe).
+	 */
+	if (!READ_ONCE(engine->execlist_first))
+		return;
+
 	last = port->request;
 	if (last)
 		/* WaIdleLiteRestore:bdw,skl
-- 
2.11.0