[Intel-gfx] [PATCH v2] drm/i915: Trim the retired request queue after submitting

[Chris Wilson]

If we submit a request and see that the previous request on this
timeline was already signaled, we first do not need to add the
dependency tracker for that completed request and secondly we know that
we there is then a large backlog in retiring requests affecting this
timeline. Given that we just submitted more work to the HW, now would be
a good time to catch up on those retirements.

v2: Try to sum up the compromises involved in flushing the retirement
queue after submission.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/i915_gem_request.c | 34 ++++++++++++++++++++++++++++-----
 1 file changed, 29 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_request.c b/drivers/gpu/drm/i915/i915_gem_request.c
index a0f451b4a4e8..77c2eba490e9 100644
--- a/drivers/gpu/drm/i915/i915_gem_request.c
+++ b/drivers/gpu/drm/i915/i915_gem_request.c
@@ -983,7 +983,8 @@ void __i915_add_request(struct drm_i915_gem_request *request, bool flush_caches)
 	lockdep_assert_held(&request->i915->drm.struct_mutex);
 	trace_i915_gem_request_add(request);
 
-	/* Make sure that no request gazumped us - if it was allocated after
+	/*
+	 * Make sure that no request gazumped us - if it was allocated after
 	 * our i915_gem_request_alloc() and called __i915_add_request() before
 	 * us, the timeline will hold its seqno which is later than ours.
 	 */
@@ -1010,7 +1011,8 @@ void __i915_add_request(struct drm_i915_gem_request *request, bool flush_caches)
 		WARN(err, "engine->emit_flush() failed: %d!\n", err);
 	}
 
-	/* Record the position of the start of the breadcrumb so that
+	/*
+	 * Record the position of the start of the breadcrumb so that
 	 * should we detect the updated seqno part-way through the
 	 * GPU processing the request, we never over-estimate the
 	 * position of the ring's HEAD.
@@ -1019,7 +1021,8 @@ void __i915_add_request(struct drm_i915_gem_request *request, bool flush_caches)
 	GEM_BUG_ON(IS_ERR(cs));
 	request->postfix = intel_ring_offset(request, cs);
 
-	/* Seal the request and mark it as pending execution. Note that
+	/*
+	 * Seal the request and mark it as pending execution. Note that
 	 * we may inspect this state, without holding any locks, during
 	 * hangcheck. Hence we apply the barrier to ensure that we do not
 	 * see a more recent value in the hws than we are tracking.
@@ -1027,7 +1030,7 @@ void __i915_add_request(struct drm_i915_gem_request *request, bool flush_caches)
 
 	prev = i915_gem_active_raw(&timeline->last_request,
 				   &request->i915->drm.struct_mutex);
-	if (prev) {
+	if (prev && !i915_gem_request_completed(prev)) {
 		i915_sw_fence_await_sw_fence(&request->submit, &prev->submit,
 					     &request->submitq);
 		if (engine->schedule)
@@ -1047,7 +1050,8 @@ void __i915_add_request(struct drm_i915_gem_request *request, bool flush_caches)
 	list_add_tail(&request->ring_link, &ring->request_list);
 	request->emitted_jiffies = jiffies;
 
-	/* Let the backend know a new request has arrived that may need
+	/*
+	 * Let the backend know a new request has arrived that may need
 	 * to adjust the existing execution schedule due to a high priority
 	 * request - i.e. we may want to preempt the current request in order
 	 * to run a high priority dependency chain *before* we can execute this
@@ -1063,6 +1067,26 @@ void __i915_add_request(struct drm_i915_gem_request *request, bool flush_caches)
 	local_bh_disable();
 	i915_sw_fence_commit(&request->submit);
 	local_bh_enable(); /* Kick the execlists tasklet if just scheduled */
+
+	/*
+	 * In typical scenarios, we do not expect the previous request on
+	 * the timeline to be still tracked by timeline->last_request if it
+	 * has been completed. If the completed request is still here, that
+	 * implies that request retirement is a long way behind submission,
+	 * suggesting that we haven't been retiring frequently enough from
+	 * the combination of retire-before-alloc, waiters and the background
+	 * retirement worker. So if the last request on this timeline was
+	 * already completed, do a catch up pass, flushing the retirement queue
+	 * up to this client. Since we have now moved the heaviest operations
+	 * during retirement onto secondary workers, such as freeing objects
+	 * or contexts, retiring a bunch of requests is mostly list management
+	 * (and cache misses), and so we should not be overly penalizing this
+	 * client by performing excess work, though we may still performing
+	 * work on behalf of others -- but instead we should benefit from
+	 * improved resource management. (Well, that's the theory at least.)
+	 */
+	if (prev && i915_gem_request_completed(prev))
+		i915_gem_request_retire_upto(prev);
 }
 
 static unsigned long local_clock_us(unsigned int *cpu)
-- 
2.15.1