drm/i915: Trim the retired request queue after submitting

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@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180207084350.3929-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_request.c

@@ -1077,6 +1077,26 @@ 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)