[Intel-gfx] [PATCH] drm/i915: Soften the tasklet flush frequency before waits

[Chris Wilson]

We include a tasklet flush before waiting on a request as a precaution
against the HW being lax in event signaling. We now have a precautionary
flush in the engine's heartbeat and so do not need to be quite so
zealous on every request wait. If we focus on the request, the only
tasklet flush that matters is if there is a delay in submitting this
request to HW, so if the request is not ready to be executed no
advantage in reducing this wait can be gained by running the tasklet.
And there is little point in doing busy work for no result.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
---
 drivers/gpu/drm/i915/i915_request.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index 3bb7320249ae..19adb7b82a2e 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -1790,6 +1790,24 @@ long i915_request_wait(struct i915_request *rq,
 	if (dma_fence_add_callback(&rq->fence, &wait.cb, request_wait_wake))
 		goto out;
 
+	/*
+	 * Flush the submission tasklet, but only if it may help this request.
+	 *
+	 * We sometimes experience some latency between the HW interrupts and
+	 * tasklet execution (mostly due to ksoftirqd latency, but it can also
+	 * be due to lazy CS events), so lets run the tasklet manually if there
+	 * is a chance it may submit this request. If the request is not ready
+	 * to run, as it is waiting for other fences to be signaled, flushing
+	 * the tasklet is busy work without any advantage for this client.
+	 *
+	 * If the HW is being lazy, this is the last chance before we go to
+	 * sleep to catch any pending events. We will check periodically in
+	 * the heartbeat to flush the submission tasklets as a last resort
+	 * for unhappy HW.
+	 */
+	if (i915_request_is_ready(rq))
+		intel_engine_flush_submission(rq->engine);
+
 	for (;;) {
 		set_current_state(state);
 
@@ -1798,8 +1816,6 @@ long i915_request_wait(struct i915_request *rq,
 			break;
 		}
 
-		intel_engine_flush_submission(rq->engine);
-
 		if (signal_pending_state(state, current)) {
 			timeout = -ERESTARTSYS;
 			break;
-- 
2.20.1