drm/i915: Flush tasklet submission before sleeping on i915_request_wait

If the system is being slow and userspace is racing ahead of the GPU and
finds itself waiting for the GPU to catch up, before the process sleeps
give the tasklet a kick, bypassing ksoftirqd. If the system is
overloaded, then ksoftirqd may be delayed incurring additional latency
to our user.

This should not be a frequent problem, but in the past we have observed
several hundred millisecond delays before ksoftirqd services an
interrupt, so burn a few cycles to lend a helping hand.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191015132606.14349-1-chris@chris-wilson.co.uk

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index b8a54572a4f87ffa675de25dd56f829d0b8255a5..f1cadad4e81cacfce23887098bf83e6c028f36a2 100644 (file)
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -1496,6 +1496,7 @@ long i915_request_wait(struct i915_request *rq,
                        break;
                }
 
+               intel_engine_flush_submission(rq->engine);
                timeout = io_schedule_timeout(timeout);
        }
        __set_current_state(TASK_RUNNING);