[Intel-gfx] [PATCH v2] drm/i915: Bump ready tasks ahead of busywaits

[Chris Wilson]

Quoting Tvrtko Ursulin (2019-04-09 16:38:37)
> 
> On 09/04/2019 16:29, Chris Wilson wrote:
> > Consider two tasks that are running in parallel on a pair of engines
> > (vcs0, vcs1), but then must complete on a shared engine (rcs0). To
> > maximise throughput, we want to run the first ready task on rcs0 (i.e.
> > the first task that completes on either of vcs0 or vcs1). When using
> > semaphores, however, we will instead queue onto rcs in submission order.
> > 
> > To resolve this incorrect ordering, we want to re-evaluate the priority
> > queue when each of the request is ready. Normally this happens because
> > we only insert into the priority queue requests that are ready, but with
> > semaphores we are inserting ahead of their readiness and to compensate
> > we penalize those tasks with reduced priority (so that tasks that do not
> > need to busywait should naturally be run first). However, given a series
> > of tasks that each use semaphores, the queue degrades into submission
> > fifo rather than readiness fifo, and so to counter this we give a small
> > boost to semaphore users as their dependent tasks are completed (and so
> > we no longer require any busywait prior to running the user task as they
> > are then ready themselves).
> > 
> > v2: Fixup irqsave for schedule_lock (Tvrtko)
> > 
> > Testcase: igt/gem_exec_schedule/semaphore-codependency
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > Cc: Dmitry Rogozhkin <dmitry.v.rogozhkin at intel.com>
> > Cc: Dmitry Ermilov <dmitry.ermilov at intel.com>
> > ---
[snip]
> Looks fine to me. Provisional r-b:
> 
> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> 
> But let's wait for a media benchmarking run to see if you have nailed 
> the regression.

Aye, but we need something like this regardless as introducing a trivial
dos is not good behaviour either. Hopefully, this will evolve into
something a lot more elegant. For now, it is just another lesson learnt.
-Chris