Re: [PATCH 4/5] drm/i915/gt: Adopt engine_park synchronisation rules for engine_retire

[Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>]

On 21/11/2019 16:24, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2019-11-21 16:17:59)
> > On 21/11/2019 14:53, Chris Wilson wrote:
> > > Quoting Tvrtko Ursulin (2019-11-21 14:42:56)
> > > > On 21/11/2019 13:51, Chris Wilson wrote:
> > > > > In the next patch, we will introduce a new asynchronous retirement
> > > > > worker, fed by execlists CS events. Here we may queue a retirement as
> > > > > soon as a request is submitted to HW (and completes instantly), and we
> > > > > also want to process that retirement as early as possible and cannot
> > > > > afford to postpone (as there may not be another opportunity to retire it
> > > > > for a few seconds). To allow the new async retirer to run in parallel
> > > > > with our submission, pull the __i915_request_queue (that passes the
> > > > > request to HW) inside the timelines spinlock so that the retirement
> > > > > cannot release the timeline before we have completed the submission.
> > > > >
> > > > > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > > > > Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> > > > > ---
> > > > >     drivers/gpu/drm/i915/gt/intel_engine_pm.c | 29 ++++++++++++++++-------
> > > > >     1 file changed, 21 insertions(+), 8 deletions(-)
> > > > >
> > > > > diff --git a/drivers/gpu/drm/i915/gt/intel_engine_pm.c b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> > > > > index 373a4b9f159c..bd0af02bea16 100644
> > > > > --- a/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> > > > > +++ b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> > > > > @@ -74,18 +74,33 @@ static inline void __timeline_mark_unlock(struct intel_context *ce,
> > > > >     #endif /* !IS_ENABLED(CONFIG_LOCKDEP) */
> > > > >     
> > > > >     static void
> > > > > -__intel_timeline_enter_and_release_pm(struct intel_timeline *tl,
> > > > > -                                   struct intel_engine_cs *engine)
> > > > > +__queue_and_release_pm(struct i915_request *rq,
> > > > > +                    struct intel_timeline *tl,
> > > > > +                    struct intel_engine_cs *engine)
> > > > >     {
> > > > >         struct intel_gt_timelines *timelines = &engine->gt->timelines;
> > > > >     
> > > > > +     /*
> > > > > +      * We have to serialise all potential retirement paths with our
> > > > > +      * submission, as we don't want to underflow either the
> > > > > +      * engine->wakeref.counter or our timeline->active_count.
> > > > > +      *
> > > > > +      * Equally, we cannot allow a new submission to start until
> > > > > +      * after we finish queueing, nor could we allow that submitter
> > > > > +      * to retire us before we are ready!
> > > > > +      */
> > > > >         spin_lock(&timelines->lock);
> > > > >     
> > > > > -     if (!atomic_fetch_inc(&tl->active_count))
> > > > > -             list_add_tail(&tl->link, &timelines->active_list);
> > > > > +     /* Hand the request over to HW and so engine_retire() */
> > > > > +     __i915_request_queue(rq, NULL);
> > > > >     
> > > > > +     /* Let new submissions commence (and maybe retire this timeline) */
> > > > >         __intel_wakeref_defer_park(&engine->wakeref);
> > > > >     
> > > > > +     /* Let intel_gt_retire_requests() retire us */
> > > > > +     if (!atomic_fetch_inc(&tl->active_count))
> > > > > +             list_add_tail(&tl->link, &timelines->active_list);
> > > > > +
> > > > >         spin_unlock(&timelines->lock);
> > > >
> > > > Now that everything is under the lock the order of operation is not
> > > > important, or it still is?
> > >
> > > queue before unpark that is required.
> > >
> > > unpark and add_to_timeline, the order is flexible as the lock governors
> > > the interactions between those and retirers. So I chose to allow the
> > > next newcomer start a few instructions earlier.
> >
> > Yes, because of different locks. So the comment above
> > __intel_wakeref_defer_park is not correct since timeline cannot be
> > retired until the lock is dropped.
>
> The goal was to indicate that the wakeref.count will allow new
> submissions to bypass the engine-pm; while also tying back to the
> retirement theme and reminding the reader that request submission also
> implies some retiring of old requests on the timeline.
>
> So I was trying to point out the connection between all steps and the
> act of retiring, since that was most pressing on my mind.
>
> > It's only preservation of timeline ordering which mandates defer_park
> > after request_queue. As far as I am able to summon my own understanding
> > from yesterday.
>
> Correct. That's the important bit from yesterday.

Phew.. thanks for re-confirming.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

Regards,

Tvrtko


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