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

[Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>]

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.

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.

Regards,

Tvrtko




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