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

[Chris Wilson]

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 at chris-wilson.co.uk>
> >>> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> >>> ---
> >>>    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.
-Chris