[Intel-gfx] [PATCH v3] drm/i915/execlists: Reclaim the hanging virtual request

[Chris Wilson]

Quoting Tvrtko Ursulin (2020-01-21 17:43:37)
> 
> On 21/01/2020 17:32, Chris Wilson wrote:
> > Quoting Tvrtko Ursulin (2020-01-21 17:19:52)
> >>
> >> On 21/01/2020 14:07, Chris Wilson wrote:
> >>> Quoting Tvrtko Ursulin (2020-01-21 13:55:29)
> >>>>
> >>>>
> >>>> On 21/01/2020 13:04, Chris Wilson wrote:
> >>>>> +             GEM_BUG_ON(!reset_in_progress(&engine->execlists));
> >>>>> +
> >>>>> +             /*
> >>>>> +              * An unsubmitted request along a virtual engine will
> >>>>> +              * remain on the active (this) engine until we are able
> >>>>> +              * to process the context switch away (and so mark the
> >>>>> +              * context as no longer in flight). That cannot have happened
> >>>>> +              * yet, otherwise we would not be hanging!
> >>>>> +              */
> >>>>> +             spin_lock_irqsave(&ve->base.active.lock, flags);
> >>>>> +             GEM_BUG_ON(intel_context_inflight(rq->context) != engine);
> >>>>> +             GEM_BUG_ON(ve->request != rq);
> >>>>> +             ve->request = NULL;
> >>>>> +             spin_unlock_irqrestore(&ve->base.active.lock, flags);
> >>>>> +
> >>>>> +             rq->engine = engine;
> >>>>
> >>>> Lets see I understand this... tasklet has been disabled and ring paused.
> >>>> But we find an uncompleted request in the ELSP context, with rq->engine
> >>>> == virtual engine. Therefore this cannot be the first request on this
> >>>> timeline but has to be later.
> >>>
> >>> Not quite.
> >>>
> >>> engine->execlists.active[] tracks the HW, it get's updated only upon
> >>> receiving HW acks (or we reset).
> >>>
> >>> So if execlists_active()->engine == virtual, it can only mean that the
> >>> inflight _hanging_ request has already been unsubmitted by an earlier
> >>> preemption in execlists_dequeue(), but that preemption has not yet been
> >>> processed by the HW. (Hence the preemption-reset underway.)
> >>>
> >>> Now while we coalesce the requests for a context into a single ELSP[]
> >>> slot, and only record the last request submitted for a context, we have
> >>> to walk back along that context's timeline to find the earliest
> >>> incomplete request and blame the hang upon it.
> >>>
> >>> For a virtual engine, it's much simpler as there is only ever one
> >>> request in flight, but I don't think that has any impact here other
> >>> than that we only need to repair the single unsubmitted request that was
> >>> returned to the virtual engine.
> >>>
> >>>> One which has been put on the runqueue but
> >>>> not yet submitted to hw. (Because one at a time.) Or it has been
> >>>> unsubmitted by __unwind_incomplete_request already. In the former case
> >>>> why move it to the physical engine? Also in the latter actually, it
> >>>> would overwrite rq->engine with the physical one.
> >>>
> >>> Yes. For incomplete preemption event, the request is *still* on this
> >>> engine and has not been released (rq->context->inflight == engine, so it
> >>> cannot be submitted to any other engine, until after we acknowledge the
> >>> context has been saved and is no longer being accessed by HW.) It is
> >>> legal for us to process the hanging request along this engine; we have a
> >>> suboptimal decision to return the request to the same engine after the
> >>> reset, but since we have replaced the hanging payload, the request is a
> >>> mere signaling placeholder (and I do not think will overly burden the
> >>> system and negatively impact other virtual engines).
> >>
> >> What if the request in elsp actually completed in the meantime eg.
> >> preemption timeout was a false positive?
> >>
> >> In execlists_capture we do:
> >>
> >>          cap->rq = execlists_active(&engine->execlists);
> >>
> >> This gets a completed request, then we do:
> >>
> >>          cap->rq = active_request(cap->rq->context->timeline, cap->rq);
> >>
> >> This walks along the virtual timeline and finds a next virtual request.
> >> It then binds this request to a physical engine and sets ve->request to
> >> NULL.
> > 
> > If we miss the completion event, then active_request() returns the
> > original request and we blame it for a having a 650ms preemption-off
> > shader with a 640ms preemption timeout.
> 
> I am thinking of this sequence of interleaved events:
> 
>         preempt_timeout
>                                 tasklet_disable
>                                 ring_pause
>                                 execlist_active
>         seqno write visible
>                                 active_request - walks the tl to next

... tries to walk to next, sees no incomplete request, returns original
request.

static struct i915_request *
active_request(const struct intel_timeline * const tl, struct i915_request *rq)
{
        struct i915_request *active = rq;
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this sneaky line

        rcu_read_lock();
        list_for_each_entry_continue_reverse(rq, &tl->requests, link) {
                if (i915_request_completed(rq))
                        break;

                active = rq;
		^^^^^^^^^^^^ these too may complete at any moment after
		our inspection


        }
        rcu_read_unlock();

        return active;
}

>                                 execlist_hold
>                                 schedule_worker
>                                 tasklet_enable
>         process_csb completed
> 
> This is not possible? Seqno write happening needs only to be roughly 
> there since as long as tasklet has been disabled execlist->active 
> remains fixed.

It's certainly possible, the requests do keep going on the HW up until
the next semaphore (which is after the seqno write). That is taken into
account in that we may end up trying to reset a completed request, which
should be avoided in execlists_reset() [after the HW has processed the
reset request], but we capture the request anyway and put it back for
execution (which is avoided in execlists_dequeue). Isn't preempt-to-busy
fun?
-Chris