Quoting Tvrtko Ursulin (2021-02-08 14:56:31)
> On 08/02/2021 10:52, Chris Wilson wrote:
> > +static bool need_preempt(const struct intel_engine_cs *engine,
> > const struct i915_request *rq)
> > {
> > const struct i915_sched *se = &engine->sched;
> > - int last_prio;
> > + const struct i915_request *first = NULL;
> > + const struct i915_request *next;
> >
> > if (!i915_sched_use_busywait(se))
> > return false;
> >
> > /*
> > - * Check if the current priority hint merits a preemption attempt.
> > - *
> > - * We record the highest value priority we saw during rescheduling
> > - * prior to this dequeue, therefore we know that if it is strictly
> > - * less than the current tail of ESLP[0], we do not need to force
> > - * a preempt-to-idle cycle.
> > - *
> > - * However, the priority hint is a mere hint that we may need to
> > - * preempt. If that hint is stale or we may be trying to preempt
> > - * ourselves, ignore the request.
> > - *
> > - * More naturally we would write
> > - * prio >= max(0, last);
> > - * except that we wish to prevent triggering preemption at the same
> > - * priority level: the task that is running should remain running
> > - * to preserve FIFO ordering of dependencies.
> > + * If this request is special and must not be interrupted at any
> > + * cost, so be it. Note we are only checking the most recent request
> > + * in the context and so may be masking an earlier vip request. It
> > + * is hoped that under the conditions where nopreempt is used, this
> > + * will not matter (i.e. all requests to that context will be
> > + * nopreempt for as long as desired).
> > */
> > - last_prio = max(effective_prio(rq), I915_PRIORITY_NORMAL - 1);
> > - if (engine->execlists.queue_priority_hint <= last_prio)
> > + if (i915_request_has_nopreempt(rq))
> > return false;
> >
> > /*
> > * Check against the first request in ELSP[1], it will, thanks to the
> > * power of PI, be the highest priority of that context.
> > */
> > - if (!list_is_last(&rq->sched.link, &se->requests) &&
> > - rq_prio(list_next_entry(rq, sched.link)) > last_prio)
> > - return true;
> > + next = next_elsp_request(se, rq);
> > + if (dl_before(next, first))
>
> Here first is always NULL so dl_before always returns true, meaning it
> appears redundant to call it.
I was applying a pattern :)
>
> > + first = next;
> >
> > /*
> > * If the inflight context did not trigger the preemption, then maybe
> > @@ -356,8 +343,31 @@ static bool need_preempt(struct intel_engine_cs *engine,
> > * ELSP[0] or ELSP[1] as, thanks again to PI, if it was the same
> > * context, it's priority would not exceed ELSP[0] aka last_prio.
> > */
> > - return max(virtual_prio(&engine->execlists),
> > - queue_prio(se)) > last_prio;
> > + next = first_request(se);
> > + if (dl_before(next, first))
> > + first = next; > +
> > + next = first_virtual(engine);
> > + if (dl_before(next, first))
> > + first = next;
> > +
> > + if (!dl_before(first, rq))
> > + return false;
>
> Ends up earliest deadline between list of picks: elsp[1] (or maybe next
> in context, depends on coalescing criteria), first in the priolist,
> first virtual.
>
> Virtual has a separate queue so that's understandable, but can "elsp[1]"
> really have an earlier deadling than first_request() (head of thepriolist)?
elsp[1] could have been promoted and thus now have an earlier deadline
than elsp[0]. Consider the heartbeat as a trivial example that is first
submitted at very low priority, but by the end has absolute priority.
> > +static u64 virtual_deadline(u64 kt, int priority)
> > +{
> > + return i915_sched_to_ticks(kt + prio_slice(priority));
> > +}
> > +
> > +u64 i915_scheduler_next_virtual_deadline(int priority)
> > +{
> > + return virtual_deadline(ktime_get_mono_fast_ns(), priority);
> > +}
>
> This helpers becomes a bit odd in that the only two callers are rewind
> and defer. And it queries ktime, while before deadline was set based on
> signalers.
>
> Where is the place which set the ktime based deadline (converted to
> ticks) for requests with no signalers?
signal_deadline() with no signalers returns now. So the first request in
a sequence is queued with virtual_deadline(now() + prio_slice()).
> > void i915_request_enqueue(struct i915_request *rq)
> > {
> > - struct intel_engine_cs *engine = rq->engine;
> > - struct i915_sched *se = intel_engine_get_scheduler(engine);
> > + struct i915_sched *se = i915_request_get_scheduler(rq);
> > + u64 dl = earliest_deadline(se, rq);
> > unsigned long flags;
> > bool kick = false;
> >
> > @@ -880,11 +1107,11 @@ void i915_request_enqueue(struct i915_request *rq)
> > list_add_tail(&rq->sched.link, &se->hold);
> > i915_request_set_hold(rq);
> > } else {
> > - queue_request(se, rq);
> > -
> > + set_bit(I915_FENCE_FLAG_PQUEUE, &rq->fence.flags);
> > + kick = __i915_request_set_deadline(se, rq,
> > + min(dl, rq_deadline(rq)));
>
> What is this min for? Dl has been computed above based on rq, so I
> wonder why rq_deadline has to be considered again.
earliest_deadline() only looks at the signalers (or now if none) and
picks the next deadline in that sequence. However, some requests we may
set the deadline explicitly (e.g. heartbeat has a known deadline, vblank
rendering we can approximate a deadline) and so we also consider what
deadline has already been specified.
> Because earliest_deadline does not actually consider rq->sched.deadline?
> So conceptually earliest_deadline would be described as what?
sequence_deadline() ?
earliest_deadline_for_this_sequence() ?
-Chris
_______________________________________________
Intel-gfx mailing list
Intel-gfx@xxxxxxxxxxxxxxxxxxxxx
https://lists.freedesktop.org/mailman/listinfo/intel-gfx
