[Intel-gfx] [PATCH 10/31] drm/i915: Fair low-latency scheduling

[Tvrtko Ursulin]

On 08/02/2021 15:29, Chris Wilson wrote:
> 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 :)

Yeah, thought so. It's fine.

> 
>>
>>> +             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.

The tree is not kept sorted at all times, or at least at the time 
need_preempt peeks at it?

> 
>>> +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()).

Ah ok.

> 
>>>    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() ?

Don't know really. Don't think it's a matter of names just me building a 
good image of the operation.

But as earliest does imply earliest, which then gets potentially 
overwritten with something even earlier, hm.. baseline? :) Default? 
Nah.. Scheduling_deadline? Tree deadline? Sorted deadline?

Regards,

Tvrtko