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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

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
				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.

>> Then on unhold ve->submit_notify is called which sets ve->request to
>> this request but the rq->engine points to the physical engine.
> 
> We don't call ve->submit_notify() on unhold, we put it back into our
> local priority queue. Keeping ownership of the request on the local
> engine seemed to the easiest way to keep track of the locking, and
> re-submitting the guilty request on the same engine should not be an
> issue.

True, I am jumping between different things and have confused this bit.

Regards,

Tvrtko
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