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

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

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.

Then on unhold ve->submit_notify is called which sets ve->request to 
this request but the rq->engine points to the physical engine.

Regards,

Tvrtko


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