[Intel-gfx] [PATCH 2/5] drm/i915/perf: allow holding preemption on filtered ctx

[Chris Wilson]

Quoting Lionel Landwerlin (2019-05-24 10:51:49)
> On 24/05/2019 10:42, Chris Wilson wrote:
> > Quoting Lionel Landwerlin (2019-05-24 10:28:16)
> >> On 21/05/2019 17:36, Chris Wilson wrote:
> >>> Quoting Lionel Landwerlin (2019-05-21 15:08:52)
> >>>> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> >>>> index f263a8374273..2ad95977f7a8 100644
> >>>> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> >>>> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> >>>> @@ -2085,7 +2085,7 @@ static int gen9_emit_bb_start(struct i915_request *rq,
> >>>>           if (IS_ERR(cs))
> >>>>                   return PTR_ERR(cs);
> >>>>    
> >>>> -       *cs++ = MI_ARB_ON_OFF | MI_ARB_ENABLE;
> >>>> +       *cs++ = MI_ARB_ON_OFF | rq->hw_context->arb_enable;
> >>> My prediction is that this will result in this context being reset due
> >>> to preemption timeouts and the context under profile being banned. Note
> >>> that preemption timeouts will be the primary means for hang detection
> >>> for endless batches.
> >>> -Chris
> >>>
> >> Another thought :
> >>
> >> What if we ran with the max priority?
> >> It would be fine to have the hangcheck preempt the workload (it's pretty
> >> short and shouldn't affect perf counters from 3d/compute pipeline much)
> >> as long as ensure nothing else runs.
> > It's certainly safer from the pov that we don't block preemption and so
> > don't incur forced resets. Not keen on the system being perturbed by the
> > act of observing it, and I still dislike the notion of permitting one
> > client to hog the GPU so easily. Makes me think of RT throttling, and
> > generally throwing out the absolute priority system (in exchange for
> > computed deadlines or something).
> > -Chris
> >
> I don't like it much either but I can't see how to do otherwise with the 
> hardware we currently have.
> 
> I'm thinking of 2 priorities values one of scheduling, one once running.

It's not quite that easy as you may start running concurrently with one
of your dependencies and must therefore manage the priority inversion if
you boost yourself. And I've just gone through and thrown out the
current complexity of manipulating priority as they run because it made
timeslicing much harder (where the priority was changing between
evaluating the need for the context switch and the context switch
occurring -- such mistakes can be noticed in throughput sensitive
transcode workloads).

> Most contexts would have both values equal.
> 
> Could mitigate the issue a bit?

A bit, it gives you a soft notion of a no-preempt flag without queue
jumping. rq_prio(rq) | intel_context->effective_priority or somesuch.
-Chris