Quoting Tvrtko Ursulin (2018-05-01 16:33:15)
>
> On 01/05/2018 11:46, Chris Wilson wrote:
> > Due to the latency of the tasklet running from ksoftirqd, by the time we
> > process the execlist dequeue may be a long time behind the GPU. If the
> > request was completed when we ran reschedule, we will not have tweaked
> > its priority, but if it is still listed as being in-flight for dequeue
> > we will use it as a reference for the rest of the queue, including
> > requests from its own context which will now be at higher priority. This
> > can cause us to issue a preempt-to-idle request, even though the request
> > we want to preempt is already complete.
> >
> > Reported-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> > ---
> > drivers/gpu/drm/i915/intel_lrc.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> > index c6464336963e..d344fbe17997 100644
> > --- a/drivers/gpu/drm/i915/intel_lrc.c
> > +++ b/drivers/gpu/drm/i915/intel_lrc.c
> > @@ -610,7 +610,8 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
> > if (!execlists_is_active(execlists, EXECLISTS_ACTIVE_HWACK))
> > goto unlock;
> >
> > - if (need_preempt(engine, last, execlists->queue_priority)) {
> > + if (need_preempt(engine, last, execlists->queue_priority) &&
> > + !i915_request_completed(last)) {
> > inject_preempt_context(engine);
> > goto unlock;
> > }
> >
>
> It makes sense on the most rudimentary level - don't preempt a context
> which is context saving itself.
>
> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
And picked up. To me the important connection is that when processing
the dependency tree we don't touch completed requests, so it should also
apply then during check of the inflight requests (as we may have skipped
them during rescheduling).
Thanks,
-Chris
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