[Intel-gfx] [PATCH] drm/i915: Copy across scheduler behaviour flags across submit fences

[Chris Wilson]

Quoting Tvrtko Ursulin (2019-11-27 14:22:37)
> 
> On 27/11/2019 14:04, Chris Wilson wrote:
> > Quoting Tvrtko Ursulin (2019-11-27 13:46:14)
> >> On 27/11/2019 11:17, Chris Wilson wrote:
> >>> We want the bonded request to have the same scheduler properties as its
> >>> master so that it is placed at the same depth in the queue. For example,
> >>> consider we have requests A, B and B', where B & B' are a bonded pair to
> >>> run in parallel on two engines.
> >>>
> >>>        A -> B
> >>>                     \- B'
> >>>
> >>> B will run after A and so may be scheduled on an idle engine and wait on
> >>> A using a semaphore. B' sees B being executed and so enters the queue on
> >>> the same engine as A. As B' did not inherit the semaphore-chain from B,
> >>> it may have higher precedence than A and so preempts execution. However,
> >>> B' then sits on a semaphore waiting for B, who is waiting for A, who is
> >>> blocked by B.
> >>>
> >>> Ergo B' needs to inherit the scheduler properties from B (i.e. the
> >>> semaphore chain) so that it is scheduled with the same priority as B and
> >>> will not be executed ahead of Bs dependencies.
> >>>
> >>> Furthermore, to prevent the priorities changing via the expose fence on
> >>> B', we need to couple in the dependencies for PI. This requires us to
> >>> relax our sanity-checks that dependencies are strictly in order.
> >>
> >> Good catch, this needed some deep thinking! And it looks okay, even
> >> though ideally we would be able to fix it not to signal the submit fence
> >> until semaphore was completed. But for that I think we would need to
> >> emit a request while emitting a request, so that the semaphore wait
> >> would be in its own.
> > 
> > At a push we could add an MI_USER_INTERRUPT after the initial breadcrumb
> > and couple the submit fence into that. That would be virtually
> > equivalent to emitting a separate request for semaphores. Something to
> > ponder over.
> 
> Hm, if not too difficult it would definitely be much preferable since 
> relying on controlling preemption decisions feels a bit fragile/hackish.
> 
> Simply moving __notify_execute_cb from __i915_request_submit to 
> intel_engine_breadcrumbs_irq, under a __i915_request_has_started check, 
> could do it?

95% of the way, yes.
-Chris