Quoting Tvrtko Ursulin (2019-10-14 10:34:31)
>
> On 13/10/2019 21:30, Chris Wilson wrote:
> > Since commit e2144503bf3b ("drm/i915: Prevent bonded requests from
> > overtaking each other on preemption") we have restricted requests to run
> > on their chosen engine across preemption events. We can take this
> > restriction into account to know that we will want to resubmit those
> > requests onto the same physical engine, and so can shortcircuit the
> > virtual engine selection process and keep the request on the same
> > engine during unwind.
> >
> > References: e2144503bf3b ("drm/i915: Prevent bonded requests from overtaking each other on preemption")
> > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> > ---
> > drivers/gpu/drm/i915/gt/intel_lrc.c | 6 +++---
> > drivers/gpu/drm/i915/i915_request.c | 2 +-
> > 2 files changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> > index e6bf633b48d5..03732e3f5ec7 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> > @@ -895,7 +895,6 @@ __unwind_incomplete_requests(struct intel_engine_cs *engine)
> > list_for_each_entry_safe_reverse(rq, rn,
> > &engine->active.requests,
> > sched.link) {
> > - struct intel_engine_cs *owner;
> >
> > if (i915_request_completed(rq))
> > continue; /* XXX */
> > @@ -910,8 +909,7 @@ __unwind_incomplete_requests(struct intel_engine_cs *engine)
> > * engine so that it can be moved across onto another physical
> > * engine as load dictates.
> > */
> > - owner = rq->hw_context->engine;
> > - if (likely(owner == engine)) {
> > + if (likely(rq->execution_mask == engine->mask)) {
> > GEM_BUG_ON(rq_prio(rq) == I915_PRIORITY_INVALID);
> > if (rq_prio(rq) != prio) {
> > prio = rq_prio(rq);
> > @@ -922,6 +920,8 @@ __unwind_incomplete_requests(struct intel_engine_cs *engine)
> > list_move(&rq->sched.link, pl);
> > active = rq;
> > } else {
> > + struct intel_engine_cs *owner = rq->hw_context->engine;
>
> I guess there is some benefit in doing fewer operations as long as we
> are fixing the engine anyway (at the moment at least).
>
> However on this branch here the concern was request completion racing
> with preemption handling and with this change the breadcrumb will not
> get canceled any longer and may get signaled on the virtual engine.
> Which then leads to the explosion this branch fixed. At least that's
> what I remembered in combination with the comment below..
No, we don't change back to the virtual engine, so that is not an issue.
The problem was only because of the rq->engine = owner where the
breadcrumbs were still on the previous engine lists and assumed to be
under that engine->breadcrumbs.lock (but would in future be assumed to be
under rq->engine->breadcrumbs.lock).
-Chris
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