[Intel-gfx] [PATCH 3/3] drm/i915/execlists: Suppress redundant preemption
Chris Wilson
chris at chris-wilson.co.uk
Wed Jan 23 14:14:05 UTC 2019
Quoting Chris Wilson (2019-01-23 13:47:29)
> Quoting Chris Wilson (2019-01-23 12:36:02)
> > On unwinding the active request we give it a small (limited to internal
> > priority levels) boost to prevent it from being gazumped a second time.
> > However, this means that it can be promoted to above the request that
> > triggered the preemption request, causing a preempt-to-idle cycle for no
> > change. We can avoid this if we take the boost into account when
> > checking if the preemption request is valid.
> >
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > ---
> > drivers/gpu/drm/i915/intel_lrc.c | 29 +++++++++++++++++++++++++----
> > 1 file changed, 25 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> > index d9d744f6ab2c..74726f647e47 100644
> > --- a/drivers/gpu/drm/i915/intel_lrc.c
> > +++ b/drivers/gpu/drm/i915/intel_lrc.c
> > @@ -163,6 +163,8 @@
> > #define WA_TAIL_DWORDS 2
> > #define WA_TAIL_BYTES (sizeof(u32) * WA_TAIL_DWORDS)
> >
> > +#define ACTIVE_PRIORITY (I915_PRIORITY_NEWCLIENT)
> > +
> > static int execlists_context_deferred_alloc(struct i915_gem_context *ctx,
> > struct intel_engine_cs *engine,
> > struct intel_context *ce);
> > @@ -181,13 +183,31 @@ static inline int rq_prio(const struct i915_request *rq)
> > return rq->sched.attr.priority;
> > }
> >
> > +static inline int active_prio(const struct i915_request *rq)
> > +{
> > + int prio = rq_prio(rq);
> > +
> > + /*
> > + * On unwinding the active request, we give it a priority bump
> > + * equivalent to a freshly submitted request. This protects it from
> > + * being gazumped again, but it would be preferrable if we didn't
> > + * let it be gazumped in the first place!
> > + *
> > + * See __unwind_incomplete_requests()
> > + */
> > + if (i915_request_started(rq))
> > + prio |= ACTIVE_PRIORITY;
>
> Hmm, actually we are put to the tail of that priolist so we don't get
> rerun ahead of the preemptee if we end up at the same priority.
> ACTIVE_PRIORITY - 1 would seem to be the right compromise.
gem_sync/switch-default says ACTIVE_PRIORITY though. Hmm.
-Chris
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