Quoting Chris Wilson (2019-01-23 14:14:05)
> 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@xxxxxxxxxxxxxxxxxx>
> > > ---
> > > 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.
The answer is don't be lazy.
- if (i915_request_started(rq))
+ if ((prio & ACTIVE_PRIORITY) != ACTIVE_PRIORITY &&
+ i915_request_started(rq)) {
prio |= ACTIVE_PRIORITY;
+ prio--;
+ }
That doesn't break switch-default while providing a more accurate
estimate of prio after preemption.
-Chris
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