Quoting Tvrtko Ursulin (2019-01-23 14:08:56)
>
> On 23/01/2019 12:36, Chris Wilson wrote:
> > In order to avoid preempting ourselves, we currently refuse to schedule
> > the tasklet if we reschedule an inflight context. However, this glosses
> > over a few issues such as what happens after a CS completion event and
> > we then preempt the newly executing context with itself, or if something
> > else causes a tasklet_schedule triggering the same evaluation to
> > preempt the active context with itself.
> >
> > To avoid the extra complications, after deciding that we have
> > potentially queued a request with higher priority than the currently
> > executing request, inspect the head of the queue to see if it is indeed
> > higher priority from another context.
> >
> > References: a2bf92e8cc16 ("drm/i915/execlists: Avoid kicking priority on the current context")
> > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> > ---
> > drivers/gpu/drm/i915/i915_scheduler.c | 20 ++++++--
> > drivers/gpu/drm/i915/intel_lrc.c | 67 ++++++++++++++++++++++++---
> > 2 files changed, 76 insertions(+), 11 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_scheduler.c b/drivers/gpu/drm/i915/i915_scheduler.c
> > index 340faea6c08a..fb5d953430e5 100644
> > --- a/drivers/gpu/drm/i915/i915_scheduler.c
> > +++ b/drivers/gpu/drm/i915/i915_scheduler.c
> > @@ -239,6 +239,18 @@ sched_lock_engine(struct i915_sched_node *node, struct intel_engine_cs *locked)
> > return engine;
> > }
> >
> > +static bool inflight(const struct i915_request *rq,
> > + const struct intel_engine_cs *engine)
> > +{
> > + const struct i915_request *active;
> > +
> > + if (!rq->global_seqno)
> > + return false;
> > +
> > + active = port_request(engine->execlists.port);
> > + return active->hw_context == rq->hw_context;
> > +}
> > +
> > static void __i915_schedule(struct i915_request *rq,
> > const struct i915_sched_attr *attr)
> > {
> > @@ -328,6 +340,7 @@ static void __i915_schedule(struct i915_request *rq,
> > INIT_LIST_HEAD(&dep->dfs_link);
> >
> > engine = sched_lock_engine(node, engine);
> > + lockdep_assert_held(&engine->timeline.lock);
> >
> > /* Recheck after acquiring the engine->timeline.lock */
> > if (prio <= node->attr.priority || node_signaled(node))
> > @@ -356,17 +369,16 @@ static void __i915_schedule(struct i915_request *rq,
> > if (prio <= engine->execlists.queue_priority)
> > continue;
> >
> > + engine->execlists.queue_priority = prio;
> > +
> > /*
> > * If we are already the currently executing context, don't
> > * bother evaluating if we should preempt ourselves.
> > */
> > - if (node_to_request(node)->global_seqno &&
> > - i915_seqno_passed(port_request(engine->execlists.port)->global_seqno,
> > - node_to_request(node)->global_seqno))
> > + if (inflight(node_to_request(node), engine))
> > continue;
> >
> > /* Defer (tasklet) submission until after all of our updates. */
> > - engine->execlists.queue_priority = prio;
> > tasklet_hi_schedule(&engine->execlists.tasklet);
> > }
> >
> > diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> > index 8aa8a4862543..d9d744f6ab2c 100644
> > --- a/drivers/gpu/drm/i915/intel_lrc.c
> > +++ b/drivers/gpu/drm/i915/intel_lrc.c
> > @@ -182,12 +182,64 @@ static inline int rq_prio(const struct i915_request *rq)
> > }
> >
> > static inline bool need_preempt(const struct intel_engine_cs *engine,
> > - const struct i915_request *last,
> > - int prio)
> > + const struct i915_request *rq,
> > + int q_prio)
> > {
> > - return (intel_engine_has_preemption(engine) &&
> > - __execlists_need_preempt(prio, rq_prio(last)) &&
> > - !i915_request_completed(last));
> > + const struct intel_context *ctx = rq->hw_context;
> > + const int last_prio = rq_prio(rq);
> > + struct rb_node *rb;
> > + int idx;
> > +
> > + if (!intel_engine_has_preemption(engine))
> > + return false;
> > +
> > + if (i915_request_completed(rq))
> > + return false;
> > +
> > + if (!__execlists_need_preempt(q_prio, last_prio))
> > + return false;
> > +
> > + /*
> > + * The queue_priority is a mere hint that we may need to preempt.
> > + * If that hint is stale or we may be trying to preempt ourselves,
> > + * ignore the request.
> > + */
> > +
> > + list_for_each_entry_continue(rq, &engine->timeline.requests, link) {
> > + GEM_BUG_ON(rq->hw_context == ctx);
>
> Why would there be no more requests belonging to the same context on the
> engine timeline after the first one? Did you mean "if (rq->hw_context ==
> ctx) continue;" ?
We enter the function with rq == execlist->port, i.e. the last request
submitted to ELSP[0]. In this loop, we iterate from the start of ELSP[1]
and all the request here belong to that context. It is illegal for
ELSP[0].lrca == ELSP[1].lrca, i.e. the context must be different.
>
> > + if (rq_prio(rq) > last_prio)
> > + return true;
> > + }
> > +
> > + rb = rb_first_cached(&engine->execlists.queue);
> > + if (!rb)
> > + return false;
> > +
> > + priolist_for_each_request(rq, to_priolist(rb), idx)
> > + return rq->hw_context != ctx && rq_prio(rq) > last_prio;
>
> This isn't equivalent to top of the queue priority
> (engine->execlists.queue_priority)? Apart from the different ctx check.
> So I guess it is easier than storing new engine->execlists.queue_top_ctx
> and wondering about the validity of that pointer.
The problem being that queue_priority may not always match the top of
the execlists->queue. Right, the first attempt I tried was to store the
queue_context matching the queue_priority, but due to the suppression of
inflight preemptions, it doesn't match for long, and is not as accurate
as one would like across CS events.
priolist_for_each_request() isn't too horrible for finding the first
pointer. I noted that we teach it to do: for(idx = __ffs(p->used); ...)
though. If we didn't care about checking hw_context, we can compute the
prio from (p->priority+1)<<SHIFT - ffs(p->used).
-Chris
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