> > > @@ -4323,6 +4340,10 @@ bool __sched yield_to(struct task_struct *p,
> > bool preempt)
> > > rq = this_rq();
> > >
> > > again:
> > > + /* optimistic test to avoid taking locks */
> > > + if (!__yield_to_candidate(curr, p))
> > > + goto out_irq;
> > > +
>
> So add something like:
>
> /* Optimistic, if we 'raced' with another yield_to(), don't bother */
> if (p_rq->cfs_rq->skip)
> goto out_irq;
> >
> >
> > > p_rq = task_rq(p);
> > > double_rq_lock(rq, p_rq);
> >
> >
> But I do have a question on this optimization though,.. Why do we check
> p_rq->cfs_rq->skip and not rq->cfs_rq->skip ?
>
> That is, I'd like to see this thing explained a little better.
>
> Does it go something like: p_rq is the runqueue of the task we'd like to
> yield to, rq is our own, they might be the same. If we have a ->skip,
> there's nothing we can do about it, OTOH p_rq having a ->skip and
> failing the yield_to() simply means us picking the next VCPU thread,
> which might be running on an entirely different cpu (rq) and could
> succeed?
>
Oh this made me look back at yield_to() again. I had misread the
yield_to_task_fair() code. I had wrongly thought that both ->skip and
->next buddies for the p_rq would be set. But it looks like only ->next
for the p_rq is set and ->skip is set for rq.
This should also explains why Andrew saw a regression when checking for
->skip flag instead of PF_VCPU.
Can we check for p_rq->cfs.next and bail out if
@@ -4820,6 +4820,23 @@ void __sched yield(void)
}
EXPORT_SYMBOL(yield);
+/*
+ * Tests preconditions required for sched_class::yield_to().
+ */
+static bool __yield_to_candidate(struct task_struct *curr, struct task_struct
*p, struct rq *p_rq)
+{
+ if (!curr->sched_class->yield_to_task)
+ return false;
+
+ if (curr->sched_class != p->sched_class)
+ return false;
+
+ if (task_running(p_rq, p) || p->state)
+ return false;
+
+ return true;
+}
+
/**
* yield_to - yield the current processor to another thread in
* your thread group, or accelerate that thread toward the
@@ -4844,20 +4861,24 @@ bool __sched yield_to(struct task_struct *p, bool
preempt)
again:
p_rq = task_rq(p);
+
+ /* optimistic test to avoid taking locks */
+ if (!__yield_to_candidate(curr, p, p_rq))
+ goto out_irq;
+
+ /* if next buddy is set, assume yield is in progress */
+ if (p_rq->cfs.next)
+ goto out_irq;
+
double_rq_lock(rq, p_rq);
while (task_rq(p) != p_rq) {
double_rq_unlock(rq, p_rq);
goto again;
}
- if (!curr->sched_class->yield_to_task)
- goto out;
-
- if (curr->sched_class != p->sched_class)
- goto out;
-
- if (task_running(p_rq, p) || p->state)
- goto out;
+ /* validate state, holding p_rq ensures p's state cannot change */
+ if (!__yield_to_candidate(curr, p, p_rq))
+ goto out_unlock;
yielded = curr->sched_class->yield_to_task(rq, p, preempt);
if (yielded) {
@@ -4877,8 +4898,9 @@ again:
rq->skip_clock_update = 0;
}
-out:
+out_unlock:
double_rq_unlock(rq, p_rq);
+out_irq:
local_irq_restore(flags);
if (yielded)
--
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