Re: [RFC][PATCH] Improving directed yield scalability for PLE handler
From: Srikar Dronamraju <hidden>
Date: 2012-09-11 07:08:41
Also in:
lkml
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@@ -4323,6 +4340,10 @@ bool __sched yield_to(struct task_struct *p,bool preempt)quoted
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;quoted
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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)