Messages in this thread | | | Subject | Re: [RFC][PATCH 5/5] sched: Reduce ttwu rq->lock contention | From | Peter Zijlstra <> | Date | Fri, 17 Dec 2010 19:15:40 +0100 |
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On Fri, 2010-12-17 at 18:43 +0100, Peter Zijlstra wrote: > > Hrmph, so is it only about serializing concurrent wakeups? If so, we > could possibly hold p->pi_lock over the wakeup.
Something like the below.. except it still suffers from the __migrate_task() hole you identified in your other email.
By fully serializing all wakeups using ->pi_lock it becomes a lot simpler (although I just realized we might have a problem with try_to_wake_up_local).
static int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) { unsigned long flags; int cpu, ret = 0;
smp_wmb(); raw_spin_lock_irqsave(&p->pi_lock, flags);
if (!(p->state & state)) goto unlock;
ret = 1; /* we qualify as a proper wakeup now */
if (p->se.on_rq && ttwu_force(p, state, wake_flags)) goto unlock;
p->sched_contributes_to_load = !!task_contributes_to_load(p);
/* * In order to serialize against other tasks wanting to task_rq_lock() * we need to wait until the current task_rq(p)->lock holder goes away, * so that the next might observe TASK_WAKING. */ p->state = TASK_WAKING; smp_wmb(); raw_spin_unlock_wait(&task_rq(p)->lock);
/* * Stable, now that TASK_WAKING is visible. */ cpu = task_cpu(p);
#ifdef CONFIG_SMP /* * Catch the case where schedule() has done the dequeue but hasn't yet * scheduled to a new task, in that case p is still being referenced * by that cpu so we cannot wake it to any other cpu. * * Here we must either do a full remote enqueue, or simply wait for * the remote cpu to finish the schedule(), the latter was found to * be cheapest. */ while (p->oncpu) cpu_relax();
if (p->sched_class->task_waking) p->sched_class->task_waking(p);
cpu = select_task_rq(p, SD_BALANCE_WAKE, wake_flags); #endif ttwu_queue(p, cpu); ttwu_stat(p, cpu, wake_flags); unlock: raw_spin_unlock_irqrestore(&p->pi_lock, flags);
return ret; }
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