lkml.org 
[lkml]   [2010]   [Dec]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [RFC][PATCH 5/5] sched: Reduce ttwu rq->lock contention
From
Date
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;
}



\
 
 \ /
  Last update: 2010-12-17 19:19    [W:1.074 / U:1.120 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site