Re: [PATCH RFC] reduce runqueue lock contention

[Mike Galbraith <efault@gmx.de>]

On Mon, 2010-12-13 at 18:41 -0800, Frank Rowand wrote:
> On 06/21/10 06:04, Peter Zijlstra wrote:
> > On Mon, 2010-06-21 at 12:54 +0200, Peter Zijlstra wrote:
> >>> It looses the ttwu task_running() check, as I must admit I'm not quite
> >>> sure what it does.. Ingo?
> > 
> > I think I figured out what its for, its for when p is prev in schedule()
> > after deactivate_task(), we have to call activate_task() it again, but
> > we cannot migrate the task because the CPU its on is still referencing
> > it.
> 
> I have not been able to make sense of the task_running() check in
> try_to_wake_up(), even with that clue.  The try_to_wake_up() code in
> question is:
> 
>         rq = task_rq_lock(p, &flags);
>         if (!(p->state & state))
>                 goto out;
> 
>         if (p->se.on_rq)
>                 goto out_running;
> 
>         cpu = task_cpu(p);
>         orig_cpu = cpu;
> 
> #ifdef CONFIG_SMP
>         if (unlikely(task_running(rq, p)))
>                 goto out_activate;
> 
> 
> The relevent code in schedule() executes with the rq lock held (many
> lines left out to emphasize the key lines):
> 
>         raw_spin_lock_irq(&rq->lock);
> 
>         if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
> 
>                        deactivate_task(rq, prev, DEQUEUE_SLEEP);
> 
>         if (likely(prev != next)) {
>                 rq->curr = next;
>                 context_switch(rq, prev, next); /* unlocks the rq */
>         } else
>                 raw_spin_unlock_irq(&rq->lock);
> 
> 
> If (p->se.on_rq) can becomes false due to deactivate_task()
> then task_running() will also become false while the rq lock is still
> held (either via "rq->curr = next" or via context_switch() updating
> p->oncpu -- which one matters depends on #ifdef __ARCH_WANT_UNLOCKED_CTXSW).
> 
> I  haven't been able to find any case where task_running() can be true
> when (p->se.on_rq) is false, while the rq lock is not being held.  Thus
> I don't think the branch to out_activate will ever be taken.
> 
> What am I missing, or is the task_running() test not needed?

Say the last runnable task passes through schedule(), is deactivated.
We hit idle_balance(), which drops/retakes rq->lock _before_ the task
schedules off.  ttwu() can acquire rq->lock in that window, p->se.on_rq
is false, p->state is true, as is task_current(rq, p).

We have to check whether the task is still current, but not enqueued,
lest the wakeup be a noop, and the wakee possibly then sleep forever.

	-Mike