linux-kernel - Re: [PATCH RFC] reduce runqueue lock contention

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Date:	Mon, 13 Dec 2010 18:41:44 -0800
From:	Frank Rowand <frank.rowand@...sony.com>
To:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>
CC:	Chris Mason <chris.mason@...cle.com>,
	"axboe@...nel.dk" <axboe@...nel.dk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>,
	Oleg Nesterov <oleg@...hat.com>, tglx <tglx@...utronix.de>
Subject: Re: [PATCH RFC] reduce runqueue lock contention

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?

Thanks!

Frank

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