linux-kernel - Re: [RFC][PATCH 5/5] sched: Reduce ttwu rq->lock contention

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Date:	Fri, 17 Dec 2010 11:06:19 +0800
From:	"Yan, Zheng" <zheng.z.yan@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Chris Mason <chris.mason@...cle.com>,
	Frank Rowand <frank.rowand@...sony.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>, Paul Turner <pjt@...gle.com>,
	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 5/5] sched: Reduce ttwu rq->lock contention

On Fri, Dec 17, 2010 at 4:32 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> @@ -953,7 +955,7 @@ static inline struct rq *__task_rq_lock(
>        for (;;) {
>                rq = task_rq(p);
>                raw_spin_lock(&rq->lock);
> -               if (likely(rq == task_rq(p)))
> +               if (likely(rq == task_rq(p)) && !task_is_waking(p))
>                        return rq;
>                raw_spin_unlock(&rq->lock);
>        }
> @@ -973,7 +975,7 @@ static struct rq *task_rq_lock(struct ta
>                local_irq_save(*flags);
>                rq = task_rq(p);
>                raw_spin_lock(&rq->lock);
> -               if (likely(rq == task_rq(p)))
> +               if (likely(rq == task_rq(p)) && !task_is_waking(p))
>                        return rq;
>                raw_spin_unlock_irqrestore(&rq->lock, *flags);
>        }

Looks like nothing prevents ttwu() from changing task's CPU while
some one else is holding task_rq_lock(). Is this OK?

Thanks
Yan, Zheng
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