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Date: Fri, 17 Dec 2010 11:06:19 +0800
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Subject: Re: [RFC][PATCH 5/5] sched: Reduce ttwu rq->lock contention
From: "Yan, Zheng" <zheng.z.yan@linux.intel.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>, Chris Mason <chris.mason@oracle.com>,
        Frank Rowand <frank.rowand@am.sony.com>, Ingo Molnar <mingo@elte.hu>,
        Thomas Gleixner <tglx@linutronix.de>, Mike Galbraith <efault@gmx.de>,
        Paul Turner <pjt@google.com>, Jens Axboe <axboe@kernel.dk>,
        linux-kernel@vger.kernel.org
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On Fri, Dec 17, 2010 at 4:32 AM, Peter Zijlstra <peterz@infradead.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