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

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Date:	Thu, 16 Dec 2010 19:42:29 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	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 12/16, Peter Zijlstra wrote:
>
> +static int ttwu_force(struct task_struct *p, int wake_flags)
> +{
> +	struct rq *rq;
> +	int ret = 0;
> +
> +	/*
> +	 * Since we've already set TASK_WAKING this task's CPU cannot
> +	 * change from under us.

I think it can. Yes, we've set TASK_WAKING. But, at least the task
itself can change its state back to TASK_RUNNING without calling
schedule. Say, __wait_event()-like code.

> +static int
> +try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>  {
> -	int cpu, orig_cpu, this_cpu, success = 0;
> +	int cpu = task_cpu(p);
>  	unsigned long flags;
> -	unsigned long en_flags = ENQUEUE_WAKEUP;
> -	struct rq *rq;
> +	int success = 0;
> +	int load;
>  
> -	this_cpu = get_cpu();
> -
> -	smp_wmb();
> -	rq = task_rq_lock(p, &flags);
> -	if (!(p->state & state))
> -		goto out;
> +	local_irq_save(flags);
> +	for (;;) {
> +		unsigned int task_state = p->state;
>  
> -	cpu = task_cpu(p);
> +		if (!(task_state & state))
> +			goto out;

Well, this surely breaks the code like

	CONDITION = true;
	wake_up_process(p);

At least we need mb() before we check task_state the first time.

Oleg.

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