linux-kernel - Re: [RFC PATCH 2/3] sched: add yield

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Date:	Wed, 08 Dec 2010 17:59:43 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Avi Kiviti <avi@...hat.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Anthony Liguori <aliguori@...ux.vnet.ibm.com>,
	Paul Turner <pjt@...gle.com>
Subject: Re: [RFC PATCH 2/3] sched: add yield_to function

On 12/08/2010 03:00 PM, Peter Zijlstra wrote:

> Anyway, complete untested and such..

Looks very promising.  I've been making a few changes in the same
direction (except for the fancy CFS bits) and have one way to solve
the one problem you pointed out in your patch.

> +void yield_to(struct task_struct *p)
> +{
...
> +	on_rq = p->se.on_rq;
> +	if (on_rq)
> +		dequeue_task(p_rq, p, 0);
> +
> +	ret = 0;
> +	if (p->sched_class == curr->sched_class&&  curr->sched_class->yield_to)
> +		curr->sched_class->yield_to(p);
> +
> +	if (on_rq)
> +		enqueue_task(p_rq, p, 0);

> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index c886717..8689bcd 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c

> +static void yield_to_fair(struct task_stuct *p)
> +{
> +	struct sched_entity *se =&current->se;
> +	struct sched_entity *p_se =&p->se;
> +	u64 lag0, p_lag0;
> +	s64 lag, p_lag;
> +
> +	lag0 = avg_vruntime(cfs_rq_of(se));
> +	p_lag0 = avg_vruntime(cfs_rq_of(p_se));
> +
> +	lag = se->vruntime - avg_vruntime(cfs_rq);
> +	p_lag = p_se->vruntime - avg_vruntime(p_cfs_rq);
> +
> +	if (p_lag>  lag) { /* if P is owed less service */
> +		se->vruntime = lag0 + p_lag;
> +		p_se->vruntime = p_lag + lag;
> +	}
> +
> +	/*
> +	 * XXX try something smarter here
> +	 */
> +	resched_task(p);
> +	resched_task(current);
> +}

If we do the dequeue_task and enqueue_task here, we can use
check_preempt_curr in yield_to_fair.

Alternatively, we can do the rescheduling from the main
yield_to function, not from yield_to_fair, by calling
check_preempt_curr on p and current after p has been
enqueued.

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