linux-kernel - Re: [RFC][PATCH 2/4] sched: Push put_prev_task() into pick_next

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Date:	Fri, 15 Jun 2012 12:03:10 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Glauber Costa <glommer@...allels.com>
Cc:	mingo@...nel.org, pjt@...gle.com, venki@...gle.com, efault@....de,
	rostedt@...dmis.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 2/4] sched: Push put_prev_task() into
 pick_next_task()

On Fri, 2012-06-15 at 13:49 +0400, Glauber Costa wrote:
> On 06/14/2012 05:29 PM, Peter Zijlstra wrote:
> > +static struct task_struct *
> > +pick_next_task_rt(struct rq *rq, struct task_struct *prev)
> >   {
> > -	struct task_struct *p = _pick_next_task_rt(rq);
> > +	struct task_struct *p;
> > +	struct rt_rq *rt_rq =&rq->rt;
> > +
> > +	if (!rt_rq->rt_nr_running)
> > +		return NULL;
> > +
> > +	if (rt_rq_throttled(rt_rq))
> > +		return NULL;
> > +
> > +	if (prev)
> > +		prev->sched_class->put_prev_task(rq, prev);
> > +
> it might be me, but this one sounds strange. If the responsibility of 
> putting the task now lays with pic_next_task, you can't return NULL 
> without doing this first.

Its the responsibility of the sched_class::pick_next_task implementation
that will return the next task. Not of the first one called.

Clearly this needs a comment.. /me adds.

---
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1096,6 +1096,11 @@ struct sched_class {
 
 	void (*check_preempt_curr) (struct rq *rq, struct task_struct *p, int flags);
 
+	/*
+	 * It is the responsibility of the pick_next_task() method that will
+	 * return the next task to call put_prev_task() on the @prev task or
+	 * something equivalent.
+	 */
 	struct task_struct * (*pick_next_task) (struct rq *rq,
 						struct task_struct *prev);
 	void (*put_prev_task) (struct rq *rq, struct task_struct *p);

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