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Subject: Re: CFS related question
From: Peter Zijlstra <peterz@infradead.org>
To: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>, LKML <linux-kernel@vger.kernel.org>
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Date: Sat, 18 Oct 2008 22:28:18 +0200
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On Sun, 2008-10-19 at 00:03 +0400, Cyrill Gorcunov wrote:
> Hi Ingo, Peter,
> 
> I just curious, look we have the following
> 
> static struct sched_entity *pick_next_entity(struct cfs_rq *cfs_rq)
> {
> 	struct sched_entity *se = NULL;
> 
> 	if (first_fair(cfs_rq)) {
> 		se = __pick_next_entity(cfs_rq);
> 		se = pick_next(cfs_rq, se);
> 		set_next_entity(cfs_rq, se);
> 	}
> 
> 	return se;
> }
> 
> which I presume may return NULL so the following piece
> could fail
> 
> static struct task_struct *pick_next_task_fair(struct rq *rq)
> {
> 	struct task_struct *p;
> 	struct cfs_rq *cfs_rq = &rq->cfs;
> 	struct sched_entity *se;
> 
> 	if (unlikely(!cfs_rq->nr_running))
> 		return NULL;
> 
> 	do {
> -->		se = pick_next_entity(cfs_rq);
> --> OOPs	cfs_rq = group_cfs_rq(se);
> 	} while (cfs_rq);
> 
> 	p = task_of(se);
> 	hrtick_start_fair(rq, p);
> 
> 	return p;
> }
> 
> Did I miss something? Or it comepletely can NOT happen?

pick_next_entity() only returns NULL when !first_fair(), which is when !
nr_running.

So the initial !nr_running check in pick_next_task_fair() will catch
that. Further nested RQs will never have !nr_running because then they
get dequeued.