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Date: Sun, 19 Oct 2008 09:29:58 +0400
From: Cyrill Gorcunov <gorcunov@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: CFS related question
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References: <20081018200319.GC8188@localhost> <1224361698.10548.38.camel@lappy.programming.kicks-ass.net>
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[Peter Zijlstra - Sat, Oct 18, 2008 at 10:28:18PM +0200]
| 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.
| 
| 
| 

Thanks Peter!

		- Cyrill -