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SubjectRe: CFS related question
[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 -


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