Messages in this thread | | | Subject | Re: CFS related question | From | Peter Zijlstra <> | 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.
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