Messages in this thread | | | Date | Mon, 17 Feb 2014 11:31:16 +0800 | From | Michael wang <> | Subject | Re: [RFC PATCH] sched: make sure sched-priority after invoke idle_balance() |
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On 02/14/2014 08:38 PM, Peter Zijlstra wrote: [snip] >> >> This patch will prevent this happen by some rechecking after idle_balance(), it >> utilize the resched-flag for the case when RT/DL task was enqueued but don't ask >> for resched (will that ever happened?). > > I'm not sure this is actually working right; the problem is that while > we do retry on need_resched() in the main schedule() loop, that last > need_resched() is on @next (then current). So clearing/resetting @prev's > need_resched() is not going to trigger that loop. > > Not to mention we explicitly clear @prev's need_resched right after > pick_next_task().
Actually it's not aim at that timing, but consider about the RT case, it won't work as expected anyway...
> > So how about something like this? > > I don't particularly like adding that condition to pick_next_task(); but > the alternative is recursively calling pick_next_task() and while > recursion is strictly limited to the number of sched_classes, it does > feel kinda icky.
Yeah...but it works, the RT stuff is inside the loop and really hard to be handled...
> > Anybody got any preferences? > > --- > Subject: sched: Guarantee task priority in pick_next_task() [snip] > pick_next_task(struct rq *rq, struct task_struct *prev) > { > - const struct sched_class *class; > + const struct sched_class *class = &fair_sched_class; > struct task_struct *p; > > /* > * Optimization: we know that if all tasks are in > * the fair class we can call that function directly: > */ > - if (likely(prev->sched_class == &fair_sched_class && > + if (likely(prev->sched_class == class && > rq->nr_running == rq->cfs.h_nr_running)) { > p = fair_sched_class.pick_next_task(rq, prev); > if (likely(p)) > - return p; > + goto got_task;
Since idle_balance() won't happen in the loop, may be we could use:
if p && p->sched_class == class return p
in here, let it fall down into the loop if p is idle, since that means we got RT/DL and will do this anyway, could save two jump work may be? (and may could combine some code below if so?)
Regards, Michael Wang
> } > > +again: > for_each_class(class) { > p = class->pick_next_task(rq, prev); > if (p) > - return p; > + goto got_task; > } > > BUG(); /* the idle class will always have a runnable task */ > + > +got_task: > + /* > + * See pick_next_task_{fair,rt}(); they return rq->idle in case > + * they want to re-start the task selection. > + */ > + if (unlikely(p->sched_class != class)) > + goto again; > + > + return p; > } > > /* > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -4684,6 +4684,7 @@ pick_next_task_fair(struct rq *rq, struc > struct cfs_rq *cfs_rq = &rq->cfs; > struct sched_entity *se; > struct task_struct *p; > + int new_tasks; > > again: > #ifdef CONFIG_FAIR_GROUP_SCHED > @@ -4782,7 +4783,20 @@ pick_next_task_fair(struct rq *rq, struc > return p; > > idle: > - if (idle_balance(rq)) /* drops rq->lock */ > + /* > + * Because idle_balance() releases (and re-acquires) rq->lock, it is > + * possible for any higher priority task to appear. In that case we > + * must re-start the pick_next_entity() loop. > + */ > + new_tasks = idle_balance(rq); > + > + /* > + * See pick_next_task(); we return rq->idle to restart task selection. > + */ > + if (rq->nr_running != rq->cfs.h_nr_running) > + return rq->idle; > + > + if (new_tasks) > goto again; > > return NULL; > --- a/kernel/sched/rt.c > +++ b/kernel/sched/rt.c > @@ -1360,8 +1360,16 @@ pick_next_task_rt(struct rq *rq, struct > struct task_struct *p; > struct rt_rq *rt_rq = &rq->rt; > > - if (need_pull_rt_task(rq, prev)) > + if (need_pull_rt_task(rq, prev)) { > pull_rt_task(rq); > + /* > + * pull_rt_task() can drop (and re-acquire) rq->lock; this > + * means a dl task can slip in, in which case we need to > + * re-start task selection. > + */ > + if (unlikely(rq->dl.dl_nr_running)) > + return rq->idle; > + } > > if (!rt_rq->rt_nr_running) > return NULL; > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ >
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