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SubjectRe: [RFC PATCH] sched: make sure sched-priority after invoke idle_balance()
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;
> --
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