Re: [PATCH V2 2/3] sched: Fix race in idle_balance()

[Preeti Murthy]

HI Daniel,

Isn't the only scenario where another cpu can put an idle task on
our runqueue, in nohz_idle_balance() where only the cpus in
the nohz.idle_cpus_mask are iterated through. But for the case
that this patch is addressing, the cpu in question is not yet a part
of the nohz.idle_cpus_mask right?

Any other case would trigger load balancing on the same cpu, but
we are preempt_disabled and interrupt disabled at this point.

Thanks

Regards
Preeti U Murthy

On Fri, Feb 7, 2014 at 4:40 AM, Daniel Lezcano
<daniel.lezcano@xxxxxxxxxx> wrote:
> The scheduler main function 'schedule()' checks if there are no more tasks
> on the runqueue. Then it checks if a task should be pulled in the current
> runqueue in idle_balance() assuming it will go to idle otherwise.
>
> But the idle_balance() releases the rq->lock in order to lookup in the sched
> domains and takes the lock again right after. That opens a window where
> another cpu may put a task in our runqueue, so we won't go to idle but
> we have filled the idle_stamp, thinking we will.
>
> This patch closes the window by checking if the runqueue has been modified
> but without pulling a task after taking the lock again, so we won't go to idle
> right after in the __schedule() function.
>
> Cc: alex.shi@xxxxxxxxxx
> Cc: peterz@xxxxxxxxxxxxx
> Cc: mingo@xxxxxxxxxx
> Signed-off-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c |    7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 428bc9d..5ebc681 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6589,6 +6589,13 @@ void idle_balance(struct rq *this_rq)
>
>         raw_spin_lock(&this_rq->lock);
>
> +       /*
> +        * While browsing the domains, we released the rq lock.
> +        * A task could have be enqueued in the meantime
> +        */
> +       if (this_rq->nr_running && !pulled_task)
> +               return;
> +
>         if (pulled_task || time_after(jiffies, this_rq->next_balance)) {
>                 /*
>                  * We are going idle. next_balance may be set based on
> --
> 1.7.9.5
>
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