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

[Daniel Lezcano]

On 02/10/2014 10:24 AM, Preeti Murthy wrote:
> HI Daniel,
>
> Isn't the only scenario where another cpu can put an idle task on
> our runqueue,

Well, I am not sure to understand what you meant, but I assume you are 
asking if it is possible to have a task to be pulled when we are idle, 
right ?

This patch fixes the race when the current cpu is *about* to enter idle 
when calling schedule().


> 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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