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

[Daniel Lezcano <daniel.lezcano@linaro.org>]

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@linaro.org> 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@linaro.org
>> Cc: peterz@infradead.org
>> Cc: mingo@kernel.org
>> Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
>> Signed-off-by: Peter Zijlstra <peterz@infradead.org>
>> ---
>>   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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