Re: [PATCH] sched: fix clear NOHZ_BALANCE_KICK

Vincent Guittot
2013-06-04T11:48:52Z

On 4 June 2013 13:19, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> On Tue, Jun 04, 2013 at 01:11:47PM +0200, Vincent Guittot wrote:
>> On 4 June 2013 12:26, Frederic Weisbecker <fweisbec@gmail.com> wrote:
>> > On Tue, Jun 04, 2013 at 11:36:11AM +0200, Peter Zijlstra wrote:
>> >>
>> >> The best I can seem to come up with is something like the below; but I think
>> >> its ghastly. Surely we can do something saner with that bit.
>> >>
>> >> Having to clear it at 3 different places is just wrong.
>> >
>> > We could clear the flag early in scheduler_ipi() and set some
>> > specific value in rq->idle_balance that tells we want nohz idle
>> > balancing from the softirq, something like this untested:
>>
>> I'm not sure that we can have less than 2 places to clear it: cancel
>> place or acknowledge place otherwise we can face a situation where
>> idle load balance will be triggered 2 consecutive times because
>> NOHZ_BALANCE_KICK will be cleared before the idle load balance has
>> been done and had a chance to migrate tasks.
>
> I guess it depends what is the minimum value of rq->next_balance, it seems
> to be large enough to avoid this kind of incident. Although I don't
> know well the whole logic with rq->next_balance and ilb trigger so I must
> defer to you.

In the trace that was showing the issue, i can see that both CPU0 and
CPU1 were trying to trig ILB almost simultaneously and the
test_and_set NOHZ_BALANCE_KICK filters one request so i would say that
clearing the bit before the end of the idle load balance sequence can
generate such sequence

In the sequence below, i have minimized the clear of NOHZ_BALANCE_KICK
in 2 places : acknowledge and cancel. I have reused part of the
proposal from peter which clears the bit if the condition doesn't
match but i have reordered the tests to done that only if all other
condition are matching

static inline bool got_nohz_idle_kick(void)
{
- int cpu = smp_processor_id();
- return idle_cpu(cpu) && test_bit(NOHZ_BALANCE_KICK, nohz_flags(cpu));
+ bool nohz_kick = test_bit(NOHZ_BALANCE_KICK, nohz_flags(cpu));
+
+ if (!nohz_kick)
+ return false;
+
+ if (idle_cpu(cpu) && !need_resched())
+ return true;
+
+ clear_bit(NOHZ_BALANCE_KICK, nohz_flags(cpu));
+ return false;
}

#else /* CONFIG_NO_HZ_COMMON */
@@ -1393,8 +1401,9 @@ static void sched_ttwu_pending(void)

void scheduler_ipi(void)
{
- if (llist_empty(&this_rq()->wake_list) && !got_nohz_idle_kick()
- && !tick_nohz_full_cpu(smp_processor_id()))
+ if (llist_empty(&this_rq()->wake_list)
+ && !tick_nohz_full_cpu(smp_processor_id())
+ && !got_nohz_idle_kick())
return;

/*
@@ -1417,7 +1426,7 @@ void scheduler_ipi(void)
/*
* Check if someone kicked us for doing the nohz idle load balance.
*/
- if (unlikely(got_nohz_idle_kick() && !need_resched())) {
+ if (unlikely(got_nohz_idle_kick())) {
this_rq()->idle_balance = 1;
raise_softirq_irqoff(SCHED_SOFTIRQ);
}