Messages in this thread | | | Date | Tue, 4 Jun 2013 13:15:10 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched: fix clear NOHZ_BALANCE_KICK |
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On Tue, Jun 04, 2013 at 12:26:22PM +0200, Frederic Weisbecker 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:
Yeah, I suppose something like that is a little better.. a few nits though:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 58453b8..330136b 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -630,15 +630,14 @@ void wake_up_nohz_cpu(int cpu) > wake_up_idle_cpu(cpu); > } > > -static inline bool got_nohz_idle_kick(void) > +static inline bool got_nohz_idle_kick(int cpu) > { > - int cpu = smp_processor_id(); > - return idle_cpu(cpu) && test_bit(NOHZ_BALANCE_KICK, nohz_flags(cpu)); > + return test_and_clear_bit(NOHZ_BALANCE_KICK, nohz_flags(cpu)); > } > > #else /* CONFIG_NO_HZ_COMMON */ > > -static inline bool got_nohz_idle_kick(void) > +static inline bool got_nohz_idle_kick(int cpu) > { > return false; > } > @@ -1393,8 +1392,12 @@ 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())) > + int cpu = smp_processor_id(); > + bool idle_kick = got_nohz_idle_kick(cpu);
This puts an unconditional atomic instruction in the IPI path. if (test) clear(); is lots cheaper, esp. since most IPIs won't have this flag set.
> + > + if (!(idle_kick && idle_cpu(cpu)) > + && llist_empty(&this_rq()->wake_list) > + && !tick_nohz_full_cpu(cpu)
What's with this weird operator first split style?
> return; > > /* > +enum idle_balance_type { > + IDLE_BALANCE = 1, > + IDLE_NOHZ_BALANCE = 2, > +};
You might want to update the rq->idle_balance assignment in scheduler_tick() to make sure it uses the right value (it does now, but there's nothing stopping people from changing the values).
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