Re: [PATCH] sched: fix clear NOHZ_BALANCE_KICK

Vincent Guittot
2013-06-04T11:11:51Z

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.

>
> 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);
> +
> + if (!(idle_kick && idle_cpu(cpu))
> + && llist_empty(&this_rq()->wake_list)
> + && !tick_nohz_full_cpu(cpu)
> return;
>
> /*
> @@ -1417,8 +1420,8 @@ void scheduler_ipi(void)
> /*
> * Check if someone kicked us for doing the nohz idle load balance.
> */
> - if (unlikely(got_nohz_idle_kick() && !need_resched())) {
> - this_rq()->idle_balance = 1;
> + if (unlikely(idle_kick && idle_cpu(cpu) && !need_resched())) {
> + this_rq()->idle_balance = IDLE_NOHZ_BALANCE;
> raise_softirq_irqoff(SCHED_SOFTIRQ);
> }
> irq_exit();
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c61a614..816e7b0 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5577,15 +5577,14 @@ out:
> * In CONFIG_NO_HZ_COMMON case, the idle balance kickee will do the
> * rebalancing for all the cpus for whom scheduler ticks are stopped.
> */
> -static void nohz_idle_balance(int this_cpu, enum cpu_idle_type idle)
> +static void nohz_idle_balance(int this_cpu)
> {
> struct rq *this_rq = cpu_rq(this_cpu);
> struct rq *rq;
> int balance_cpu;
>
> - if (idle != CPU_IDLE ||
> - !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)))
> - goto end;
> + if (this_rq->idle_balance != IDLE_NOHZ_BALANCE)
> + return;
>
> for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
> if (balance_cpu == this_cpu || !idle_cpu(balance_cpu))
> @@ -5612,8 +5611,12 @@ static void nohz_idle_balance(int this_cpu, enum cpu_idle_type idle)
> this_rq->next_balance = rq->next_balance;
> }
> nohz.next_balance = this_rq->next_balance;
> -end:
> - clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
> +
> + /* There could be concurrent updates from irqs but we don't care */
> + if (idle_cpu(this_cpu))
> + this_rq->idle_balance = IDLE_BALANCE;
> + else
> + this_rq->idle_balance = 0;
> }
>
> /*
> @@ -5679,7 +5682,7 @@ need_kick:
> return 1;
> }
> #else
> -static void nohz_idle_balance(int this_cpu, enum cpu_idle_type idle) { }
> +static void nohz_idle_balance(int this_cpu) { }
> #endif
>
> /*
> @@ -5700,7 +5703,7 @@ static void run_rebalance_domains(struct softirq_action *h)
> * balancing on behalf of the other idle cpus whose ticks are
> * stopped.
> */
> - nohz_idle_balance(this_cpu, idle);
> + nohz_idle_balance(this_cpu);
> }
>
> static inline int on_null_domain(int cpu)
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index ce39224..e9de976 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -387,6 +387,11 @@ extern struct root_domain def_root_domain;
>
> #endif /* CONFIG_SMP */
>
> +enum idle_balance_type {
> + IDLE_BALANCE = 1,
> + IDLE_NOHZ_BALANCE = 2,
> +};
> +
> /*
> * This is the main, per-CPU runqueue data structure.
> *
> @@ -458,7 +463,7 @@ struct rq {
>
> unsigned long cpu_power;
>
> - unsigned char idle_balance;
> + enum idle_balance_type idle_balance;
> /* For active balancing */
> int post_schedule;
> int active_balance;