Re: [PATCH] sched: fix clear NOHZ_BALANCE_KICK

Frederic Weisbecker
2013-06-04T10:26:28Z

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:

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;