Re: [PATCH] sched: fix clear NOHZ_BALANCE_KICK

Peter Zijlstra
2013-06-04T09:36:31Z

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.

---
kernel/sched/core.c | 35 +++++++++++++++++++++++++----------
kernel/sched/fair.c | 6 ++++--
2 files changed, 29 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 142c682..4846223 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -630,15 +630,23 @@ 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));
+ bool nohz_kick = test_bit(NOHZ_BALANCE_KICK, nohz_flags(cpu));
+
+ if (!nohz_kick)
+ return false;
+
+ if (idle_cpu(cpu))
+ return true;
+
+ clear_bit(NOHZ_BALANCE_KICK, nohz_flags(cpu));
+ return false;
}

#else /* CONFIG_NO_HZ_COMMON */

-static inline bool got_nohz_idle_kick(void)
+static inline bool got_nohz_idle_kick(int cpu)
{
return false;
}
@@ -1395,8 +1403,11 @@ 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 this_cpu = smp_processor_id();
+ bool nohz_kick = got_nohz_idle_kick(this_cpu);
+
+ if (llist_empty(&this_rq()->wake_list) && !nohz_kick &&
+ !tick_nohz_full_cpu(this_cpu))
return;

/*
@@ -1413,15 +1424,19 @@ void scheduler_ipi(void)
* somewhat pessimize the simple resched case.
*/
irq_enter();
- tick_nohz_full_check();
+ tick_nohz_full_check(); /* restart tick if required */
sched_ttwu_pending();

/*
* 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;
- raise_softirq_irqoff(SCHED_SOFTIRQ);
+ if (unlikely(nohz_kick)) {
+ if (!need_resched()) {
+ this_rq()->idle_balance = 1;
+ raise_softirq_irqoff(SCHED_SOFTIRQ);
+ } else {
+ clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
+ }
}
irq_exit();
}
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 143dcdb..4e6cff4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5600,8 +5600,10 @@ static void nohz_idle_balance(int this_cpu, enum cpu_idle_type idle)
struct rq *rq;
int balance_cpu;

- if (idle != CPU_IDLE ||
- !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)))
+ if (!test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)))
+ return;
+
+ if (idle != CPU_IDLE)
goto end;

for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {