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SubjectRe: [patch 1/2] sched: use resched IPI to kick off the nohz idle balance
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Date

Something like so in fact,.. except for the MB bits..

---
Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -1404,6 +1404,18 @@ void wake_up_idle_cpu(int cpu)
smp_send_reschedule(cpu);
}

+static inline bool got_nohz_kick(void)
+{
+ return !!this_rq()->nohz_balance_kick;
+}
+
+#else /* CONFIG_NO_HZ */
+
+static inline bool got_nohz_kick(void)
+{
+ return false;
+}
+
#endif /* CONFIG_NO_HZ */

static u64 sched_avg_period(void)
@@ -2717,7 +2729,7 @@ static void sched_ttwu_pending(void)

void scheduler_ipi(void)
{
- if (llist_empty(&this_rq()->wake_list))
+ if (llist_empty(&this_rq()->wake_list) && got_nohz_kick())
return;

/*
@@ -2734,7 +2746,14 @@ void scheduler_ipi(void)
* somewhat pessimize the simple resched case.
*/
irq_enter();
- sched_ttwu_pending();
+ if (list)
+ sched_ttwu_do_pending(list);
+
+ /*
+ * Check if someone kicked us for doing the nohz idle load balance.
+ */
+ if (unlikely(got_nohz_kick() && idle_cpu(smp_processor_id()) && !need_resched()))
+ raise_softirq_irqoff(SCHED_SOFTIRQ);
irq_exit();
}

@@ -8288,7 +8307,6 @@ void __init sched_init(void)
rq_attach_root(rq, &def_root_domain);
#ifdef CONFIG_NO_HZ
rq->nohz_balance_kick = 0;
- init_sched_softirq_csd(&per_cpu(remote_sched_softirq_cb, i));
#endif
#endif
init_rq_hrtick(rq);
Index: linux-2.6/kernel/sched_fair.c
===================================================================
--- linux-2.6.orig/kernel/sched_fair.c
+++ linux-2.6/kernel/sched_fair.c
@@ -4269,22 +4269,6 @@ static int active_load_balance_cpu_stop(
}

#ifdef CONFIG_NO_HZ
-
-static DEFINE_PER_CPU(struct call_single_data, remote_sched_softirq_cb);
-
-static void trigger_sched_softirq(void *data)
-{
- raise_softirq_irqoff(SCHED_SOFTIRQ);
-}
-
-static inline void init_sched_softirq_csd(struct call_single_data *csd)
-{
- csd->func = trigger_sched_softirq;
- csd->info = NULL;
- csd->flags = 0;
- csd->priv = 0;
-}
-
/*
* idle load balancing details
* - One of the idle CPUs nominates itself as idle load_balancer, while
@@ -4450,11 +4434,14 @@ static void nohz_balancer_kick(int cpu)
}

if (!cpu_rq(ilb_cpu)->nohz_balance_kick) {
- struct call_single_data *cp;
-
cpu_rq(ilb_cpu)->nohz_balance_kick = 1;
- cp = &per_cpu(remote_sched_softirq_cb, cpu);
- __smp_call_function_single(ilb_cpu, cp, 0);
+ /*
+ * Use kick_process instead of resched_cpu.
+ * This way we generate a sched IPI on the target cpu which
+ * is idle. And the softirq performing nohz idle load balance
+ * will be run before returning from the IPI.
+ */
+ kick_process(idle_task(ilb_cpu));
}
return;
}


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