[patch 3/6] sched: use rq->avg_event to resurrect nohz ratelimiting

[Mike Galbraith]

Entering nohz at high frequency eats cycles, ratelimit it
just as we do idle_balance(), and for the same reason.

Signed-off-by: Mike Galbraith <efault@xxxxxx>

---
 include/linux/sched.h    |    2 ++
 kernel/sched.c           |    7 +++++++
 kernel/time/tick-sched.c |    2 +-
 3 files changed, 10 insertions(+), 1 deletion(-)

Index: linux-3.2.git/include/linux/sched.h
===================================================================
--- linux-3.2.git.orig/include/linux/sched.h
+++ linux-3.2.git/include/linux/sched.h
@@ -1979,8 +1979,10 @@ static inline void idle_task_exit(void)
 
 #if defined(CONFIG_NO_HZ) && defined(CONFIG_SMP)
 extern void wake_up_idle_cpu(int cpu);
+extern int sched_needs_cpu(int cpu);
 #else
 static inline void wake_up_idle_cpu(int cpu) { }
+static inline void sched_needs_cpu(int cpu) { }
 #endif
 
 extern unsigned int sysctl_sched_latency;
Index: linux-3.2.git/kernel/sched.c
===================================================================
--- linux-3.2.git.orig/kernel/sched.c
+++ linux-3.2.git/kernel/sched.c
@@ -1419,6 +1419,13 @@ static inline bool got_nohz_idle_kick(vo
 	return idle_cpu(smp_processor_id()) && this_rq()->nohz_balance_kick;
 }
 
+#ifdef CONFIG_SMP
+int sched_needs_cpu(int cpu)
+{
+	return cpu_rq(cpu)->avg_event < sysctl_sched_migration_cost;
+}
+#endif
+
 #else /* CONFIG_NO_HZ */
 
 static inline bool got_nohz_idle_kick(void)
Index: linux-3.2.git/kernel/time/tick-sched.c
===================================================================
--- linux-3.2.git.orig/kernel/time/tick-sched.c
+++ linux-3.2.git/kernel/time/tick-sched.c
@@ -352,7 +352,7 @@ void tick_nohz_stop_sched_tick(int inidl
 	} while (read_seqretry(&xtime_lock, seq));
 
 	if (rcu_needs_cpu(cpu) || printk_needs_cpu(cpu) ||
-	    arch_needs_cpu(cpu)) {
+	    arch_needs_cpu(cpu) || sched_needs_cpu(cpu)) {
 		next_jiffies = last_jiffies + 1;
 		delta_jiffies = 1;
 	} else {


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