sched: ratelimit nohz

Entering nohz code on every micro-idle is too expensive to bear.

Signed-off-by: Mike Galbraith <efault@gmx.de>
---
 include/linux/sched.h    |    5 +++++
 kernel/sched/core.c      |    8 ++++++++
 kernel/time/tick-sched.c |    2 +-
 3 files changed, 14 insertions(+), 1 deletion(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2286,6 +2286,11 @@ static inline int set_cpus_allowed_ptr(s
 #ifdef CONFIG_NO_HZ_COMMON
 void calc_load_enter_idle(void);
 void calc_load_exit_idle(void);
+#ifdef CONFIG_SMP
+extern int sched_needs_cpu(int cpu);
+#else
+static inline int sched_needs_cpu(int cpu) { return 0; }
+#endif
 #else
 static inline void calc_load_enter_idle(void) { }
 static inline void calc_load_exit_idle(void) { }
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -577,6 +577,14 @@ static inline bool got_nohz_idle_kick(vo
 	return false;
 }
 
+int sched_needs_cpu(int cpu)
+{
+	if (tick_nohz_full_cpu(cpu))
+		return 0;
+
+	return  cpu_rq(cpu)->avg_idle < sysctl_sched_migration_cost;
+}
+
 #else /* CONFIG_NO_HZ_COMMON */
 
 static inline bool got_nohz_idle_kick(void)
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -676,7 +676,7 @@ static ktime_t tick_nohz_stop_sched_tick
 	} while (read_seqretry(&jiffies_lock, seq));
 	ts->last_jiffies = basejiff;
 
-	if (rcu_needs_cpu(basemono, &next_rcu) ||
+	if (sched_needs_cpu(cpu) || rcu_needs_cpu(basemono, &next_rcu) ||
 	    arch_needs_cpu() || irq_work_needs_cpu()) {
 		next_tick = basemono + TICK_NSEC;
 	} else {