[PATCH 3/5] nohz: Allow to check if remote CPU tick is stopped

[Frederic Weisbecker]

This check is racy but provides a good heuristic to determine whether
a CPU may need a remote tick or not.

Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Chris Metcalf <cmetcalf@xxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Luiz Capitulino <lcapitulino@xxxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Wanpeng Li <kernellwp@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
---
 include/linux/tick.h     | 2 ++
 kernel/time/tick-sched.c | 7 +++++++
 2 files changed, 9 insertions(+)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index f442d1a..417f8d2 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -114,6 +114,7 @@ enum tick_dep_bits {
 #ifdef CONFIG_NO_HZ_COMMON
 extern bool tick_nohz_enabled;
 extern int tick_nohz_tick_stopped(void);
+extern int tick_nohz_tick_stopped_cpu(int cpu);
 extern void tick_nohz_idle_enter(void);
 extern void tick_nohz_idle_exit(void);
 extern void tick_nohz_irq_exit(void);
@@ -124,6 +125,7 @@ extern u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time);
 #else /* !CONFIG_NO_HZ_COMMON */
 #define tick_nohz_enabled (0)
 static inline int tick_nohz_tick_stopped(void) { return 0; }
+static inline int tick_nohz_tick_stopped_cpu(int cpu) { return 0; }
 static inline void tick_nohz_idle_enter(void) { }
 static inline void tick_nohz_idle_exit(void) { }
 
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 99578f0..ad07d6e 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -486,6 +486,13 @@ int tick_nohz_tick_stopped(void)
 	return __this_cpu_read(tick_cpu_sched.tick_stopped);
 }
 
+int tick_nohz_tick_stopped_cpu(int cpu)
+{
+	struct tick_sched *ts = per_cpu_ptr(&tick_cpu_sched, cpu);
+
+	return ts->tick_stopped;
+}
+
 /**
  * tick_nohz_update_jiffies - update jiffies when idle was interrupted
  *
-- 
2.7.4