sched: New helper to prevent from stopping the tick in full dynticks

Provide a new helper to be called from the full dynticks engine
before stopping the tick in order to make sure we don't stop
it when there is more than one task running on the CPU.

This way we make sure that the tick stays alive to maintain
fairness.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Geoff Levand <geoff@infradead.org>
Cc: Gilad Ben Yossef <gilad@benyossef.com>
Cc: Hakan Akkan <hakanakkan@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1ff9e0a5de27f95e08452275b4e8d82bf6b45a27..a74adedcdd10b0bea041ad1aa5bc814eb718458d 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1856,6 +1856,12 @@ extern void wake_up_nohz_cpu(int cpu);
 static inline void wake_up_nohz_cpu(int cpu) { }
 #endif
 
+#ifdef CONFIG_NO_HZ_FULL
+extern bool sched_can_stop_tick(void);
+#else
+static inline bool sched_can_stop_tick(void) { return false; }
+#endif
+
 #ifdef CONFIG_SCHED_AUTOGROUP
 extern void sched_autogroup_create_attach(struct task_struct *p);
 extern void sched_autogroup_detach(struct task_struct *p);

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0f0a5b3fd62c8c6ddeffc2d0ee1ec840f596eafc..69f71335984f42463c48f4b9e3d81e9b23297d55 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -650,6 +650,24 @@ static inline bool got_nohz_idle_kick(void)
 
 #endif /* CONFIG_NO_HZ_COMMON */
 
+#ifdef CONFIG_NO_HZ_FULL
+bool sched_can_stop_tick(void)
+{
+       struct rq *rq;
+
+       rq = this_rq();
+
+       /* Make sure rq->nr_running update is visible after the IPI */
+       smp_rmb();
+
+       /* More than one running task need preemption */
+       if (rq->nr_running > 1)
+               return false;
+
+       return true;
+}
+#endif /* CONFIG_NO_HZ_FULL */
+
 void sched_avg_update(struct rq *rq)
 {
        s64 period = sched_avg_period();