[tip:timers/core] tick/nohz: Optimize nohz idle enter

[tip-bot for Gaurav Jindal <tipbot@zytor.com>]

Commit-ID:  1f3b0f8243cb934307f59bd4d8e43b868e61d4d9
Gitweb:     http://git.kernel.org/tip/1f3b0f8243cb934307f59bd4d8e43b868e61d4d9
Author:     Gaurav Jindal <Gaurav.Jindal@spreadtrum.com>
AuthorDate: Thu, 14 Jul 2016 12:04:20 +0000
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Tue, 19 Jul 2016 13:48:24 +0200

tick/nohz: Optimize nohz idle enter

tick_nohz_start_idle is called before checking whether the idle tick can be
stopped. If the tick cannot be stopped, calling tick_nohz_start_idle() is
pointless and just wasting CPU cycles.

Only invoke tick_nohz_start_idle() when can_stop_idle_tick() returns true. A
short one minute observation of the effect on ARM64 shows a reduction of calls
by 1.5% thus optimizing the idle entry sequence.

[tglx: Massaged changelog ]

Co-developed-by: Sanjeev Yadav<sanjeev.yadav@spreadtrum.com>
Signed-off-by: Gaurav Jindal<gaurav.jindal@spreadtrum.com>
Link: http://lkml.kernel.org/r/20160714120416.GB21099@gaurav.jindal@spreadtrum.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

---
 kernel/time/tick-sched.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 2ec7c00..204fdc8 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -908,11 +908,10 @@ static void __tick_nohz_idle_enter(struct tick_sched *ts)
 	ktime_t now, expires;
 	int cpu = smp_processor_id();
 
-	now = tick_nohz_start_idle(ts);
-
 	if (can_stop_idle_tick(cpu, ts)) {
 		int was_stopped = ts->tick_stopped;
 
+		now = tick_nohz_start_idle(ts);
 		ts->idle_calls++;
 
 		expires = tick_nohz_stop_sched_tick(ts, now, cpu);