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>

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 2ec7c00228f3c981568b1dc911b4a91a83148d91..204fdc86863d71368ef61faacdb0d5458e78039f 100644 (file)
--- 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);