nohz: Update tick instead of restarting tick in tick_nohz_idle_exit()

[Yunfeng Ye]

In realtime scenarios, the "nohz_full" parameter is configured. Tick
interference is not expected when there is only one realtime thread.
But when the idle thread is switched to the realtime thread, the tick
timer is restarted always.

So on the nohz full mode, it is unnecessary to restart the tick timer
when there is only one realtime thread. Adding can_stop_full_tick()
before restarting the tick, if it return true, keep tick stopped.

Signed-off-by: Yunfeng Ye <yeyunfeng@xxxxxxxxxx>
---
 kernel/time/tick-sched.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index cc7cba20382e..f664c7735cab 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1208,6 +1208,20 @@ void tick_nohz_idle_restart_tick(void)
 		__tick_nohz_idle_restart_tick(ts, ktime_get());
 }

+static void tick_nohz_update_tick(struct tick_sched *ts, ktime_t now)
+{
+#ifdef CONFIG_NO_HZ_FULL
+	int cpu = smp_processor_id();
+
+	if (tick_nohz_full_cpu(cpu) && can_stop_full_tick(cpu, ts)) {
+		tick_nohz_stop_sched_tick(ts, cpu);
+		tick_nohz_account_idle_ticks(ts);
+		return;
+	}
+#endif
+	__tick_nohz_idle_restart_tick(ts, now);
+}
+
 /**
  * tick_nohz_idle_exit - restart the idle tick from the idle task
  *
@@ -1237,7 +1251,7 @@ void tick_nohz_idle_exit(void)
 		tick_nohz_stop_idle(ts, now);

 	if (tick_stopped)
-		__tick_nohz_idle_restart_tick(ts, now);
+		tick_nohz_update_tick(ts, now);

 	local_irq_enable();
 }
-- 
2.18.4