linux-kernel - The tick when exit idle for nohz

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Date:	Thu, 3 Sep 2015 01:46:24 +0000
From:	"Jiang, Yunhong" <yunhong.jiang@...el.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>
Subject: The tick when exit idle for nohz_full cpu

Hi, Frederic
	I noticed currently the tick_nohz_idle_exit() will always restart the sched_tick, even when it's on the nohz_full cpu. I'm not sure if we can keep the sched_tick stopped if it's on a nohz_full_cpu. The sched tick will be enabled on tick_nohz_task_switch() if needed. Is it ok to keep the sched tick off in the scheduling process?

	Something like followed patch (just to show the idea).

Thanks
--jyh

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 3319e16..7519a0f 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -909,8 +909,9 @@ void tick_nohz_idle_exit(void)
                tick_nohz_stop_idle(ts, now);

        if (ts->tick_stopped) {
-               tick_nohz_restart_sched_tick(ts, now);
                tick_nohz_account_idle_ticks(ts);
+               if (!tick_nohz_full_cpu(smp_processor_id()))
+                       tick_nohz_restart_sched_tick(ts, now);
        }
 
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