Re: [PATCH] sched: idle: Avoid retaining the tick when it has been stopped
From: Rafael J. Wysocki <hidden>
Date: 2018-08-17 09:34:23
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On Thursday, August 16, 2018 3:27:24 PM CEST Frederic Weisbecker wrote:
On Thu, Aug 09, 2018 at 07:08:34PM +0200, Rafael J. Wysocki wrote:quoted
From: Rafael J. Wysocki <redacted> If the tick has been stopped already, but the governor has not asked to stop it (which it can do sometimes), the idle loop should invoke tick_nohz_idle_stop_tick(), to let tick_nohz_stop_tick() take care of this case properly. Fixes: 554c8aa8ecad (sched: idle: Select idle state before stopping the tick) Signed-off-by: Rafael J. Wysocki <redacted> --- kernel/sched/idle.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) Index: linux-pm/kernel/sched/idle.c ===================================================================--- linux-pm.orig/kernel/sched/idle.c +++ linux-pm/kernel/sched/idle.c@@ -190,7 +190,7 @@ static void cpuidle_idle_call(void) */ next_state = cpuidle_select(drv, dev, &stop_tick); - if (stop_tick) + if (stop_tick || tick_nohz_tick_stopped()) tick_nohz_idle_stop_tick(); else tick_nohz_idle_retain_tick();So what if tick_nohz_idle_stop_tick() sees no timer to schedule and cancels it, we may remain idle in a shallow state for a long while?
Yes, but the governor is expected to avoid using shallow states when the tick is stopped already.
quoted hunk ↗ jump to hunk
Otherwise we can have something like this:diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index da9455a..408c985 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c@@ -806,6 +806,9 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu) static void tick_nohz_retain_tick(struct tick_sched *ts) { ts->timer_expires_base = 0; + + if (ts->tick_stopped) + tick_nohz_restart(ts, ktime_get()); } #ifdef CONFIG_NO_HZ_FULL
We could do that, but my concern with that approach is that we may end up stopping and starting the tick back and forth without exiting the loop in do_idle() just because somebody uses a periodic timer behind our back and the governor gets confused. Besides, that would be a change in behavior, while the $subject patch simply fixes a mistake in the original design. Cheers, Rafael