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: > > From: Rafael J. Wysocki <rafael.j.wyso...@intel.com> > > > > 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 <rafael.j.wyso...@intel.com> > > --- > > 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. > 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