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Date:   Thu, 09 Aug 2018 23:31:46 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH] sched: idle: Reenable sched tick for cpuidle request

On Thursday, August 9, 2018 6:43:55 PM CEST Rafael J. Wysocki wrote:
> On Thu, Aug 9, 2018 at 6:29 PM,  <leo.yan@...aro.org> wrote:
> > On Thu, Aug 09, 2018 at 05:42:30PM +0200, Rafael J. Wysocki wrote:
> >
> > [...]
> >
> >> >> This issue can be easily reproduce with the case on Arm Hikey board: use
> >> >> CPU0 to send IPI to CPU7, CPU7 receives the IPI and in the callback
> >> >> function it start a hrtimer with 4ms, so the 4ms timer delta value can
> >> >> let 'menu' governor to choose deepest state in the next entering idle
> >> >> time.  From then on, CPU7 restarts hrtimer with 1ms interval for total
> >> >> 10 times, so this can utilize the typical pattern in 'menu' governor to
> >> >> have prediction for 1ms duration, finally idle governor is easily to
> >> >> select a shallow state, on Hikey board it usually is to select CPU off
> >> >> state.  From then on, CPU7 stays in this shallow state for long time
> >> >> until there have other interrupts on it.
> >> >
> >> > And which means that the above-mentioned code misses this case.
> >>
> >> And I don't really understand how this happens. :-/
> >>
> >> If menu sees that the tick has been stopped, it sets
> >> data->predicted_us to the minimum of TICK_USEC and
> >> ktime_to_us(delta_next) and the latency requirements comes from PM QoS
> >> (no interactivity boost).  Thus the only case when it will say "do not
> >> stop the tick" is when delta_next is below the tick period length, but
> >> that's OK, because it means that there is a timer pending that much
> >> time away, so it doesn't make sense to select a deeper idle state
> >> then.
> >>
> >> If there is a short-interval timer pending every time we go idle, it
> >> doesn't matter that the tick is stopped really, because the other
> >> timer will wake the CPU up anyway.
> >>
> >> Have I missed anything?
> >
> > Yeah, you miss one case is if there haven't anymore timer event, for this
> > case the ktime_to_us(delta_next) is a quite large value and
> > data->predicted_us will be to set TICK_USEC; if HZ=1000 then TICK_USEC is
> > 1000us, on Hikey board if data->predicted_us is 1000us then it's easily
> > to set shallow state (C1) rather than C2.  Unfortunately, this is the
> > last time the CPU can predict idle state before it will stay in idle
> > for long period.
> 
> Fair enough, but in that case the governor will want the tick to be
> stopped, because expected_interval is TICK_USEC then, so I'm not sure
> how the patch helps?
> 
> > [...]
> >
> >> >> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> >> >> index 1a3e9bd..802286e 100644
> >> >> --- a/kernel/sched/idle.c
> >> >> +++ b/kernel/sched/idle.c
> >> >> @@ -190,10 +190,18 @@ static void cpuidle_idle_call(void)
> >> >>                */
> >> >>               next_state = cpuidle_select(drv, dev, &stop_tick);
> >> >>
> >> >> -             if (stop_tick)
> >> >> +             if (stop_tick) {
> >> >>                       tick_nohz_idle_stop_tick();
> >> >> -             else
> >> >> +             } else {
> >> >> +                     /*
> >> >> +                      * The cpuidle framework says to not stop tick but
> >> >> +                      * the tick has been stopped yet, so restart it.
> >> >> +                      */
> >> >> +                     if (tick_nohz_tick_stopped())
> >> >> +                             tick_nohz_idle_restart_tick();
> >> >
> >> > You need an "else" here IMO as Peter said.
> >
> > Yeah, I have replied to Peter, after we restart the tick, I found must to
> > call tick_retain() to clear 'ts->timer_expires_base' to 0, otherwise
> > tick_nohz_idle_exit() reports warning when exit from idle loop.
> 
> I see now, thanks.
> 
> >> And I really would prefer to avoid restarting the tick here, because
> >> it is overhead and quite likely unnecessary.
> >
> > I understand the logic when read the code, actually I did some experiments
> > on the function menu_select(), in menu_select() function it discards the
> > consideration for typical pattern interval and it also tries to avoid to
> > enable tick and select more shallow state at the bottom of function.  So I
> > agree that in the middle of idles it's redundant to reenable tick and the
> > code is careful thought.
> >
> > But this patch tries to rescue the case at the last time the CPU enter one
> > shallow idle state but without wake up event.
> 
> It is better to avoid entering a shallow state IMO.  Let me think
> about that a bit.

The simple change below should address this issue and I don't quite see
what it can break.  It may cause deeper idle states to be selected with
the tick already stopped, but that really shouldn't be problematic, as
(since the tick has been stopped) there are no strict latency constraints,
so even if there is an early wakeup, we should be able to tolerate the
extra latency just fine.

---
 drivers/cpuidle/governors/menu.c |   10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

Index: linux-pm/drivers/cpuidle/governors/menu.c
===================================================================
--- linux-pm.orig/drivers/cpuidle/governors/menu.c
+++ linux-pm/drivers/cpuidle/governors/menu.c
@@ -349,14 +349,12 @@ static int menu_select(struct cpuidle_dr
 		 * If the tick is already stopped, the cost of possible short
 		 * idle duration misprediction is much higher, because the CPU
 		 * may be stuck in a shallow idle state for a long time as a
-		 * result of it.  In that case say we might mispredict and try
-		 * to force the CPU into a state for which we would have stopped
-		 * the tick, unless a timer is going to expire really soon
-		 * anyway.
+		 * result of it.  In that case say we might mispredict and use
+		 * the known time to the closest timer event for the idle state
+		 * selection.
 		 */
 		if (data->predicted_us < TICK_USEC)
-			data->predicted_us = min_t(unsigned int, TICK_USEC,
-						   ktime_to_us(delta_next));
+			data->predicted_us = ktime_to_us(delta_next);
 	} else {
 		/*
 		 * Use the performance multiplier and the user-configurable

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