Date:   Fri, 10 Aug 2018 20:31:43 +0800
From:   leo.yan@...aro.org
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH v2] cpuidle: menu: Handle stopped tick more aggressively

On Fri, Aug 10, 2018 at 01:04:22PM +0200, Rafael J. Wysocki wrote:
> On Fri, Aug 10, 2018 at 11:20 AM <leo.yan@...aro.org> wrote:
> >
> > On Fri, Aug 10, 2018 at 09:57:18AM +0200, Rafael J . Wysocki wrote:
> > > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > Subject: [PATCH] cpuidle: menu: Handle stopped tick more aggressively
> > >
> > > Commit 87c9fe6ee495 (cpuidle: menu: Avoid selecting shallow states
> > > with stopped tick) missed the case when the target residencies of
> > > deep idle states of CPUs are above the tick boundary which may cause
> > > the CPU to get stuck in a shallow idle state for a long time.
> > >
> > > Say there are two CPU idle states available: one shallow, with the
> > > target residency much below the tick boundary and one deep, with
> > > the target residency significantly above the tick boundary.  In
> > > that case, if the tick has been stopped already and the expected
> > > next timer event is relatively far in the future, the governor will
> > > assume the idle duration to be equal to TICK_USEC and it will select
> > > the idle state for the CPU accordingly.  However, that will cause the
> > > shallow state to be selected even though it would have been more
> > > energy-efficient to select the deep one.
> > >
> > > To address this issue, modify the governor to always assume idle
> > > duration to be equal to the time till the closest timer event if
> > > the tick is not running which will cause the selected idle states
> > > to always match the known CPU wakeup time.
> > >
> > > Also make it always indicate that the tick should be stopped in
> > > that case for consistency.
> > >
> > > Fixes: 87c9fe6ee495 (cpuidle: menu: Avoid selecting shallow states with stopped tick)
> > > Reported-by: Leo Yan <leo.yan@...aro.org>
> > > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > ---
> > >
> > > -> v2: Initialize first_idx properly in the stopped tick case.
> > >
> > > ---
> > >  drivers/cpuidle/governors/menu.c |   55 +++++++++++++++++----------------------
> > >  1 file changed, 25 insertions(+), 30 deletions(-)
> > >
> > > Index: linux-pm/drivers/cpuidle/governors/menu.c
> > > ===================================================================
> > > --- linux-pm.orig/drivers/cpuidle/governors/menu.c
> > > +++ linux-pm/drivers/cpuidle/governors/menu.c
> > > @@ -285,9 +285,8 @@ static int menu_select(struct cpuidle_dr
> > >  {
> > >       struct menu_device *data = this_cpu_ptr(&menu_devices);
> > >       int latency_req = cpuidle_governor_latency_req(dev->cpu);
> > > -     int i;
> > > -     int first_idx;
> > > -     int idx;
> > > +     int first_idx = 0;
> > > +     int idx, i;
> > >       unsigned int interactivity_req;
> > >       unsigned int expected_interval;
> > >       unsigned long nr_iowaiters, cpu_load;
> > > @@ -307,6 +306,18 @@ static int menu_select(struct cpuidle_dr
> > >       /* determine the expected residency time, round up */
> > >       data->next_timer_us = ktime_to_us(tick_nohz_get_sleep_length(&delta_next));
> > >
> > > +     /*
> > > +      * 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 use the known time till the closest
> > > +      * timer event for the idle state selection.
> > > +      */
> > > +     if (tick_nohz_tick_stopped()) {
> > > +             data->predicted_us = ktime_to_us(delta_next);
> > > +             goto select;
> > > +     }
> > > +
> >
> > This introduce two potential issues:
> >
> > - This will totally ignore the typical pattern in idle loop; I
> >   observed on the mmc driver can trigger multiple times (> 10 times)
> >   with consistent interval;
> 
> I'm not sure what you mean by "ignore".

You could see after move code from blow to this position, the typical
pattern interval will not be accounted; so if in the middle of idles
there have a bunch of interrupts with fix pattern, the upper code
cannot detect this pattern anymore.

[...]

> > > -     if ((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) ||
> > > -         expected_interval < TICK_USEC) {
> > > +     if (((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) ||
> > > +         expected_interval < TICK_USEC) && !tick_nohz_tick_stopped()) {
> >
> > I am not sure this logic is right... Why not use below checking, so
> > for POLLING state we will never ask to stop the tick?
> >
> >         if (drv->states[idx].flags & CPUIDLE_FLAG_POLLING ||
> >             (expected_interval < TICK_USEC && !tick_nohz_tick_stopped())) {
> >
> 
> The only effect of it would be setting stop_tick to false, but why
> would that matter?

Please consider below situation, not sure if this case is existed or
not:

  step1: first time: enter one idle state with stopping tick;
  step2: second time: select POLLING state and tick_nohz_tick_stopped()
  is true;

So in step2, it cannot set stop_tick to false with below sentence.

> > >               unsigned int delta_next_us = ktime_to_us(delta_next);
> > >
> > >               *stop_tick = false;

[...]

Thanks,
Leo Yan

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