linux-kernel - Re: [PATCH v1 1/4] cpuidle: governors: teo: Drop incorrect target residency check

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Message-ID: <CAJZ5v0hzuHd9MjHdovFZZwf=wHAQQsVKKbtkHCNtX5M+vZck3g@mail.gmail.com>
Date: Thu, 13 Nov 2025 12:47:30 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Christian Loehle <christian.loehle@....com>
Cc: Linux PM <linux-pm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Reka Norman <rekanorman@...omium.org>
Subject: Re: [PATCH v1 1/4] cpuidle: governors: teo: Drop incorrect target
 residency check

On Thu, Nov 13, 2025 at 12:41 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Thu, Nov 13, 2025 at 12:32 PM Christian Loehle
> <christian.loehle@....com> wrote:
> >
> > On 11/12/25 16:22, Rafael J. Wysocki wrote:
> > > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > >
> > > When the target residency of the current candidate idle state is
> > > greater than the expected time till the closest timer (the sleep
> > > length), it does not matter whether or not the tick has already
> > > been stopped or if it is going to be stopped.  The closest timer
> > > will trigger anyway at its due time, so it does not make sense to
> > > select an idle state with target residency above the sleep length.
> > >
> > > Accordingly, drop the teo_state_ok() check done in that case and
> > > let the governor use the teo_find_shallower_state() return value
> > > as the new candidate idle state index.
> > >
> > > Fixes: 21d28cd2fa5f ("cpuidle: teo: Do not call tick_nohz_get_sleep_length() upfront")
> > > Cc: All applicable <stable@...r.kernel.org>
> > > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > ---
> > >  drivers/cpuidle/governors/teo.c |    7 ++-----
> > >  1 file changed, 2 insertions(+), 5 deletions(-)
> > >
> > > --- a/drivers/cpuidle/governors/teo.c
> > > +++ b/drivers/cpuidle/governors/teo.c
> > > @@ -458,11 +458,8 @@ static int teo_select(struct cpuidle_dri
> > >        * If the closest expected timer is before the target residency of the
> > >        * candidate state, a shallower one needs to be found.
> > >        */
> > > -     if (drv->states[idx].target_residency_ns > duration_ns) {
> > > -             i = teo_find_shallower_state(drv, dev, idx, duration_ns, false);
> > > -             if (teo_state_ok(i, drv))
> > > -                     idx = i;
> > > -     }
> > > +     if (drv->states[idx].target_residency_ns > duration_ns)
> > > +             idx = teo_find_shallower_state(drv, dev, idx, duration_ns, false);
> > >
> > >       /*
> > >        * If the selected state's target residency is below the tick length
> > >
> > >
> > >
> >
> > AFAICT this check was to not be stuck in a shallow state when tick is already disabled.
> > There might be a timer armed in t+500us but that might still get cancelled, which
> > is why we didn't think a below TICK_NSEC 'shallow' state is acceptable?
>
> This is all about hrtimers which are not expected to be canceled too
> often and real energy is wasted here by going too deep if the timer is
> not canceled.

Overall, both teo and menu assume that the timers reported by
tick_nohz_get_sleep_length() will trigger.  Otherwise, calling it
would be kind of pointless ...