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

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Message-ID: <CAJZ5v0giKy-Jm6eXAwvToJhAaWVa1jFU6s_aixCU8BzmPR4+cw@mail.gmail.com>
Date: Thu, 13 Nov 2025 14:26:39 +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 12:47 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> 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 ...

Anyway, I've sent a v2 of the $subject patch with a more elaborate changelog:

https://lore.kernel.org/linux-pm/5955081.DvuYhMxLoT@rafael.j.wysocki/

Hopefully, it is more convincing.