linux-kernel - Re: [PATCH v1] cpuidle: Do not return from cpuidle_play

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Message-ID: <CAJZ5v0gfSpzjD1PDhMOmqV_wcnCtr=m12noAqVpQkDsjetu+Ug@mail.gmail.com>
Date: Fri, 15 Nov 2024 13:46:29 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>, Linux PM <linux-pm@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, x86 Maintainers <x86@...nel.org>, 
	Patryk Wlazlyn <patryk.wlazlyn@...ux.intel.com>, 
	"Gautham R. Shenoy" <gautham.shenoy@....com>, 
	Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>, 
	Mario Limonciello <mario.limonciello@....com>
Subject: Re: [PATCH v1] cpuidle: Do not return from cpuidle_play_dead() on
 callback failures

On Fri, Nov 15, 2024 at 11:14 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Nov 14, 2024 at 06:46:20PM +0100, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > If the :enter_dead() idle state callback fails for a certain state,
> > there may be still a shallower state for which it will work.
> >
> > Because the only caller of cpuidle_play_dead(), native_play_dead(),
> > falls back to hlt_play_dead() if it returns an error, it should
> > better try all of the idle states for which :enter_dead() is present
> > before failing, so change it accordingly.
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
> >  drivers/cpuidle/cpuidle.c |    7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > Index: linux-pm/drivers/cpuidle/cpuidle.c
> > ===================================================================
> > --- linux-pm.orig/drivers/cpuidle/cpuidle.c
> > +++ linux-pm/drivers/cpuidle/cpuidle.c
> > @@ -70,9 +70,10 @@ int cpuidle_play_dead(void)
> >               return -ENODEV;
> >
> >       /* Find lowest-power state that supports long-term idle */
> > -     for (i = drv->state_count - 1; i >= 0; i--)
> > -             if (drv->states[i].enter_dead)
> > -                     return drv->states[i].enter_dead(dev, i);
> > +     for (i = drv->state_count - 1; i >= 0; i--) {
> > +             if (drv->states[i].enter_dead && !drv->states[i].enter_dead(dev, i))
> > +                     return 0;
> > +     }
>
> Hmm, strictly speaking there is no 'success' return from play_dead(). On
> success, the CPU is dead :-)

Well, would you prefer something like

for (i = drv->state_count - 1; i >= 0; i--) {
        if (drv->states[i].enter_dead)
                drv->states[i].enter_dead(dev, i);
}

and adding a comment before the final return statement that
:enter_dead() only returns on failure?