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

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Message-ID: <20241115125523.GD22801@noisy.programming.kicks-ass.net>
Date: Fri, 15 Nov 2024 13:55:23 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: "Rafael J. Wysocki" <rafael@...nel.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 01:46:29PM +0100, Rafael J. Wysocki wrote:
> 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?

Yeah, but perhaps remove the return value entirely if we're going to
ignore it anyway. And then assume that any return is a failure to die.

I mean, something like:

	if (drv->states[i].enter_dead && !drv->states[i].enter_dead(dev, i))
		panic("Dead CPU walking...");

is 'fun' but not very useful.