Re: [PATCH v1] cpuidle: Do not return from cpuidle_play_dead() on callback failures
From: Peter Zijlstra
Date: Fri Nov 15 2024 - 07:55:55 EST
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@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, Nov 14, 2024 at 06:46:20PM +0100, Rafael J. Wysocki wrote:
> > > From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > >
> > > 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@xxxxxxxxx>
> > > ---
> > > 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.