Re: [PATCH v1] cpuidle: Add sanity check for exit latency and target residency

[Rafael J. Wysocki]

On Fri, Nov 7, 2025 at 7:47 PM Artem Bityutskiy
<artem.bityutskiy@xxxxxxxxxxxxxxx> wrote:
>
> On Fri, 2025-11-07 at 19:19 +0100, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> >
> > Make __cpuidle_driver_init() sanitize CPU idle states so that the exit
> > latency of a given state is not greater than its target residency which
> > would break cpuidle assumptions.
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > ---
> >  drivers/cpuidle/driver.c |   11 +++++++++++
> >  1 file changed, 11 insertions(+)
> >
> > --- a/drivers/cpuidle/driver.c
> > +++ b/drivers/cpuidle/driver.c
> > @@ -193,6 +193,16 @@ static void __cpuidle_driver_init(struct
> >                       s->exit_latency_ns =  0;
> >               else
> >                       s->exit_latency = div_u64(s->exit_latency_ns, NSEC_PER_USEC);
> > +
> > +             /*
> > +              * Ensure that the exit latency of a CPU idle state does not
> > +              * exceed its target residency which is assumed in cpuidle in
> > +              * multiple places.
> > +              */
> > +             if (s->exit_latency_ns > s->target_residency_ns) {
> > +                     s->target_residency_ns = s->exit_latency_ns;
> > +                     s->target_residency = s->exit_latency;
> > +             }
>
> I suggest to error out instead of capping it. Because as soon as you
> cap it, you may end up with the target residency of the next C-state.

Good point.

> Just erroring out is very explicit, no surprises, and the table
> provider will have to just fix the table.

Alternatively, the state may just be dropped, but let's try with
failing to begin with.