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

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Message-ID: <CAJZ5v0hVPbU__=LZA1una8+1JSmGFytpBjBEXq2EuL_VOSYw2A@mail.gmail.com>
Date: Fri, 7 Nov 2025 19:58:44 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Linux PM <linux-pm@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH v1] cpuidle: Add sanity check for exit latency and target residency

On Fri, Nov 7, 2025 at 7:47 PM Artem Bityutskiy
<artem.bityutskiy@...ux.intel.com> wrote:
>
> On Fri, 2025-11-07 at 19:19 +0100, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > 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@...el.com>
> > ---
> >  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.