linux-kernel - Re: [PATCH v2] cpuidle: Add sanity check for exit latency and target residency

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Message-ID: <CAJZ5v0i9DkZR=ZnJ-+VDm-2wk-ab2X=RM69uiSTgwuhGnf8zVg@mail.gmail.com>
Date: Sat, 8 Nov 2025 12:48:22 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
Cc: Linux PM <linux-pm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH v2] cpuidle: Add sanity check for exit latency and target residency

On Sat, Nov 8, 2025 at 12:02 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Sat, Nov 8, 2025 at 9:49 AM Artem Bityutskiy
> <artem.bityutskiy@...ux.intel.com> wrote:
> >
> > On Fri, 2025-11-07 at 20:07 +0100, Rafael J. Wysocki wrote:
> > > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > >
> > > Make __cpuidle_driver_init() fail if the exit latency of one of the
> > > driver's idle states is less than its exit latency which would break
> > > cpuidle assumptions.
> > >
> > > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > LGTM
> >
> > Reviewed-by: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
> >
> >
> > By the way, I have a more paranoid validation patch, which validates
> > latency and also more. Sure I can rebase it later on top of this
> > patch.
>
> That should be rather straightforward.
>
> > I did not have time to polish it yet, but sharing just in case there is
> > a quick feedback.
> >
> > From: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
> > Subject: [PATCH] cpuidle: Add idle states validation
> >
> > Validate the idle states table provided by the underlying idle driver. For
> > example, validate that deeper idle states have greater latency and target
> > residency compared to shallower states.
> >
> > Signed-off-by: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
> > ---
> >  drivers/cpuidle/driver.c | 58 ++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 56 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/cpuidle/driver.c b/drivers/cpuidle/driver.c
> > index 9bbfa594c4425..6bcedad534dd9 100644
> > --- a/drivers/cpuidle/driver.c
> > +++ b/drivers/cpuidle/driver.c
> > @@ -20,6 +20,10 @@
> >
> >  #include "cpuidle.h"
> >
> > +/* Maximum allowed latency and target residency values */
> > +#define MAX_LATENCY 50000 /* 50 milliseconds */
> > +#define MAX_RESIDENCY 1000000 /* 1 second */
> > +
> >  DEFINE_SPINLOCK(cpuidle_driver_lock);
> >
> >  #ifdef CONFIG_CPU_IDLE_MULTIPLE_DRIVERS
> > @@ -148,11 +152,46 @@ static void cpuidle_setup_broadcast_timer(void *arg)
> >                 tick_broadcast_disable();
> >  }
> >
> > +/**
> > + * validate_state - Validate an idle state.
> > + * @state: The C-state to validate.
> > + * @prev_state: The previous idle state or NULL.
> > + *
> > + * Return: 0 if the idle state is valid or -EINVAL otherwise.
> > + */
> > +static int validate_state(struct cpuidle_state *s, struct cpuidle_state *prev_s)
> > +{
> > +       if (s->exit_latency == 0)
> > +               return -EINVAL;
>
> The change above will break the polling state AFAICS.
>
> > +
> > +       if (s->exit_latency > MAX_LATENCY)
> > +               return -EINVAL;
> > +
> > +       if (s->target_residency > MAX_RESIDENCY)
> > +               return -EINVAL;
> > +
> > +       if (s->target_residency < s->exit_latency)
> > +               return -EINVAL;
> > +
> > +       if (!prev_s)
> > +               return 0;
> > +
> > +       if (s->exit_latency <= prev_s->exit_latency)
> > +               return -EINVAL;
>
> Well, is this really necessary?  Nothing depends on this ordering AFAICS.

Yes, it is, there are assumptions in the governors regarding this.
Sorry for the noise.