Re: [PATCH 1/2] cpuidle: Skip miss accounting for timer broadcast fallbacks

[Rafael J. Wysocki (Intel)]

On Fri, Oct 30, 2026 at 9:36 AM Christian Loehle
<christian.loehle@xxxxxxx> wrote:
>
> A failed tick_broadcast_enter() can force a shallower state than the
> governor selected. Exclude successful broadcast fallbacks from above/below
> miss accounting while retaining their usage, residency and governor
> feedback.

Is this a theoretical problem, or can it be observed in practice?

In either case, it would be good to add some information on that here.

> Fixes: 04dab58a39d4 ("cpuidle: Add 'above' and 'below' idle state metrics")
> Signed-off-by: Christian Loehle <christian.loehle@xxxxxxx>
> ---
>  drivers/cpuidle/cpuidle.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index 2d2f40a2cb81..faa29f58738e 100644
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -222,6 +222,7 @@ noinstr int cpuidle_enter_state(struct cpuidle_device *dev,
>
>         struct cpuidle_state *target_state = &drv->states[index];
>         bool broadcast = !!(target_state->flags & CPUIDLE_FLAG_TIMER_STOP);
> +       bool broadcast_failed = false;
>         ktime_t time_start, time_end;
>
>         instrumentation_begin();
> @@ -232,6 +233,7 @@ noinstr int cpuidle_enter_state(struct cpuidle_device *dev,
>          * CPU as a broadcast timer, this call may fail if it is not available.
>          */
>         if (broadcast && tick_broadcast_enter()) {
> +               broadcast_failed = true;
>                 index = find_deepest_state(drv, dev, target_state->exit_latency_ns,
>                                            CPUIDLE_FLAG_TIMER_STOP, false);
>
> @@ -306,6 +308,13 @@ noinstr int cpuidle_enter_state(struct cpuidle_device *dev,
>                 dev->states_usage[entered_state].time_ns += diff;
>                 dev->states_usage[entered_state].usage++;
>
> +               /*
> +                * The broadcast constraint forced a shallower state, do not
> +                * account for it as a cpu_idle_miss.
> +                */
> +               if (broadcast_failed)
> +                       goto out;
> +
>                 if (diff < drv->states[entered_state].target_residency_ns) {
>                         for (i = entered_state - 1; i >= 0; i--) {
>                                 if (dev->states_usage[i].disable)
> @@ -338,6 +347,7 @@ noinstr int cpuidle_enter_state(struct cpuidle_device *dev,
>                 dev->states_usage[index].rejected++;
>         }
>
> +out:
>         instrumentation_end();
>
>         return entered_state;
> --
> 2.34.1
>