linux-kernel - Re: [RFC/PATCH 3/3] cpuidle: Respect the system-wakeup QoS limit for s2idle

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Message-ID: <CAJZ5v0i0h1RZxr47AVdLsNp6xq1dbKE4xTw6AmEGOr+NPAqD0w@mail.gmail.com>
Date: Mon, 21 Jul 2025 18:04:22 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Ulf Hansson <ulf.hansson@...aro.org>
Cc: "Rafael J . Wysocki" <rafael@...nel.org>, linux-pm@...r.kernel.org, 
	Kevin Hilman <khilman@...libre.com>, Pavel Machek <pavel@...nel.org>, Len Brown <len.brown@...el.com>, 
	Daniel Lezcano <daniel.lezcano@...aro.org>, Saravana Kannan <saravanak@...gle.com>, 
	Maulik Shah <quic_mkshah@...cinc.com>, Prasad Sodagudi <psodagud@...cinc.com>, 
	linux-kernel@...r.kernel.org
Subject: Re: [RFC/PATCH 3/3] cpuidle: Respect the system-wakeup QoS limit for s2idle

On Wed, Jul 16, 2025 at 2:33 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:
>
> A system-wakeup QoS limit may have been requested by user-space. To avoid
> entering a too deep state during s2idle, let's start to take into account
> the QoS limit when selecting a suitable low-power-state.
>
> Signed-off-by: Ulf Hansson <ulf.hansson@...aro.org>
> ---
>  drivers/cpuidle/cpuidle.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index 0835da449db8..5f6dacb5b134 100644
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -190,14 +190,15 @@ static noinstr void enter_s2idle_proper(struct cpuidle_driver *drv,
>   */
>  int cpuidle_enter_s2idle(struct cpuidle_driver *drv, struct cpuidle_device *dev)
>  {
> +       u64 constraint_ns = system_wakeup_latency_qos_limit() * NSEC_PER_USEC;
>         int index;
>
>         /*
> -        * Find the deepest state with ->enter_s2idle present, which guarantees
> -        * that interrupts won't be enabled when it exits and allows the tick to
> -        * be frozen safely.
> +        * Find the deepest state with ->enter_s2idle present that meets the
> +        * system-wakeup QoS limit, which guarantees that interrupts won't be
> +        * enabled when it exits and allows the tick to be frozen safely.
>          */
> -       index = find_deepest_state(drv, dev, U64_MAX, 0, true);
> +       index = find_deepest_state(drv, dev, constraint_ns, 0, true);
>         if (index > 0) {
>                 enter_s2idle_proper(drv, dev, index);
>                 local_irq_enable();

This is not the only place that needs to be patched this way.
cpuidle_idle_call() is another one AFAICS.