Re: [patch 2/2] sched/idle: Make default_idle_call() NOHZ aware

Re: [patch 2/2] sched/idle: Make default_idle_call() NOHZ aware

[Rafael J. Wysocki]

On Wednesday, March 4, 2026 4:03:06 AM CET Qais Yousef wrote:
> On 03/02/26 22:25, Rafael J. Wysocki wrote:
> > On Mon, Mar 2, 2026 at 12:04 PM Christian Loehle
> > <christian.loehle@arm.com> wrote:
> > >
> > > On 3/1/26 19:30, Thomas Gleixner wrote:
> > > > Guests fall back to default_idle_call() as there is no cpuidle driver
> > > > available to them by default. That causes a problem in fully loaded
> > > > scenarios where CPUs go briefly idle for a couple of microseconds:
> > > >
> > > > tick_nohz_idle_stop_tick() is invoked unconditionally which means unless
> > > > there is timer pending in the next tick, the tick is stopped and a couple
> > > > of microseconds later when the idle condition goes away restarted. That
> > > > requires to program the clockevent device twice which implies a VM exit for
> > > > each reprogramming.
> > > >
> > > > It was suggested to remove the tick_nohz_idle_stop_tick() invocation from
> > > > the default idle code, but would be counterproductive. It would not allow
> > > > the host to go into deeper idle states when the guest CPU is fully idle as
> > > > it has to maintain the periodic tick.
> > > >
> > > > Cure this by implementing a trivial moving average filter which keeps track
> > > > of the recent idle recidency time and only stop the tick when the average
> > > > is larger than a tick.
> > > >
> > > > Signed-off-by: Thomas Gleixner <tglx@kernel.org>
> > > > ---
> > > >  kernel/sched/idle.c |   65 +++++++++++++++++++++++++++++++++++++++++++++-------
> > > >  1 file changed, 57 insertions(+), 8 deletions(-)
> > > >
> > > > --- a/kernel/sched/idle.c
> > > > +++ b/kernel/sched/idle.c
> > > > @@ -105,12 +105,7 @@ static inline void cond_tick_broadcast_e
> > > >  static inline void cond_tick_broadcast_exit(void) { }
> > > >  #endif /* !CONFIG_GENERIC_CLOCKEVENTS_BROADCAST_IDLE */
> > > >
> > > > -/**
> > > > - * default_idle_call - Default CPU idle routine.
> > > > - *
> > > > - * To use when the cpuidle framework cannot be used.
> > > > - */
> > > > -static void __cpuidle default_idle_call(void)
> > > > +static void __cpuidle __default_idle_call(void)
> > > >  {
> > > >       instrumentation_begin();
> > > >       if (!current_clr_polling_and_test()) {
> > > > @@ -130,6 +125,61 @@ static void __cpuidle default_idle_call(
> > > >       instrumentation_end();
> > > >  }
> > > >
> > > > +#ifdef CONFIG_NO_HZ_COMMON
> > > > +
> > > > +/* Limit to 4 entries so it fits in a cache line */
> > > > +#define IDLE_DUR_ENTRIES     4
> > > > +#define IDLE_DUR_MASK                (IDLE_DUR_ENTRIES - 1)
> > > > +
> > > > +struct idle_nohz_data {
> > > > +     u64             duration[IDLE_DUR_ENTRIES];
> > > > +     u64             entry_time;
> > > > +     u64             sum;
> > > > +     unsigned int    idx;
> > > > +};
> > > > +
> > > > +static DEFINE_PER_CPU_ALIGNED(struct idle_nohz_data, nohz_data);
> > > > +
> > > > +/**
> > > > + * default_idle_call - Default CPU idle routine.
> > > > + *
> > > > + * To use when the cpuidle framework cannot be used.
> > > > + */
> > > > +static void default_idle_call(void)
> > > > +{
> > > > +     struct idle_nohz_data *nd = this_cpu_ptr(&nohz_data);
> > > > +     unsigned int idx = nd->idx;
> > > > +     s64 delta;
> > > > +
> > > > +     /*
> > > > +      * If the CPU spends more than a tick on average in idle, try to stop
> > > > +      * the tick.
> > > > +      */
> > > > +     if (nd->sum > TICK_NSEC * IDLE_DUR_ENTRIES)
> > > > +             tick_nohz_idle_stop_tick();
> > > > +
> > > > +     __default_idle_call();
> > > > +
> > > > +     /*
> > > > +      * Build a moving average of the time spent in idle to prevent stopping
> > > > +      * the tick on a loaded system which only goes idle briefly.
> > > > +      */
> > > > +     delta = max(sched_clock() - nd->entry_time, 0);
> > > > +     nd->sum += delta - nd->duration[idx];
> > > > +     nd->duration[idx] = delta;
> > > > +     nd->idx = (idx + 1) & IDLE_DUR_MASK;
> > > > +}
> > > > +
> > > > +static void default_idle_enter(void)
> > > > +{
> > > > +     this_cpu_write(nohz_data.entry_time, sched_clock());
> > > > +}
> > > > +
> > > > +#else  /* CONFIG_NO_HZ_COMMON */
> > > > +static inline void default_idle_call(void { __default_idle_call(); }
> > > > +static inline void default_idle_enter(void) { }
> > > > +#endif /* !CONFIG_NO_HZ_COMMON */
> > > > +
> > > >  static int call_cpuidle_s2idle(struct cpuidle_driver *drv,
> > > >                              struct cpuidle_device *dev,
> > > >                              u64 max_latency_ns)
> > > > @@ -186,8 +236,6 @@ static void cpuidle_idle_call(void)
> > > >       }
> > > >
> > > >       if (cpuidle_not_available(drv, dev)) {
> > > > -             tick_nohz_idle_stop_tick();
> > > > -
> > > >               default_idle_call();
> > > >               goto exit_idle;
> > > >       }
> > > > @@ -276,6 +324,7 @@ static void do_idle(void)
> > > >
> > > >       __current_set_polling();
> > > >       tick_nohz_idle_enter();
> > > > +     default_idle_enter();
> > > >
> > > >       while (!need_resched()) {
> > > >
> > > >
> > >
> > > How does this work? We don't stop the tick until the average idle time is larger,
> > > but if we don't stop the tick how is that possible?
> > >
> > > Why don't we just require one or two consecutive tick wakeups before stopping?
> > 
> > Exactly my thought and I think one should be sufficient.
> 
> I concur. From our experience with TEO util threshold these averages can
> backfire. I think one tick is sufficient delay to not be obviously broken.

So if I'm not mistaken, it would be something like the appended prototype
(completely untested, but it builds for me).

---
 drivers/cpuidle/cpuidle.c |   10 ----------
 kernel/sched/idle.c       |   32 ++++++++++++++++++++++++--------
 2 files changed, 24 insertions(+), 18 deletions(-)

--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c
@@ -359,16 +359,6 @@ noinstr int cpuidle_enter_state(struct c
 int cpuidle_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 		   bool *stop_tick)
 {
-	/*
-	 * If there is only a single idle state (or none), there is nothing
-	 * meaningful for the governor to choose. Skip the governor and
-	 * always use state 0 with the tick running.
-	 */
-	if (drv->state_count <= 1) {
-		*stop_tick = false;
-		return 0;
-	}
-
 	return cpuidle_curr_governor->select(drv, dev, stop_tick);
 }
 
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -161,6 +161,14 @@ static int call_cpuidle(struct cpuidle_d
 	return cpuidle_enter(drv, dev, next_state);
 }
 
+static void idle_call_stop_or_retain_tick(bool stop_tick)
+{
+	if (stop_tick || tick_nohz_tick_stopped())
+		tick_nohz_idle_stop_tick();
+	else
+		tick_nohz_idle_retain_tick();
+}
+
 /**
  * cpuidle_idle_call - the main idle function
  *
@@ -170,7 +178,7 @@ static int call_cpuidle(struct cpuidle_d
  * set, and it returns with polling set.  If it ever stops polling, it
  * must clear the polling bit.
  */
-static void cpuidle_idle_call(void)
+static void cpuidle_idle_call(bool got_tick)
 {
 	struct cpuidle_device *dev = cpuidle_get_device();
 	struct cpuidle_driver *drv = cpuidle_get_cpu_driver(dev);
@@ -186,7 +194,7 @@ static void cpuidle_idle_call(void)
 	}
 
 	if (cpuidle_not_available(drv, dev)) {
-		tick_nohz_idle_stop_tick();
+		idle_call_stop_or_retain_tick(!got_tick);
 
 		default_idle_call();
 		goto exit_idle;
@@ -221,7 +229,7 @@ static void cpuidle_idle_call(void)
 
 		next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
 		call_cpuidle(drv, dev, next_state);
-	} else {
+	} else if (drv->state_count > 1) {
 		bool stop_tick = true;
 
 		/*
@@ -229,16 +237,22 @@ static void cpuidle_idle_call(void)
 		 */
 		next_state = cpuidle_select(drv, dev, &stop_tick);
 
-		if (stop_tick || tick_nohz_tick_stopped())
-			tick_nohz_idle_stop_tick();
-		else
-			tick_nohz_idle_retain_tick();
+		idle_call_stop_or_retain_tick(stop_tick);
 
 		entered_state = call_cpuidle(drv, dev, next_state);
 		/*
 		 * Give the governor an opportunity to reflect on the outcome
 		 */
 		cpuidle_reflect(dev, entered_state);
+	} else {
+		/*
+		 * If there is only a single idle state (or none), there is
+		 * nothing meaningful for the governor to choose.  Skip the
+		 * governor and always use state 0.
+		 */
+		idle_call_stop_or_retain_tick(!got_tick);
+
+		call_cpuidle(drv, dev, 0);
 	}
 
 exit_idle:
@@ -259,6 +273,7 @@ exit_idle:
 static void do_idle(void)
 {
 	int cpu = smp_processor_id();
+	bool got_tick = false;
 
 	/*
 	 * Check if we need to update blocked load
@@ -329,8 +344,9 @@ static void do_idle(void)
 			tick_nohz_idle_restart_tick();
 			cpu_idle_poll();
 		} else {
-			cpuidle_idle_call();
+			cpuidle_idle_call(got_tick);
 		}
+		got_tick = tick_nohz_idle_got_tick();
 		arch_cpu_idle_exit();
 	}
 

Re: [patch 2/2] sched/idle: Make default_idle_call() NOHZ aware

[Rafael J. Wysocki]

On Fri, Mar 6, 2026 at 10:21 PM Rafael J. Wysocki <rafael@kernel.org> wrote:
>
> On Wednesday, March 4, 2026 4:03:06 AM CET Qais Yousef wrote:
> > On 03/02/26 22:25, Rafael J. Wysocki wrote:
> > > On Mon, Mar 2, 2026 at 12:04 PM Christian Loehle
> > > <christian.loehle@arm.com> wrote:
> > > >
> > > > On 3/1/26 19:30, Thomas Gleixner wrote:
> > > > > Guests fall back to default_idle_call() as there is no cpuidle driver
> > > > > available to them by default. That causes a problem in fully loaded
> > > > > scenarios where CPUs go briefly idle for a couple of microseconds:
> > > > >
> > > > > tick_nohz_idle_stop_tick() is invoked unconditionally which means unless
> > > > > there is timer pending in the next tick, the tick is stopped and a couple
> > > > > of microseconds later when the idle condition goes away restarted. That
> > > > > requires to program the clockevent device twice which implies a VM exit for
> > > > > each reprogramming.
> > > > >
> > > > > It was suggested to remove the tick_nohz_idle_stop_tick() invocation from
> > > > > the default idle code, but would be counterproductive. It would not allow
> > > > > the host to go into deeper idle states when the guest CPU is fully idle as
> > > > > it has to maintain the periodic tick.
> > > > >
> > > > > Cure this by implementing a trivial moving average filter which keeps track
> > > > > of the recent idle recidency time and only stop the tick when the average
> > > > > is larger than a tick.
> > > > >
> > > > > Signed-off-by: Thomas Gleixner <tglx@kernel.org>
> > > > > ---
> > > > >  kernel/sched/idle.c |   65 +++++++++++++++++++++++++++++++++++++++++++++-------
> > > > >  1 file changed, 57 insertions(+), 8 deletions(-)
> > > > >
> > > > > --- a/kernel/sched/idle.c
> > > > > +++ b/kernel/sched/idle.c
> > > > > @@ -105,12 +105,7 @@ static inline void cond_tick_broadcast_e
> > > > >  static inline void cond_tick_broadcast_exit(void) { }
> > > > >  #endif /* !CONFIG_GENERIC_CLOCKEVENTS_BROADCAST_IDLE */
> > > > >
> > > > > -/**
> > > > > - * default_idle_call - Default CPU idle routine.
> > > > > - *
> > > > > - * To use when the cpuidle framework cannot be used.
> > > > > - */
> > > > > -static void __cpuidle default_idle_call(void)
> > > > > +static void __cpuidle __default_idle_call(void)
> > > > >  {
> > > > >       instrumentation_begin();
> > > > >       if (!current_clr_polling_and_test()) {
> > > > > @@ -130,6 +125,61 @@ static void __cpuidle default_idle_call(
> > > > >       instrumentation_end();
> > > > >  }
> > > > >
> > > > > +#ifdef CONFIG_NO_HZ_COMMON
> > > > > +
> > > > > +/* Limit to 4 entries so it fits in a cache line */
> > > > > +#define IDLE_DUR_ENTRIES     4
> > > > > +#define IDLE_DUR_MASK                (IDLE_DUR_ENTRIES - 1)
> > > > > +
> > > > > +struct idle_nohz_data {
> > > > > +     u64             duration[IDLE_DUR_ENTRIES];
> > > > > +     u64             entry_time;
> > > > > +     u64             sum;
> > > > > +     unsigned int    idx;
> > > > > +};
> > > > > +
> > > > > +static DEFINE_PER_CPU_ALIGNED(struct idle_nohz_data, nohz_data);
> > > > > +
> > > > > +/**
> > > > > + * default_idle_call - Default CPU idle routine.
> > > > > + *
> > > > > + * To use when the cpuidle framework cannot be used.
> > > > > + */
> > > > > +static void default_idle_call(void)
> > > > > +{
> > > > > +     struct idle_nohz_data *nd = this_cpu_ptr(&nohz_data);
> > > > > +     unsigned int idx = nd->idx;
> > > > > +     s64 delta;
> > > > > +
> > > > > +     /*
> > > > > +      * If the CPU spends more than a tick on average in idle, try to stop
> > > > > +      * the tick.
> > > > > +      */
> > > > > +     if (nd->sum > TICK_NSEC * IDLE_DUR_ENTRIES)
> > > > > +             tick_nohz_idle_stop_tick();
> > > > > +
> > > > > +     __default_idle_call();
> > > > > +
> > > > > +     /*
> > > > > +      * Build a moving average of the time spent in idle to prevent stopping
> > > > > +      * the tick on a loaded system which only goes idle briefly.
> > > > > +      */
> > > > > +     delta = max(sched_clock() - nd->entry_time, 0);
> > > > > +     nd->sum += delta - nd->duration[idx];
> > > > > +     nd->duration[idx] = delta;
> > > > > +     nd->idx = (idx + 1) & IDLE_DUR_MASK;
> > > > > +}
> > > > > +
> > > > > +static void default_idle_enter(void)
> > > > > +{
> > > > > +     this_cpu_write(nohz_data.entry_time, sched_clock());
> > > > > +}
> > > > > +
> > > > > +#else  /* CONFIG_NO_HZ_COMMON */
> > > > > +static inline void default_idle_call(void { __default_idle_call(); }
> > > > > +static inline void default_idle_enter(void) { }
> > > > > +#endif /* !CONFIG_NO_HZ_COMMON */
> > > > > +
> > > > >  static int call_cpuidle_s2idle(struct cpuidle_driver *drv,
> > > > >                              struct cpuidle_device *dev,
> > > > >                              u64 max_latency_ns)
> > > > > @@ -186,8 +236,6 @@ static void cpuidle_idle_call(void)
> > > > >       }
> > > > >
> > > > >       if (cpuidle_not_available(drv, dev)) {
> > > > > -             tick_nohz_idle_stop_tick();
> > > > > -
> > > > >               default_idle_call();
> > > > >               goto exit_idle;
> > > > >       }
> > > > > @@ -276,6 +324,7 @@ static void do_idle(void)
> > > > >
> > > > >       __current_set_polling();
> > > > >       tick_nohz_idle_enter();
> > > > > +     default_idle_enter();
> > > > >
> > > > >       while (!need_resched()) {
> > > > >
> > > > >
> > > >
> > > > How does this work? We don't stop the tick until the average idle time is larger,
> > > > but if we don't stop the tick how is that possible?
> > > >
> > > > Why don't we just require one or two consecutive tick wakeups before stopping?
> > >
> > > Exactly my thought and I think one should be sufficient.
> >
> > I concur. From our experience with TEO util threshold these averages can
> > backfire. I think one tick is sufficient delay to not be obviously broken.
>
> So if I'm not mistaken, it would be something like the appended prototype
> (completely untested, but it builds for me).
>
> ---
>  drivers/cpuidle/cpuidle.c |   10 ----------
>  kernel/sched/idle.c       |   32 ++++++++++++++++++++++++--------
>  2 files changed, 24 insertions(+), 18 deletions(-)
>
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -359,16 +359,6 @@ noinstr int cpuidle_enter_state(struct c
>  int cpuidle_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
>                    bool *stop_tick)
>  {
> -       /*
> -        * If there is only a single idle state (or none), there is nothing
> -        * meaningful for the governor to choose. Skip the governor and
> -        * always use state 0 with the tick running.
> -        */
> -       if (drv->state_count <= 1) {
> -               *stop_tick = false;
> -               return 0;
> -       }
> -
>         return cpuidle_curr_governor->select(drv, dev, stop_tick);
>  }
>
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -161,6 +161,14 @@ static int call_cpuidle(struct cpuidle_d
>         return cpuidle_enter(drv, dev, next_state);
>  }
>
> +static void idle_call_stop_or_retain_tick(bool stop_tick)
> +{
> +       if (stop_tick || tick_nohz_tick_stopped())
> +               tick_nohz_idle_stop_tick();
> +       else
> +               tick_nohz_idle_retain_tick();
> +}
> +
>  /**
>   * cpuidle_idle_call - the main idle function
>   *
> @@ -170,7 +178,7 @@ static int call_cpuidle(struct cpuidle_d
>   * set, and it returns with polling set.  If it ever stops polling, it
>   * must clear the polling bit.
>   */
> -static void cpuidle_idle_call(void)
> +static void cpuidle_idle_call(bool got_tick)
>  {
>         struct cpuidle_device *dev = cpuidle_get_device();
>         struct cpuidle_driver *drv = cpuidle_get_cpu_driver(dev);
> @@ -186,7 +194,7 @@ static void cpuidle_idle_call(void)
>         }
>
>         if (cpuidle_not_available(drv, dev)) {
> -               tick_nohz_idle_stop_tick();
> +               idle_call_stop_or_retain_tick(!got_tick);

Oh, I got this backwards (here and below).

The tick should be stopped if we've got the tick previously, but you
get the idea.

[Note to self: Don't send patches in the night.]

>                 default_idle_call();
>                 goto exit_idle;
> @@ -221,7 +229,7 @@ static void cpuidle_idle_call(void)
>
>                 next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
>                 call_cpuidle(drv, dev, next_state);
> -       } else {
> +       } else if (drv->state_count > 1) {
>                 bool stop_tick = true;
>
>                 /*
> @@ -229,16 +237,22 @@ static void cpuidle_idle_call(void)
>                  */
>                 next_state = cpuidle_select(drv, dev, &stop_tick);
>
> -               if (stop_tick || tick_nohz_tick_stopped())
> -                       tick_nohz_idle_stop_tick();
> -               else
> -                       tick_nohz_idle_retain_tick();
> +               idle_call_stop_or_retain_tick(stop_tick);
>
>                 entered_state = call_cpuidle(drv, dev, next_state);
>                 /*
>                  * Give the governor an opportunity to reflect on the outcome
>                  */
>                 cpuidle_reflect(dev, entered_state);
> +       } else {
> +               /*
> +                * If there is only a single idle state (or none), there is
> +                * nothing meaningful for the governor to choose.  Skip the
> +                * governor and always use state 0.
> +                */
> +               idle_call_stop_or_retain_tick(!got_tick);
> +
> +               call_cpuidle(drv, dev, 0);
>         }
>
>  exit_idle:
> @@ -259,6 +273,7 @@ exit_idle:
>  static void do_idle(void)
>  {
>         int cpu = smp_processor_id();
> +       bool got_tick = false;
>
>         /*
>          * Check if we need to update blocked load
> @@ -329,8 +344,9 @@ static void do_idle(void)
>                         tick_nohz_idle_restart_tick();
>                         cpu_idle_poll();
>                 } else {
> -                       cpuidle_idle_call();
> +                       cpuidle_idle_call(got_tick);
>                 }
> +               got_tick = tick_nohz_idle_got_tick();
>                 arch_cpu_idle_exit();
>         }

[PATCH v1] sched: idle: Make skipping governor callbacks more consistent

[Rafael J. Wysocki]

From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

If the cpuidle governor .select() callback is skipped because there
is only one idle state in the cpuidle driver, the .reflect() callback
should be skipped as well, at least for consistency (if not for
correctness), so do it.

Fixes: e5c9ffc6ae1b ("cpuidle: Skip governor when only one idle state is available")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
 drivers/cpuidle/cpuidle.c |   10 ----------
 kernel/sched/idle.c       |   11 ++++++++++-
 2 files changed, 10 insertions(+), 11 deletions(-)

--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c
@@ -359,16 +359,6 @@ noinstr int cpuidle_enter_state(struct c
 int cpuidle_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 		   bool *stop_tick)
 {
-	/*
-	 * If there is only a single idle state (or none), there is nothing
-	 * meaningful for the governor to choose. Skip the governor and
-	 * always use state 0 with the tick running.
-	 */
-	if (drv->state_count <= 1) {
-		*stop_tick = false;
-		return 0;
-	}
-
 	return cpuidle_curr_governor->select(drv, dev, stop_tick);
 }
 
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -221,7 +221,7 @@ static void cpuidle_idle_call(void)
 
 		next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
 		call_cpuidle(drv, dev, next_state);
-	} else {
+	} else if (drv->state_count > 1) {
 		bool stop_tick = true;
 
 		/*
@@ -239,6 +239,15 @@ static void cpuidle_idle_call(void)
 		 * Give the governor an opportunity to reflect on the outcome
 		 */
 		cpuidle_reflect(dev, entered_state);
+	} else {
+		tick_nohz_idle_retain_tick();
+
+		/*
+		 * If there is only a single idle state (or none), there is
+		 * nothing meaningful for the governor to choose.  Skip the
+		 * governor and always use state 0.
+		 */
+		call_cpuidle(drv, dev, 0);
 	}
 
 exit_idle:

Re: [patch 2/2] sched/idle: Make default_idle_call() NOHZ aware

[Rafael J. Wysocki]

On Friday, March 6, 2026 10:31:49 PM CET Rafael J. Wysocki wrote:
> On Fri, Mar 6, 2026 at 10:21 PM Rafael J. Wysocki <rafael@kernel.org> wrote:
> > On Wednesday, March 4, 2026 4:03:06 AM CET Qais Yousef wrote:
> > > On 03/02/26 22:25, Rafael J. Wysocki wrote:
> > > > On Mon, Mar 2, 2026 at 12:04 PM Christian Loehle

[cut]

> > > > >
> > > > > Why don't we just require one or two consecutive tick wakeups before stopping?
> > > >
> > > > Exactly my thought and I think one should be sufficient.
> > >
> > > I concur. From our experience with TEO util threshold these averages can
> > > backfire. I think one tick is sufficient delay to not be obviously broken.
> >
> > So if I'm not mistaken, it would be something like the appended prototype
> > (completely untested, but it builds for me).
> >
> > ---
> >  drivers/cpuidle/cpuidle.c |   10 ----------
> >  kernel/sched/idle.c       |   32 ++++++++++++++++++++++++--------
> >  2 files changed, 24 insertions(+), 18 deletions(-)
> >
> > --- a/drivers/cpuidle/cpuidle.c
> > +++ b/drivers/cpuidle/cpuidle.c
> > @@ -359,16 +359,6 @@ noinstr int cpuidle_enter_state(struct c
> >  int cpuidle_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
> >                    bool *stop_tick)
> >  {
> > -       /*
> > -        * If there is only a single idle state (or none), there is nothing
> > -        * meaningful for the governor to choose. Skip the governor and
> > -        * always use state 0 with the tick running.
> > -        */
> > -       if (drv->state_count <= 1) {
> > -               *stop_tick = false;
> > -               return 0;
> > -       }
> > -
> >         return cpuidle_curr_governor->select(drv, dev, stop_tick);
> >  }
> >
> > --- a/kernel/sched/idle.c
> > +++ b/kernel/sched/idle.c
> > @@ -161,6 +161,14 @@ static int call_cpuidle(struct cpuidle_d
> >         return cpuidle_enter(drv, dev, next_state);
> >  }
> >
> > +static void idle_call_stop_or_retain_tick(bool stop_tick)
> > +{
> > +       if (stop_tick || tick_nohz_tick_stopped())
> > +               tick_nohz_idle_stop_tick();
> > +       else
> > +               tick_nohz_idle_retain_tick();
> > +}
> > +
> >  /**
> >   * cpuidle_idle_call - the main idle function
> >   *
> > @@ -170,7 +178,7 @@ static int call_cpuidle(struct cpuidle_d
> >   * set, and it returns with polling set.  If it ever stops polling, it
> >   * must clear the polling bit.
> >   */
> > -static void cpuidle_idle_call(void)
> > +static void cpuidle_idle_call(bool got_tick)
> >  {
> >         struct cpuidle_device *dev = cpuidle_get_device();
> >         struct cpuidle_driver *drv = cpuidle_get_cpu_driver(dev);
> > @@ -186,7 +194,7 @@ static void cpuidle_idle_call(void)
> >         }
> >
> >         if (cpuidle_not_available(drv, dev)) {
> > -               tick_nohz_idle_stop_tick();
> > +               idle_call_stop_or_retain_tick(!got_tick);
> 
> Oh, I got this backwards (here and below).
> 
> The tick should be stopped if we've got the tick previously, but you
> get the idea.

In the meantime I realized that if the .select() governor
callback is skipped, its .reflect() callback should be skipped
either, so I've posted this:

https://lkml.org/lkml/2026/3/7/569

and here's a fixed version of the last patch on top of the above (for
completeness):

---
 kernel/sched/idle.c |   25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -161,6 +161,14 @@ static int call_cpuidle(struct cpuidle_d
 	return cpuidle_enter(drv, dev, next_state);
 }
 
+static void idle_call_stop_or_retain_tick(bool stop_tick)
+{
+	if (stop_tick || tick_nohz_tick_stopped())
+		tick_nohz_idle_stop_tick();
+	else
+		tick_nohz_idle_retain_tick();
+}
+
 /**
  * cpuidle_idle_call - the main idle function
  *
@@ -170,7 +178,7 @@ static int call_cpuidle(struct cpuidle_d
  * set, and it returns with polling set.  If it ever stops polling, it
  * must clear the polling bit.
  */
-static void cpuidle_idle_call(void)
+static void cpuidle_idle_call(bool stop_tick)
 {
 	struct cpuidle_device *dev = cpuidle_get_device();
 	struct cpuidle_driver *drv = cpuidle_get_cpu_driver(dev);
@@ -186,7 +194,7 @@ static void cpuidle_idle_call(void)
 	}
 
 	if (cpuidle_not_available(drv, dev)) {
-		tick_nohz_idle_stop_tick();
+		idle_call_stop_or_retain_tick(stop_tick);
 
 		default_idle_call();
 		goto exit_idle;
@@ -222,17 +230,14 @@ static void cpuidle_idle_call(void)
 		next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
 		call_cpuidle(drv, dev, next_state);
 	} else if (drv->state_count > 1) {
-		bool stop_tick = true;
+		stop_tick = true;
 
 		/*
 		 * Ask the cpuidle framework to choose a convenient idle state.
 		 */
 		next_state = cpuidle_select(drv, dev, &stop_tick);
 
-		if (stop_tick || tick_nohz_tick_stopped())
-			tick_nohz_idle_stop_tick();
-		else
-			tick_nohz_idle_retain_tick();
+		idle_call_stop_or_retain_tick(stop_tick);
 
 		entered_state = call_cpuidle(drv, dev, next_state);
 		/*
@@ -240,7 +245,7 @@ static void cpuidle_idle_call(void)
 		 */
 		cpuidle_reflect(dev, entered_state);
 	} else {
-		tick_nohz_idle_retain_tick();
+		idle_call_stop_or_retain_tick(stop_tick);
 
 		/*
 		 * If there is only a single idle state (or none), there is
@@ -268,6 +273,7 @@ exit_idle:
 static void do_idle(void)
 {
 	int cpu = smp_processor_id();
+	bool got_tick = false;
 
 	/*
 	 * Check if we need to update blocked load
@@ -338,8 +344,9 @@ static void do_idle(void)
 			tick_nohz_idle_restart_tick();
 			cpu_idle_poll();
 		} else {
-			cpuidle_idle_call();
+			cpuidle_idle_call(got_tick);
 		}
+		got_tick = tick_nohz_idle_got_tick();
 		arch_cpu_idle_exit();
 	}
 

Re: [PATCH v1] sched: idle: Make skipping governor callbacks more consistent

[Christian Loehle]

On 3/7/26 16:12, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> 
> If the cpuidle governor .select() callback is skipped because there
> is only one idle state in the cpuidle driver, the .reflect() callback
> should be skipped as well, at least for consistency (if not for
> correctness), so do it.
> 
> Fixes: e5c9ffc6ae1b ("cpuidle: Skip governor when only one idle state is available")
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
>  drivers/cpuidle/cpuidle.c |   10 ----------
>  kernel/sched/idle.c       |   11 ++++++++++-
>  2 files changed, 10 insertions(+), 11 deletions(-)
> 
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -359,16 +359,6 @@ noinstr int cpuidle_enter_state(struct c
>  int cpuidle_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
>  		   bool *stop_tick)
>  {
> -	/*
> -	 * If there is only a single idle state (or none), there is nothing
> -	 * meaningful for the governor to choose. Skip the governor and
> -	 * always use state 0 with the tick running.
> -	 */
> -	if (drv->state_count <= 1) {
> -		*stop_tick = false;
> -		return 0;
> -	}
> -
>  	return cpuidle_curr_governor->select(drv, dev, stop_tick);
>  }
>  
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -221,7 +221,7 @@ static void cpuidle_idle_call(void)
>  
>  		next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
>  		call_cpuidle(drv, dev, next_state);
> -	} else {
> +	} else if (drv->state_count > 1) {
>  		bool stop_tick = true;
>  
>  		/*
> @@ -239,6 +239,15 @@ static void cpuidle_idle_call(void)
>  		 * Give the governor an opportunity to reflect on the outcome
>  		 */
>  		cpuidle_reflect(dev, entered_state);
> +	} else {
> +		tick_nohz_idle_retain_tick();
> +
> +		/*
> +		 * If there is only a single idle state (or none), there is
> +		 * nothing meaningful for the governor to choose.  Skip the
> +		 * governor and always use state 0.
> +		 */
> +		call_cpuidle(drv, dev, 0);
>  	}
>  
>  exit_idle:
> 
> 
> 

Duh, good catch.
Reviewed-by: Christian Loehle <christian.loehle@arm.com>

Re: [PATCH v1] sched: idle: Make skipping governor callbacks more consistent

[Rafael J. Wysocki]

On Mon, Mar 9, 2026 at 10:13 AM Christian Loehle
<christian.loehle@arm.com> wrote:
>
> On 3/7/26 16:12, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> >
> > If the cpuidle governor .select() callback is skipped because there
> > is only one idle state in the cpuidle driver, the .reflect() callback
> > should be skipped as well, at least for consistency (if not for
> > correctness), so do it.
> >
> > Fixes: e5c9ffc6ae1b ("cpuidle: Skip governor when only one idle state is available")
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > ---
> >  drivers/cpuidle/cpuidle.c |   10 ----------
> >  kernel/sched/idle.c       |   11 ++++++++++-
> >  2 files changed, 10 insertions(+), 11 deletions(-)
> >
> > --- a/drivers/cpuidle/cpuidle.c
> > +++ b/drivers/cpuidle/cpuidle.c
> > @@ -359,16 +359,6 @@ noinstr int cpuidle_enter_state(struct c
> >  int cpuidle_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
> >                  bool *stop_tick)
> >  {
> > -     /*
> > -      * If there is only a single idle state (or none), there is nothing
> > -      * meaningful for the governor to choose. Skip the governor and
> > -      * always use state 0 with the tick running.
> > -      */
> > -     if (drv->state_count <= 1) {
> > -             *stop_tick = false;
> > -             return 0;
> > -     }
> > -
> >       return cpuidle_curr_governor->select(drv, dev, stop_tick);
> >  }
> >
> > --- a/kernel/sched/idle.c
> > +++ b/kernel/sched/idle.c
> > @@ -221,7 +221,7 @@ static void cpuidle_idle_call(void)
> >
> >               next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
> >               call_cpuidle(drv, dev, next_state);
> > -     } else {
> > +     } else if (drv->state_count > 1) {
> >               bool stop_tick = true;
> >
> >               /*
> > @@ -239,6 +239,15 @@ static void cpuidle_idle_call(void)
> >                * Give the governor an opportunity to reflect on the outcome
> >                */
> >               cpuidle_reflect(dev, entered_state);
> > +     } else {
> > +             tick_nohz_idle_retain_tick();
> > +
> > +             /*
> > +              * If there is only a single idle state (or none), there is
> > +              * nothing meaningful for the governor to choose.  Skip the
> > +              * governor and always use state 0.
> > +              */
> > +             call_cpuidle(drv, dev, 0);
> >       }
> >
> >  exit_idle:
> >
> >
> >
>
> Duh, good catch.
> Reviewed-by: Christian Loehle <christian.loehle@arm.com>

OK, so any objections or concerns from anyone?

I'm about to queue this up for the next -rc.

Re: [PATCH v1] sched: idle: Make skipping governor callbacks more consistent

[Aboorva Devarajan]

On Sat, 2026-03-07 at 17:12 +0100, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> 
> If the cpuidle governor .select() callback is skipped because there
> is only one idle state in the cpuidle driver, the .reflect() callback
> should be skipped as well, at least for consistency (if not for
> correctness), so do it.
> 
> Fixes: e5c9ffc6ae1b ("cpuidle: Skip governor when only one idle state
> is available")
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
>  drivers/cpuidle/cpuidle.c |   10 ----------
>  kernel/sched/idle.c       |   11 ++++++++++-
>  2 files changed, 10 insertions(+), 11 deletions(-)
> 
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -359,16 +359,6 @@ noinstr int cpuidle_enter_state(struct c
>  int cpuidle_select(struct cpuidle_driver *drv, struct cpuidle_device
> *dev,
>  		   bool *stop_tick)
>  {
> -	/*
> -	 * If there is only a single idle state (or none), there is
> nothing
> -	 * meaningful for the governor to choose. Skip the governor
> and
> -	 * always use state 0 with the tick running.
> -	 */
> -	if (drv->state_count <= 1) {
> -		*stop_tick = false;
> -		return 0;
> -	}
> -
>  	return cpuidle_curr_governor->select(drv, dev, stop_tick);
>  }
>  
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -221,7 +221,7 @@ static void cpuidle_idle_call(void)
>  
>  		next_state = cpuidle_find_deepest_state(drv, dev,
> max_latency_ns);
>  		call_cpuidle(drv, dev, next_state);
> -	} else {
> +	} else if (drv->state_count > 1) {
>  		bool stop_tick = true;
>  
>  		/*
> @@ -239,6 +239,15 @@ static void cpuidle_idle_call(void)
>  		 * Give the governor an opportunity to reflect on
> the outcome
>  		 */
>  		cpuidle_reflect(dev, entered_state);
> +	} else {
> +		tick_nohz_idle_retain_tick();
> +
> +		/*
> +		 * If there is only a single idle state (or none),
> there is
> +		 * nothing meaningful for the governor to choose. 
> Skip the
> +		 * governor and always use state 0.
> +		 */
> +		call_cpuidle(drv, dev, 0);
>  	}
>  
>  exit_idle:
> 
> 

Hi Rafael,

Thanks for fixing this, sorry I missed it earlier.

Reviewed-by: Aboorva Devarajan <aboorvad@linux.ibm.com>

Regards,
Aboorva

Re: [patch 2/2] sched/idle: Make default_idle_call() NOHZ aware

[Qais Yousef]

On 03/07/26 17:25, Rafael J. Wysocki wrote:

> In the meantime I realized that if the .select() governor
> callback is skipped, its .reflect() callback should be skipped
> either, so I've posted this:
> 
> https://lkml.org/lkml/2026/3/7/569
> 
> and here's a fixed version of the last patch on top of the above (for
> completeness):
> 
> ---
>  kernel/sched/idle.c |   25 ++++++++++++++++---------
>  1 file changed, 16 insertions(+), 9 deletions(-)
> 
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -161,6 +161,14 @@ static int call_cpuidle(struct cpuidle_d
>  	return cpuidle_enter(drv, dev, next_state);
>  }
>  
> +static void idle_call_stop_or_retain_tick(bool stop_tick)
> +{
> +	if (stop_tick || tick_nohz_tick_stopped())
> +		tick_nohz_idle_stop_tick();
> +	else
> +		tick_nohz_idle_retain_tick();
> +}
> +
>  /**
>   * cpuidle_idle_call - the main idle function
>   *
> @@ -170,7 +178,7 @@ static int call_cpuidle(struct cpuidle_d
>   * set, and it returns with polling set.  If it ever stops polling, it
>   * must clear the polling bit.
>   */
> -static void cpuidle_idle_call(void)
> +static void cpuidle_idle_call(bool stop_tick)
>  {
>  	struct cpuidle_device *dev = cpuidle_get_device();
>  	struct cpuidle_driver *drv = cpuidle_get_cpu_driver(dev);
> @@ -186,7 +194,7 @@ static void cpuidle_idle_call(void)
>  	}
>  
>  	if (cpuidle_not_available(drv, dev)) {
> -		tick_nohz_idle_stop_tick();
> +		idle_call_stop_or_retain_tick(stop_tick);
>  
>  		default_idle_call();
>  		goto exit_idle;
> @@ -222,17 +230,14 @@ static void cpuidle_idle_call(void)
>  		next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
>  		call_cpuidle(drv, dev, next_state);
>  	} else if (drv->state_count > 1) {
> -		bool stop_tick = true;
> +		stop_tick = true;

Silly question, but wouldn't this benefit the normal path too to delay for one
tick? This will only matter for the cases where the governor doesn't explicitly
set stop_tick to either true or false - which I am not sure what they are :)

>  
>  		/*
>  		 * Ask the cpuidle framework to choose a convenient idle state.
>  		 */
>  		next_state = cpuidle_select(drv, dev, &stop_tick);
>  
> -		if (stop_tick || tick_nohz_tick_stopped())
> -			tick_nohz_idle_stop_tick();
> -		else
> -			tick_nohz_idle_retain_tick();
> +		idle_call_stop_or_retain_tick(stop_tick);
>  
>  		entered_state = call_cpuidle(drv, dev, next_state);
>  		/*
> @@ -240,7 +245,7 @@ static void cpuidle_idle_call(void)
>  		 */
>  		cpuidle_reflect(dev, entered_state);
>  	} else {
> -		tick_nohz_idle_retain_tick();
> +		idle_call_stop_or_retain_tick(stop_tick);
>  
>  		/*
>  		 * If there is only a single idle state (or none), there is
> @@ -268,6 +273,7 @@ exit_idle:
>  static void do_idle(void)
>  {
>  	int cpu = smp_processor_id();
> +	bool got_tick = false;
>  
>  	/*
>  	 * Check if we need to update blocked load
> @@ -338,8 +344,9 @@ static void do_idle(void)
>  			tick_nohz_idle_restart_tick();
>  			cpu_idle_poll();
>  		} else {
> -			cpuidle_idle_call();
> +			cpuidle_idle_call(got_tick);
>  		}
> +		got_tick = tick_nohz_idle_got_tick();
>  		arch_cpu_idle_exit();
>  	}
>  
> 
> 
> 

Re: [PATCH v1] sched: idle: Make skipping governor callbacks more consistent

[Qais Yousef]

On 03/09/26 13:26, Rafael J. Wysocki wrote:
> On Mon, Mar 9, 2026 at 10:13 AM Christian Loehle
> <christian.loehle@arm.com> wrote:
> >
> > On 3/7/26 16:12, Rafael J. Wysocki wrote:
> > > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > >
> > > If the cpuidle governor .select() callback is skipped because there
> > > is only one idle state in the cpuidle driver, the .reflect() callback
> > > should be skipped as well, at least for consistency (if not for
> > > correctness), so do it.
> > >
> > > Fixes: e5c9ffc6ae1b ("cpuidle: Skip governor when only one idle state is available")
> > > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > > ---
> > >  drivers/cpuidle/cpuidle.c |   10 ----------
> > >  kernel/sched/idle.c       |   11 ++++++++++-
> > >  2 files changed, 10 insertions(+), 11 deletions(-)
> > >
> > > --- a/drivers/cpuidle/cpuidle.c
> > > +++ b/drivers/cpuidle/cpuidle.c
> > > @@ -359,16 +359,6 @@ noinstr int cpuidle_enter_state(struct c
> > >  int cpuidle_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
> > >                  bool *stop_tick)
> > >  {
> > > -     /*
> > > -      * If there is only a single idle state (or none), there is nothing
> > > -      * meaningful for the governor to choose. Skip the governor and
> > > -      * always use state 0 with the tick running.
> > > -      */
> > > -     if (drv->state_count <= 1) {
> > > -             *stop_tick = false;
> > > -             return 0;
> > > -     }
> > > -
> > >       return cpuidle_curr_governor->select(drv, dev, stop_tick);
> > >  }
> > >
> > > --- a/kernel/sched/idle.c
> > > +++ b/kernel/sched/idle.c
> > > @@ -221,7 +221,7 @@ static void cpuidle_idle_call(void)
> > >
> > >               next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
> > >               call_cpuidle(drv, dev, next_state);
> > > -     } else {
> > > +     } else if (drv->state_count > 1) {
> > >               bool stop_tick = true;
> > >
> > >               /*
> > > @@ -239,6 +239,15 @@ static void cpuidle_idle_call(void)
> > >                * Give the governor an opportunity to reflect on the outcome
> > >                */
> > >               cpuidle_reflect(dev, entered_state);
> > > +     } else {
> > > +             tick_nohz_idle_retain_tick();
> > > +
> > > +             /*
> > > +              * If there is only a single idle state (or none), there is
> > > +              * nothing meaningful for the governor to choose.  Skip the
> > > +              * governor and always use state 0.
> > > +              */
> > > +             call_cpuidle(drv, dev, 0);
> > >       }
> > >
> > >  exit_idle:
> > >
> > >
> > >
> >
> > Duh, good catch.
> > Reviewed-by: Christian Loehle <christian.loehle@arm.com>
> 
> OK, so any objections or concerns from anyone?
> 
> I'm about to queue this up for the next -rc.

LGTM too

Re: [patch 2/2] sched/idle: Make default_idle_call() NOHZ aware

[Christian Loehle]

On 3/10/26 03:54, Qais Yousef wrote:
> On 03/07/26 17:25, Rafael J. Wysocki wrote:
> 
>> In the meantime I realized that if the .select() governor
>> callback is skipped, its .reflect() callback should be skipped
>> either, so I've posted this:
>>
>> https://lkml.org/lkml/2026/3/7/569
>>
>> and here's a fixed version of the last patch on top of the above (for
>> completeness):
>>
>> ---
>>  kernel/sched/idle.c |   25 ++++++++++++++++---------
>>  1 file changed, 16 insertions(+), 9 deletions(-)
>>
>> --- a/kernel/sched/idle.c
>> +++ b/kernel/sched/idle.c
>> @@ -161,6 +161,14 @@ static int call_cpuidle(struct cpuidle_d
>>  	return cpuidle_enter(drv, dev, next_state);
>>  }
>>  
>> +static void idle_call_stop_or_retain_tick(bool stop_tick)
>> +{
>> +	if (stop_tick || tick_nohz_tick_stopped())
>> +		tick_nohz_idle_stop_tick();
>> +	else
>> +		tick_nohz_idle_retain_tick();
>> +}
>> +
>>  /**
>>   * cpuidle_idle_call - the main idle function
>>   *
>> @@ -170,7 +178,7 @@ static int call_cpuidle(struct cpuidle_d
>>   * set, and it returns with polling set.  If it ever stops polling, it
>>   * must clear the polling bit.
>>   */
>> -static void cpuidle_idle_call(void)
>> +static void cpuidle_idle_call(bool stop_tick)
>>  {
>>  	struct cpuidle_device *dev = cpuidle_get_device();
>>  	struct cpuidle_driver *drv = cpuidle_get_cpu_driver(dev);
>> @@ -186,7 +194,7 @@ static void cpuidle_idle_call(void)
>>  	}
>>  
>>  	if (cpuidle_not_available(drv, dev)) {
>> -		tick_nohz_idle_stop_tick();
>> +		idle_call_stop_or_retain_tick(stop_tick);
>>  
>>  		default_idle_call();
>>  		goto exit_idle;
>> @@ -222,17 +230,14 @@ static void cpuidle_idle_call(void)
>>  		next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
>>  		call_cpuidle(drv, dev, next_state);
>>  	} else if (drv->state_count > 1) {
>> -		bool stop_tick = true;
>> +		stop_tick = true;
> 
> Silly question, but wouldn't this benefit the normal path too to delay for one
> tick? This will only matter for the cases where the governor doesn't explicitly
> set stop_tick to either true or false - which I am not sure what they are :)
> 
Right now the governors will always set stop_tick explicitly (and overriding
that might confuse the governor-internal state).

Re: [PATCH v1] sched: idle: Make skipping governor callbacks more consistent

[Frederic Weisbecker]

Le Sat, Mar 07, 2026 at 05:12:05PM +0100, Rafael J. Wysocki a écrit :
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> 
> If the cpuidle governor .select() callback is skipped because there
> is only one idle state in the cpuidle driver, the .reflect() callback
> should be skipped as well, at least for consistency (if not for
> correctness), so do it.
> 
> Fixes: e5c9ffc6ae1b ("cpuidle: Skip governor when only one idle state is available")
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

-- 
Frederic Weisbecker
SUSE Labs

Re: [patch 2/2] sched/idle: Make default_idle_call() NOHZ aware

[Qais Yousef]

On 03/10/26 09:18, Christian Loehle wrote:
> On 3/10/26 03:54, Qais Yousef wrote:
> > On 03/07/26 17:25, Rafael J. Wysocki wrote:
> > 
> >> In the meantime I realized that if the .select() governor
> >> callback is skipped, its .reflect() callback should be skipped
> >> either, so I've posted this:
> >>
> >> https://lkml.org/lkml/2026/3/7/569
> >>
> >> and here's a fixed version of the last patch on top of the above (for
> >> completeness):
> >>
> >> ---
> >>  kernel/sched/idle.c |   25 ++++++++++++++++---------
> >>  1 file changed, 16 insertions(+), 9 deletions(-)
> >>
> >> --- a/kernel/sched/idle.c
> >> +++ b/kernel/sched/idle.c
> >> @@ -161,6 +161,14 @@ static int call_cpuidle(struct cpuidle_d
> >>  	return cpuidle_enter(drv, dev, next_state);
> >>  }
> >>  
> >> +static void idle_call_stop_or_retain_tick(bool stop_tick)
> >> +{
> >> +	if (stop_tick || tick_nohz_tick_stopped())
> >> +		tick_nohz_idle_stop_tick();
> >> +	else
> >> +		tick_nohz_idle_retain_tick();
> >> +}
> >> +
> >>  /**
> >>   * cpuidle_idle_call - the main idle function
> >>   *
> >> @@ -170,7 +178,7 @@ static int call_cpuidle(struct cpuidle_d
> >>   * set, and it returns with polling set.  If it ever stops polling, it
> >>   * must clear the polling bit.
> >>   */
> >> -static void cpuidle_idle_call(void)
> >> +static void cpuidle_idle_call(bool stop_tick)
> >>  {
> >>  	struct cpuidle_device *dev = cpuidle_get_device();
> >>  	struct cpuidle_driver *drv = cpuidle_get_cpu_driver(dev);
> >> @@ -186,7 +194,7 @@ static void cpuidle_idle_call(void)
> >>  	}
> >>  
> >>  	if (cpuidle_not_available(drv, dev)) {
> >> -		tick_nohz_idle_stop_tick();
> >> +		idle_call_stop_or_retain_tick(stop_tick);
> >>  
> >>  		default_idle_call();
> >>  		goto exit_idle;
> >> @@ -222,17 +230,14 @@ static void cpuidle_idle_call(void)
> >>  		next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
> >>  		call_cpuidle(drv, dev, next_state);
> >>  	} else if (drv->state_count > 1) {
> >> -		bool stop_tick = true;
> >> +		stop_tick = true;
> > 
> > Silly question, but wouldn't this benefit the normal path too to delay for one
> > tick? This will only matter for the cases where the governor doesn't explicitly
> > set stop_tick to either true or false - which I am not sure what they are :)
> > 
> Right now the governors will always set stop_tick explicitly (and overriding
> that might confuse the governor-internal state).

So we can drop this hunk then

Re: [patch 2/2] sched/idle: Make default_idle_call() NOHZ aware

[Rafael J. Wysocki]

On Tue, Mar 10, 2026 at 4:03 PM Qais Yousef <qyousef@layalina.io> wrote:
>
> On 03/10/26 09:18, Christian Loehle wrote:
> > On 3/10/26 03:54, Qais Yousef wrote:
> > > On 03/07/26 17:25, Rafael J. Wysocki wrote:
> > >
> > >> In the meantime I realized that if the .select() governor
> > >> callback is skipped, its .reflect() callback should be skipped
> > >> either, so I've posted this:
> > >>
> > >> https://lkml.org/lkml/2026/3/7/569
> > >>
> > >> and here's a fixed version of the last patch on top of the above (for
> > >> completeness):
> > >>
> > >> ---
> > >>  kernel/sched/idle.c |   25 ++++++++++++++++---------
> > >>  1 file changed, 16 insertions(+), 9 deletions(-)
> > >>
> > >> --- a/kernel/sched/idle.c
> > >> +++ b/kernel/sched/idle.c
> > >> @@ -161,6 +161,14 @@ static int call_cpuidle(struct cpuidle_d
> > >>    return cpuidle_enter(drv, dev, next_state);
> > >>  }
> > >>
> > >> +static void idle_call_stop_or_retain_tick(bool stop_tick)
> > >> +{
> > >> +  if (stop_tick || tick_nohz_tick_stopped())
> > >> +          tick_nohz_idle_stop_tick();
> > >> +  else
> > >> +          tick_nohz_idle_retain_tick();
> > >> +}
> > >> +
> > >>  /**
> > >>   * cpuidle_idle_call - the main idle function
> > >>   *
> > >> @@ -170,7 +178,7 @@ static int call_cpuidle(struct cpuidle_d
> > >>   * set, and it returns with polling set.  If it ever stops polling, it
> > >>   * must clear the polling bit.
> > >>   */
> > >> -static void cpuidle_idle_call(void)
> > >> +static void cpuidle_idle_call(bool stop_tick)
> > >>  {
> > >>    struct cpuidle_device *dev = cpuidle_get_device();
> > >>    struct cpuidle_driver *drv = cpuidle_get_cpu_driver(dev);
> > >> @@ -186,7 +194,7 @@ static void cpuidle_idle_call(void)
> > >>    }
> > >>
> > >>    if (cpuidle_not_available(drv, dev)) {
> > >> -          tick_nohz_idle_stop_tick();
> > >> +          idle_call_stop_or_retain_tick(stop_tick);
> > >>
> > >>            default_idle_call();
> > >>            goto exit_idle;
> > >> @@ -222,17 +230,14 @@ static void cpuidle_idle_call(void)
> > >>            next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
> > >>            call_cpuidle(drv, dev, next_state);
> > >>    } else if (drv->state_count > 1) {
> > >> -          bool stop_tick = true;
> > >> +          stop_tick = true;
> > >
> > > Silly question, but wouldn't this benefit the normal path too to delay for one
> > > tick? This will only matter for the cases where the governor doesn't explicitly
> > > set stop_tick to either true or false - which I am not sure what they are :)
> > >
> > Right now the governors will always set stop_tick explicitly (and overriding
> > that might confuse the governor-internal state).
>
> So we can drop this hunk then

Not really.

The governors expect that stop_tick is true by default and they clear
it if needed/desired.  They may be confused if it is false to start
with (theoretically, a governor may select an idle state with target
residency beyond the tick period length then which won't make sense).

Re: [patch 2/2] sched/idle: Make default_idle_call() NOHZ aware

[Qais Yousef]

On 03/10/26 16:09, Rafael J. Wysocki wrote:
> On Tue, Mar 10, 2026 at 4:03 PM Qais Yousef <qyousef@layalina.io> wrote:
> >
> > On 03/10/26 09:18, Christian Loehle wrote:
> > > On 3/10/26 03:54, Qais Yousef wrote:
> > > > On 03/07/26 17:25, Rafael J. Wysocki wrote:
> > > >
> > > >> In the meantime I realized that if the .select() governor
> > > >> callback is skipped, its .reflect() callback should be skipped
> > > >> either, so I've posted this:
> > > >>
> > > >> https://lkml.org/lkml/2026/3/7/569
> > > >>
> > > >> and here's a fixed version of the last patch on top of the above (for
> > > >> completeness):
> > > >>
> > > >> ---
> > > >>  kernel/sched/idle.c |   25 ++++++++++++++++---------
> > > >>  1 file changed, 16 insertions(+), 9 deletions(-)
> > > >>
> > > >> --- a/kernel/sched/idle.c
> > > >> +++ b/kernel/sched/idle.c
> > > >> @@ -161,6 +161,14 @@ static int call_cpuidle(struct cpuidle_d
> > > >>    return cpuidle_enter(drv, dev, next_state);
> > > >>  }
> > > >>
> > > >> +static void idle_call_stop_or_retain_tick(bool stop_tick)
> > > >> +{
> > > >> +  if (stop_tick || tick_nohz_tick_stopped())
> > > >> +          tick_nohz_idle_stop_tick();
> > > >> +  else
> > > >> +          tick_nohz_idle_retain_tick();
> > > >> +}
> > > >> +
> > > >>  /**
> > > >>   * cpuidle_idle_call - the main idle function
> > > >>   *
> > > >> @@ -170,7 +178,7 @@ static int call_cpuidle(struct cpuidle_d
> > > >>   * set, and it returns with polling set.  If it ever stops polling, it
> > > >>   * must clear the polling bit.
> > > >>   */
> > > >> -static void cpuidle_idle_call(void)
> > > >> +static void cpuidle_idle_call(bool stop_tick)
> > > >>  {
> > > >>    struct cpuidle_device *dev = cpuidle_get_device();
> > > >>    struct cpuidle_driver *drv = cpuidle_get_cpu_driver(dev);
> > > >> @@ -186,7 +194,7 @@ static void cpuidle_idle_call(void)
> > > >>    }
> > > >>
> > > >>    if (cpuidle_not_available(drv, dev)) {
> > > >> -          tick_nohz_idle_stop_tick();
> > > >> +          idle_call_stop_or_retain_tick(stop_tick);
> > > >>
> > > >>            default_idle_call();
> > > >>            goto exit_idle;
> > > >> @@ -222,17 +230,14 @@ static void cpuidle_idle_call(void)
> > > >>            next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
> > > >>            call_cpuidle(drv, dev, next_state);
> > > >>    } else if (drv->state_count > 1) {
> > > >> -          bool stop_tick = true;
> > > >> +          stop_tick = true;
> > > >
> > > > Silly question, but wouldn't this benefit the normal path too to delay for one
> > > > tick? This will only matter for the cases where the governor doesn't explicitly
> > > > set stop_tick to either true or false - which I am not sure what they are :)
> > > >
> > > Right now the governors will always set stop_tick explicitly (and overriding
> > > that might confuse the governor-internal state).
> >
> > So we can drop this hunk then
> 
> Not really.
> 
> The governors expect that stop_tick is true by default and they clear
> it if needed/desired.  They may be confused if it is false to start
> with (theoretically, a governor may select an idle state with target
> residency beyond the tick period length then which won't make sense).

I see, thanks for the explanation. It could be me, but if you think a comment
is worthwhile to document this expectation, would be nice to have.

LGTM anyway.

Cheers