[PATCH] sched/cpufreq: Reevaluate frequency before tickless idle

[PATCH] sched/cpufreq: Reevaluate frequency before tickless idle

[Christian Loehle]

sugov_hold_freq() can preserve a UCLAMP_MIN-driven high frequency when
the runqueue goes idle. If cpuidle then stops the tick, no later
utilization update is guaranteed and a CPU using WFI can remain at an
unnecessarily high voltage for the entire idle period.

Issue a final cpufreq update when the idle tick actually transitions to
stopped and force single-policy schedutil past its rate limit. Keep the
existing hold behavior when the tick is retained.

Signed-off-by: Christian Loehle <christian.loehle@arm.com>
---
 include/linux/sched/cpufreq.h    |  1 +
 kernel/sched/cpufreq_schedutil.c |  5 ++++-
 kernel/sched/idle.c              | 29 +++++++++++++++++++++++++++--
 3 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/include/linux/sched/cpufreq.h b/include/linux/sched/cpufreq.h
index bdd31ab93bc5..0814f6c79315 100644
--- a/include/linux/sched/cpufreq.h
+++ b/include/linux/sched/cpufreq.h
@@ -9,6 +9,7 @@
  */
 
 #define SCHED_CPUFREQ_IOWAIT	(1U << 0)
+#define SCHED_CPUFREQ_IDLE	(1U << 1)
 
 #ifdef CONFIG_CPU_FREQ
 struct cpufreq_policy;
diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index a1782755efcc..90bf8d8bffb7 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -100,7 +100,7 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
 
 		return true;
 	} else if (sg_policy->need_freq_update) {
-		/* ignore_dl_rate_limit() wants a new frequency to be found. */
+		/* A forced update needs a new frequency to be found. */
 		return true;
 	}
 
@@ -407,6 +407,9 @@ static inline bool sugov_update_single_common(struct sugov_cpu *sg_cpu,
 	sugov_iowait_boost(sg_cpu, time, flags);
 	sg_cpu->last_update = time;
 
+	if (flags & SCHED_CPUFREQ_IDLE)
+		sg_cpu->sg_policy->need_freq_update = true;
+
 	ignore_dl_rate_limit(sg_cpu);
 
 	if (!sugov_should_update_freq(sg_cpu->sg_policy, time))
diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index eb73b65ce6c4..36f8840d0562 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -161,10 +161,35 @@ static int call_cpuidle(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 	return cpuidle_enter(drv, dev, next_state);
 }
 
+static void idle_stop_tick(void)
+{
+#ifdef CONFIG_CPU_FREQ
+	bool was_stopped = tick_nohz_tick_stopped();
+#endif
+
+	tick_nohz_idle_stop_tick();
+
+#ifdef CONFIG_CPU_FREQ
+	/*
+	 * Run one last cpufreq update before entering idle with the tick
+	 * stopped, because no later update is guaranteed.
+	 */
+	if (!was_stopped && tick_nohz_tick_stopped()) {
+		struct rq *rq = this_rq();
+		struct rq_flags rf;
+
+		rq_lock(rq, &rf);
+		update_rq_clock(rq);
+		cpufreq_update_util(rq, SCHED_CPUFREQ_IDLE);
+		rq_unlock(rq, &rf);
+	}
+#endif
+}
+
 static void idle_call_stop_or_retain_tick(bool stop_tick)
 {
 	if (stop_tick || tick_nohz_tick_stopped())
-		tick_nohz_idle_stop_tick();
+		idle_stop_tick();
 	else
 		tick_nohz_idle_retain_tick();
 }
@@ -225,7 +250,7 @@ static void cpuidle_idle_call(bool stop_tick)
 			max_latency_ns = dev->forced_idle_latency_limit_ns;
 		}
 
-		tick_nohz_idle_stop_tick();
+		idle_stop_tick();
 
 		next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
 		call_cpuidle(drv, dev, next_state);
-- 
2.34.1

Re: [PATCH] sched/cpufreq: Reevaluate frequency before tickless idle

[Christian Loehle]

On 8/24/26 14:18, Christian Loehle wrote:
> sugov_hold_freq() can preserve a UCLAMP_MIN-driven high frequency when
> the runqueue goes idle. If cpuidle then stops the tick, no later
> utilization update is guaranteed and a CPU using WFI can remain at an
> unnecessarily high voltage for the entire idle period.
> 
> Issue a final cpufreq update when the idle tick actually transitions to
> stopped and force single-policy schedutil past its rate limit. Keep the
> existing hold behavior when the tick is retained.
> 
> Signed-off-by: Christian Loehle <christian.loehle@arm.com>
> ---
>  include/linux/sched/cpufreq.h    |  1 +
>  kernel/sched/cpufreq_schedutil.c |  5 ++++-
>  kernel/sched/idle.c              | 29 +++++++++++++++++++++++++++--
>  3 files changed, 32 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/sched/cpufreq.h b/include/linux/sched/cpufreq.h
> index bdd31ab93bc5..0814f6c79315 100644
> --- a/include/linux/sched/cpufreq.h
> +++ b/include/linux/sched/cpufreq.h
> @@ -9,6 +9,7 @@
>   */
>  
>  #define SCHED_CPUFREQ_IOWAIT	(1U << 0)
> +#define SCHED_CPUFREQ_IDLE	(1U << 1)
>  
>  #ifdef CONFIG_CPU_FREQ
>  struct cpufreq_policy;
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index a1782755efcc..90bf8d8bffb7 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -100,7 +100,7 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
>  
>  		return true;
>  	} else if (sg_policy->need_freq_update) {
> -		/* ignore_dl_rate_limit() wants a new frequency to be found. */
> +		/* A forced update needs a new frequency to be found. */
>  		return true;
>  	}
>  
> @@ -407,6 +407,9 @@ static inline bool sugov_update_single_common(struct sugov_cpu *sg_cpu,
>  	sugov_iowait_boost(sg_cpu, time, flags);
>  	sg_cpu->last_update = time;
>  
> +	if (flags & SCHED_CPUFREQ_IDLE)
> +		sg_cpu->sg_policy->need_freq_update = true;
> +
>  	ignore_dl_rate_limit(sg_cpu);
>  
>  	if (!sugov_should_update_freq(sg_cpu->sg_policy, time))
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index eb73b65ce6c4..36f8840d0562 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -161,10 +161,35 @@ static int call_cpuidle(struct cpuidle_driver *drv, struct cpuidle_device *dev,
>  	return cpuidle_enter(drv, dev, next_state);
>  }
>  
> +static void idle_stop_tick(void)
> +{
> +#ifdef CONFIG_CPU_FREQ
> +	bool was_stopped = tick_nohz_tick_stopped();
> +#endif
> +
> +	tick_nohz_idle_stop_tick();
> +
> +#ifdef CONFIG_CPU_FREQ
> +	/*
> +	 * Run one last cpufreq update before entering idle with the tick
> +	 * stopped, because no later update is guaranteed.
> +	 */
> +	if (!was_stopped && tick_nohz_tick_stopped()) {
> +		struct rq *rq = this_rq();
> +		struct rq_flags rf;
> +
> +		rq_lock(rq, &rf);
> +		update_rq_clock(rq);
> +		cpufreq_update_util(rq, SCHED_CPUFREQ_IDLE);

Sashiko correctly noted that this may lead to an infinite loop on
slow-switch platforms where the pre-idle update results in another
sugov kthread wakeup which in turn will result in a new pre-idle
cpufreq update.

I will just track if sugov actually held a frequency and gate
the update on that in v2.

Re: [PATCH] sched/cpufreq: Reevaluate frequency before tickless idle

[Hongyan Xia]

On 8/24/2026 9:18 PM, Christian Loehle wrote:
> sugov_hold_freq() can preserve a UCLAMP_MIN-driven high frequency when
> the runqueue goes idle. If cpuidle then stops the tick, no later
> utilization update is guaranteed and a CPU using WFI can remain at an
> unnecessarily high voltage for the entire idle period.
> 
> Issue a final cpufreq update when the idle tick actually transitions to
> stopped and force single-policy schedutil past its rate limit. Keep the
> existing hold behavior when the tick is retained.
> 
> Signed-off-by: Christian Loehle <christian.loehle@arm.com>
> ---
>   include/linux/sched/cpufreq.h    |  1 +
>   kernel/sched/cpufreq_schedutil.c |  5 ++++-
>   kernel/sched/idle.c              | 29 +++++++++++++++++++++++++++--
>   3 files changed, 32 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/sched/cpufreq.h b/include/linux/sched/cpufreq.h
> index bdd31ab93bc5..0814f6c79315 100644
> --- a/include/linux/sched/cpufreq.h
> +++ b/include/linux/sched/cpufreq.h
> @@ -9,6 +9,7 @@
>    */
>   
>   #define SCHED_CPUFREQ_IOWAIT	(1U << 0)
> +#define SCHED_CPUFREQ_IDLE	(1U << 1)
>   
>   #ifdef CONFIG_CPU_FREQ
>   struct cpufreq_policy;
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index a1782755efcc..90bf8d8bffb7 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -100,7 +100,7 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
>   
>   		return true;
>   	} else if (sg_policy->need_freq_update) {
> -		/* ignore_dl_rate_limit() wants a new frequency to be found. */
> +		/* A forced update needs a new frequency to be found. */
>   		return true;
>   	}
>   
> @@ -407,6 +407,9 @@ static inline bool sugov_update_single_common(struct sugov_cpu *sg_cpu,
>   	sugov_iowait_boost(sg_cpu, time, flags);
>   	sg_cpu->last_update = time;
>   
> +	if (flags & SCHED_CPUFREQ_IDLE)
> +		sg_cpu->sg_policy->need_freq_update = true;
> +
>   	ignore_dl_rate_limit(sg_cpu);
>   
>   	if (!sugov_should_update_freq(sg_cpu->sg_policy, time))
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index eb73b65ce6c4..36f8840d0562 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -161,10 +161,35 @@ static int call_cpuidle(struct cpuidle_driver *drv, struct cpuidle_device *dev,
>   	return cpuidle_enter(drv, dev, next_state);
>   }
>   
> +static void idle_stop_tick(void)
> +{
> +#ifdef CONFIG_CPU_FREQ
> +	bool was_stopped = tick_nohz_tick_stopped();
> +#endif
> +
> +	tick_nohz_idle_stop_tick();
> +
> +#ifdef CONFIG_CPU_FREQ
> +	/*
> +	 * Run one last cpufreq update before entering idle with the tick
> +	 * stopped, because no later update is guaranteed.
> +	 */
> +	if (!was_stopped && tick_nohz_tick_stopped()) {
> +		struct rq *rq = this_rq();
> +		struct rq_flags rf;
> +
> +		rq_lock(rq, &rf);
> +		update_rq_clock(rq);
> +		cpufreq_update_util(rq, SCHED_CPUFREQ_IDLE);
> +		rq_unlock(rq, &rf);
> +	}
> +#endif
> +}
> +
>   static void idle_call_stop_or_retain_tick(bool stop_tick)
>   {
>   	if (stop_tick || tick_nohz_tick_stopped())
> -		tick_nohz_idle_stop_tick();
> +		idle_stop_tick();
>   	else
>   		tick_nohz_idle_retain_tick();
>   }
> @@ -225,7 +250,7 @@ static void cpuidle_idle_call(bool stop_tick)
>   			max_latency_ns = dev->forced_idle_latency_limit_ns;
>   		}
>   
> -		tick_nohz_idle_stop_tick();
> +		idle_stop_tick();
>   
>   		next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
>   		call_cpuidle(drv, dev, next_state);

LGTM, minus the Sashiko issue.

One side note is that I briefly looked at similar things before, not 
just for sugov_hold_freq() but for going idle in general. It might be 
desirable to predict the util *after* wake-up and use it to drive 
frequency one last time before going idle for any CPU. This avoids 
holding high frequencies in update_single() and also avoids idling CPUs 
holding high frequencies for the entire cluster in update_shared().

But predictions are hard and the energy savings aren't much so I didn't 
spend much time on it. Not sure if there are better ideas.

Re: [PATCH] sched/cpufreq: Reevaluate frequency before tickless idle

[Christian Loehle]

On 8/25/26 03:51, Hongyan Xia wrote:
> On 8/24/2026 9:18 PM, Christian Loehle wrote:
>> sugov_hold_freq() can preserve a UCLAMP_MIN-driven high frequency when
>> the runqueue goes idle. If cpuidle then stops the tick, no later
>> utilization update is guaranteed and a CPU using WFI can remain at an
>> unnecessarily high voltage for the entire idle period.
>>
>> Issue a final cpufreq update when the idle tick actually transitions to
>> stopped and force single-policy schedutil past its rate limit. Keep the
>> existing hold behavior when the tick is retained.
>>
>> Signed-off-by: Christian Loehle <christian.loehle@arm.com>
>> ---
>>   include/linux/sched/cpufreq.h    |  1 +
>>   kernel/sched/cpufreq_schedutil.c |  5 ++++-
>>   kernel/sched/idle.c              | 29 +++++++++++++++++++++++++++--
>>   3 files changed, 32 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/sched/cpufreq.h b/include/linux/sched/cpufreq.h
>> index bdd31ab93bc5..0814f6c79315 100644
>> --- a/include/linux/sched/cpufreq.h
>> +++ b/include/linux/sched/cpufreq.h
>> @@ -9,6 +9,7 @@
>>    */
>>   
>>   #define SCHED_CPUFREQ_IOWAIT	(1U << 0)
>> +#define SCHED_CPUFREQ_IDLE	(1U << 1)
>>   
>>   #ifdef CONFIG_CPU_FREQ
>>   struct cpufreq_policy;
>> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
>> index a1782755efcc..90bf8d8bffb7 100644
>> --- a/kernel/sched/cpufreq_schedutil.c
>> +++ b/kernel/sched/cpufreq_schedutil.c
>> @@ -100,7 +100,7 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
>>   
>>   		return true;
>>   	} else if (sg_policy->need_freq_update) {
>> -		/* ignore_dl_rate_limit() wants a new frequency to be found. */
>> +		/* A forced update needs a new frequency to be found. */
>>   		return true;
>>   	}
>>   
>> @@ -407,6 +407,9 @@ static inline bool sugov_update_single_common(struct sugov_cpu *sg_cpu,
>>   	sugov_iowait_boost(sg_cpu, time, flags);
>>   	sg_cpu->last_update = time;
>>   
>> +	if (flags & SCHED_CPUFREQ_IDLE)
>> +		sg_cpu->sg_policy->need_freq_update = true;
>> +
>>   	ignore_dl_rate_limit(sg_cpu);
>>   
>>   	if (!sugov_should_update_freq(sg_cpu->sg_policy, time))
>> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
>> index eb73b65ce6c4..36f8840d0562 100644
>> --- a/kernel/sched/idle.c
>> +++ b/kernel/sched/idle.c
>> @@ -161,10 +161,35 @@ static int call_cpuidle(struct cpuidle_driver *drv, struct cpuidle_device *dev,
>>   	return cpuidle_enter(drv, dev, next_state);
>>   }
>>   
>> +static void idle_stop_tick(void)
>> +{
>> +#ifdef CONFIG_CPU_FREQ
>> +	bool was_stopped = tick_nohz_tick_stopped();
>> +#endif
>> +
>> +	tick_nohz_idle_stop_tick();
>> +
>> +#ifdef CONFIG_CPU_FREQ
>> +	/*
>> +	 * Run one last cpufreq update before entering idle with the tick
>> +	 * stopped, because no later update is guaranteed.
>> +	 */
>> +	if (!was_stopped && tick_nohz_tick_stopped()) {
>> +		struct rq *rq = this_rq();
>> +		struct rq_flags rf;
>> +
>> +		rq_lock(rq, &rf);
>> +		update_rq_clock(rq);
>> +		cpufreq_update_util(rq, SCHED_CPUFREQ_IDLE);
>> +		rq_unlock(rq, &rf);
>> +	}
>> +#endif
>> +}
>> +
>>   static void idle_call_stop_or_retain_tick(bool stop_tick)
>>   {
>>   	if (stop_tick || tick_nohz_tick_stopped())
>> -		tick_nohz_idle_stop_tick();
>> +		idle_stop_tick();
>>   	else
>>   		tick_nohz_idle_retain_tick();
>>   }
>> @@ -225,7 +250,7 @@ static void cpuidle_idle_call(bool stop_tick)
>>   			max_latency_ns = dev->forced_idle_latency_limit_ns;
>>   		}
>>   
>> -		tick_nohz_idle_stop_tick();
>> +		idle_stop_tick();
>>   
>>   		next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
>>   		call_cpuidle(drv, dev, next_state);
> 
> LGTM, minus the Sashiko issue.
> 
> One side note is that I briefly looked at similar things before, not 
> just for sugov_hold_freq() but for going idle in general. It might be 
> desirable to predict the util *after* wake-up and use it to drive 
> frequency one last time before going idle for any CPU. This avoids 
> holding high frequencies in update_single() and also avoids idling CPUs 
> holding high frequencies for the entire cluster in update_shared().
> 
> But predictions are hard and the energy savings aren't much so I didn't 
> spend much time on it. Not sure if there are better ideas.

Funnily enough the slow-switch case makes this a lot more complicated,
in particular with PREEMPT_RT :/
My current approach is to have an irq work for the next tick for
the sugov_hold_freq() case, that checks if it hasn't seen another update
for the tick duration and then drops the hold_freq.
It's not ideal because of course it disabled the tick-stop by one tick,
but it's the best approach I can come up with now.
We could do something similar for decayed blocked utilization (i.e. have
an irq work for when utilization would reach lowest OPP (or an intermediate
step?) and decay that then).

That being said I do think the hold_freq and the blocked utilization are
separate issues, they just might have a similar-looking solution...
I can post an RFC for the latter, that being said I'm not entirely convinced
myself that it's worth it, while I think the hold_freq is just clearly wrong
behavior.

Re: [PATCH] sched/cpufreq: Reevaluate frequency before tickless idle

[Vincent Guittot]

On Tue, 25 Aug 2026 at 11:00, Christian Loehle <christian.loehle@arm.com> wrote:
>
> On 8/25/26 03:51, Hongyan Xia wrote:
> > On 8/24/2026 9:18 PM, Christian Loehle wrote:
> >> sugov_hold_freq() can preserve a UCLAMP_MIN-driven high frequency when
> >> the runqueue goes idle. If cpuidle then stops the tick, no later
> >> utilization update is guaranteed and a CPU using WFI can remain at an
> >> unnecessarily high voltage for the entire idle period.
> >>
> >> Issue a final cpufreq update when the idle tick actually transitions to
> >> stopped and force single-policy schedutil past its rate limit. Keep the
> >> existing hold behavior when the tick is retained.
> >>
> >> Signed-off-by: Christian Loehle <christian.loehle@arm.com>
> >> ---
> >>   include/linux/sched/cpufreq.h    |  1 +
> >>   kernel/sched/cpufreq_schedutil.c |  5 ++++-
> >>   kernel/sched/idle.c              | 29 +++++++++++++++++++++++++++--
> >>   3 files changed, 32 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/include/linux/sched/cpufreq.h b/include/linux/sched/cpufreq.h
> >> index bdd31ab93bc5..0814f6c79315 100644
> >> --- a/include/linux/sched/cpufreq.h
> >> +++ b/include/linux/sched/cpufreq.h
> >> @@ -9,6 +9,7 @@
> >>    */
> >>
> >>   #define SCHED_CPUFREQ_IOWAIT       (1U << 0)
> >> +#define SCHED_CPUFREQ_IDLE  (1U << 1)
> >>
> >>   #ifdef CONFIG_CPU_FREQ
> >>   struct cpufreq_policy;
> >> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> >> index a1782755efcc..90bf8d8bffb7 100644
> >> --- a/kernel/sched/cpufreq_schedutil.c
> >> +++ b/kernel/sched/cpufreq_schedutil.c
> >> @@ -100,7 +100,7 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
> >>
> >>              return true;
> >>      } else if (sg_policy->need_freq_update) {
> >> -            /* ignore_dl_rate_limit() wants a new frequency to be found. */
> >> +            /* A forced update needs a new frequency to be found. */
> >>              return true;
> >>      }
> >>
> >> @@ -407,6 +407,9 @@ static inline bool sugov_update_single_common(struct sugov_cpu *sg_cpu,
> >>      sugov_iowait_boost(sg_cpu, time, flags);
> >>      sg_cpu->last_update = time;
> >>
> >> +    if (flags & SCHED_CPUFREQ_IDLE)
> >> +            sg_cpu->sg_policy->need_freq_update = true;
> >> +
> >>      ignore_dl_rate_limit(sg_cpu);
> >>
> >>      if (!sugov_should_update_freq(sg_cpu->sg_policy, time))
> >> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> >> index eb73b65ce6c4..36f8840d0562 100644
> >> --- a/kernel/sched/idle.c
> >> +++ b/kernel/sched/idle.c
> >> @@ -161,10 +161,35 @@ static int call_cpuidle(struct cpuidle_driver *drv, struct cpuidle_device *dev,
> >>      return cpuidle_enter(drv, dev, next_state);
> >>   }
> >>
> >> +static void idle_stop_tick(void)
> >> +{
> >> +#ifdef CONFIG_CPU_FREQ
> >> +    bool was_stopped = tick_nohz_tick_stopped();
> >> +#endif
> >> +
> >> +    tick_nohz_idle_stop_tick();
> >> +
> >> +#ifdef CONFIG_CPU_FREQ
> >> +    /*
> >> +     * Run one last cpufreq update before entering idle with the tick
> >> +     * stopped, because no later update is guaranteed.
> >> +     */
> >> +    if (!was_stopped && tick_nohz_tick_stopped()) {
> >> +            struct rq *rq = this_rq();
> >> +            struct rq_flags rf;
> >> +
> >> +            rq_lock(rq, &rf);
> >> +            update_rq_clock(rq);
> >> +            cpufreq_update_util(rq, SCHED_CPUFREQ_IDLE);
> >> +            rq_unlock(rq, &rf);
> >> +    }
> >> +#endif
> >> +}
> >> +
> >>   static void idle_call_stop_or_retain_tick(bool stop_tick)
> >>   {
> >>      if (stop_tick || tick_nohz_tick_stopped())
> >> -            tick_nohz_idle_stop_tick();
> >> +            idle_stop_tick();
> >>      else
> >>              tick_nohz_idle_retain_tick();
> >>   }
> >> @@ -225,7 +250,7 @@ static void cpuidle_idle_call(bool stop_tick)
> >>                      max_latency_ns = dev->forced_idle_latency_limit_ns;
> >>              }
> >>
> >> -            tick_nohz_idle_stop_tick();
> >> +            idle_stop_tick();
> >>
> >>              next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
> >>              call_cpuidle(drv, dev, next_state);
> >
> > LGTM, minus the Sashiko issue.
> >
> > One side note is that I briefly looked at similar things before, not
> > just for sugov_hold_freq() but for going idle in general. It might be
> > desirable to predict the util *after* wake-up and use it to drive
> > frequency one last time before going idle for any CPU. This avoids
> > holding high frequencies in update_single() and also avoids idling CPUs
> > holding high frequencies for the entire cluster in update_shared().
> >
> > But predictions are hard and the energy savings aren't much so I didn't
> > spend much time on it. Not sure if there are better ideas.
>
> Funnily enough the slow-switch case makes this a lot more complicated,
> in particular with PREEMPT_RT :/
> My current approach is to have an irq work for the next tick for
> the sugov_hold_freq() case, that checks if it hasn't seen another update
> for the tick duration and then drops the hold_freq.
> It's not ideal because of course it disabled the tick-stop by one tick,
> but it's the best approach I can come up with now.
> We could do something similar for decayed blocked utilization (i.e. have
> an irq work for when utilization would reach lowest OPP (or an intermediate
> step?) and decay that then).
>
> That being said I do think the hold_freq and the blocked utilization are
> separate issues, they just might have a similar-looking solution...
> I can post an RFC for the latter, that being said I'm not entirely convinced
> myself that it's worth it, while I think the hold_freq is just clearly wrong
> behavior.

IIRC some past discussions, keeping the frequency when going idle was
a desired behavior because switching back to a high frequency took
time and slow down the wakeup (in particular with the cpufreq slow
path), and the power cost was not huge because long idle times would
power off the power domain, and idle time in WFI would be corrected by
the next tick. Do I understand correctly that in your case you only
have WFI, so you never power down domain for long sleep ?

I'm not sure I get why you want to decrease frequency in WFI instead
of letting the tick update the utilization. Or Am I missing something
in your use case ?

Re: [PATCH] sched/cpufreq: Reevaluate frequency before tickless idle

[Christian Loehle]

On 8/25/26 13:52, Vincent Guittot wrote:
> On Tue, 25 Aug 2026 at 11:00, Christian Loehle <christian.loehle@arm.com> wrote:
>>
>> On 8/25/26 03:51, Hongyan Xia wrote:
>>> On 8/24/2026 9:18 PM, Christian Loehle wrote:
>>>> sugov_hold_freq() can preserve a UCLAMP_MIN-driven high frequency when
>>>> the runqueue goes idle. If cpuidle then stops the tick, no later
>>>> utilization update is guaranteed and a CPU using WFI can remain at an
>>>> unnecessarily high voltage for the entire idle period.
>>>>
>>>> Issue a final cpufreq update when the idle tick actually transitions to
>>>> stopped and force single-policy schedutil past its rate limit. Keep the
>>>> existing hold behavior when the tick is retained.
>>>>
>>>> Signed-off-by: Christian Loehle <christian.loehle@arm.com>
>>>> ---
>>>>   include/linux/sched/cpufreq.h    |  1 +
>>>>   kernel/sched/cpufreq_schedutil.c |  5 ++++-
>>>>   kernel/sched/idle.c              | 29 +++++++++++++++++++++++++++--
>>>>   3 files changed, 32 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/include/linux/sched/cpufreq.h b/include/linux/sched/cpufreq.h
>>>> index bdd31ab93bc5..0814f6c79315 100644
>>>> --- a/include/linux/sched/cpufreq.h
>>>> +++ b/include/linux/sched/cpufreq.h
>>>> @@ -9,6 +9,7 @@
>>>>    */
>>>>
>>>>   #define SCHED_CPUFREQ_IOWAIT       (1U << 0)
>>>> +#define SCHED_CPUFREQ_IDLE  (1U << 1)
>>>>
>>>>   #ifdef CONFIG_CPU_FREQ
>>>>   struct cpufreq_policy;
>>>> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
>>>> index a1782755efcc..90bf8d8bffb7 100644
>>>> --- a/kernel/sched/cpufreq_schedutil.c
>>>> +++ b/kernel/sched/cpufreq_schedutil.c
>>>> @@ -100,7 +100,7 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
>>>>
>>>>              return true;
>>>>      } else if (sg_policy->need_freq_update) {
>>>> -            /* ignore_dl_rate_limit() wants a new frequency to be found. */
>>>> +            /* A forced update needs a new frequency to be found. */
>>>>              return true;
>>>>      }
>>>>
>>>> @@ -407,6 +407,9 @@ static inline bool sugov_update_single_common(struct sugov_cpu *sg_cpu,
>>>>      sugov_iowait_boost(sg_cpu, time, flags);
>>>>      sg_cpu->last_update = time;
>>>>
>>>> +    if (flags & SCHED_CPUFREQ_IDLE)
>>>> +            sg_cpu->sg_policy->need_freq_update = true;
>>>> +
>>>>      ignore_dl_rate_limit(sg_cpu);
>>>>
>>>>      if (!sugov_should_update_freq(sg_cpu->sg_policy, time))
>>>> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
>>>> index eb73b65ce6c4..36f8840d0562 100644
>>>> --- a/kernel/sched/idle.c
>>>> +++ b/kernel/sched/idle.c
>>>> @@ -161,10 +161,35 @@ static int call_cpuidle(struct cpuidle_driver *drv, struct cpuidle_device *dev,
>>>>      return cpuidle_enter(drv, dev, next_state);
>>>>   }
>>>>
>>>> +static void idle_stop_tick(void)
>>>> +{
>>>> +#ifdef CONFIG_CPU_FREQ
>>>> +    bool was_stopped = tick_nohz_tick_stopped();
>>>> +#endif
>>>> +
>>>> +    tick_nohz_idle_stop_tick();
>>>> +
>>>> +#ifdef CONFIG_CPU_FREQ
>>>> +    /*
>>>> +     * Run one last cpufreq update before entering idle with the tick
>>>> +     * stopped, because no later update is guaranteed.
>>>> +     */
>>>> +    if (!was_stopped && tick_nohz_tick_stopped()) {
>>>> +            struct rq *rq = this_rq();
>>>> +            struct rq_flags rf;
>>>> +
>>>> +            rq_lock(rq, &rf);
>>>> +            update_rq_clock(rq);
>>>> +            cpufreq_update_util(rq, SCHED_CPUFREQ_IDLE);
>>>> +            rq_unlock(rq, &rf);
>>>> +    }
>>>> +#endif
>>>> +}
>>>> +
>>>>   static void idle_call_stop_or_retain_tick(bool stop_tick)
>>>>   {
>>>>      if (stop_tick || tick_nohz_tick_stopped())
>>>> -            tick_nohz_idle_stop_tick();
>>>> +            idle_stop_tick();
>>>>      else
>>>>              tick_nohz_idle_retain_tick();
>>>>   }
>>>> @@ -225,7 +250,7 @@ static void cpuidle_idle_call(bool stop_tick)
>>>>                      max_latency_ns = dev->forced_idle_latency_limit_ns;
>>>>              }
>>>>
>>>> -            tick_nohz_idle_stop_tick();
>>>> +            idle_stop_tick();
>>>>
>>>>              next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
>>>>              call_cpuidle(drv, dev, next_state);
>>>
>>> LGTM, minus the Sashiko issue.
>>>
>>> One side note is that I briefly looked at similar things before, not
>>> just for sugov_hold_freq() but for going idle in general. It might be
>>> desirable to predict the util *after* wake-up and use it to drive
>>> frequency one last time before going idle for any CPU. This avoids
>>> holding high frequencies in update_single() and also avoids idling CPUs
>>> holding high frequencies for the entire cluster in update_shared().
>>>
>>> But predictions are hard and the energy savings aren't much so I didn't
>>> spend much time on it. Not sure if there are better ideas.
>>
>> Funnily enough the slow-switch case makes this a lot more complicated,
>> in particular with PREEMPT_RT :/
>> My current approach is to have an irq work for the next tick for
>> the sugov_hold_freq() case, that checks if it hasn't seen another update
>> for the tick duration and then drops the hold_freq.
>> It's not ideal because of course it disabled the tick-stop by one tick,
>> but it's the best approach I can come up with now.
>> We could do something similar for decayed blocked utilization (i.e. have
>> an irq work for when utilization would reach lowest OPP (or an intermediate
>> step?) and decay that then).
>>
>> That being said I do think the hold_freq and the blocked utilization are
>> separate issues, they just might have a similar-looking solution...
>> I can post an RFC for the latter, that being said I'm not entirely convinced
>> myself that it's worth it, while I think the hold_freq is just clearly wrong
>> behavior.
> 
> IIRC some past discussions, keeping the frequency when going idle was
> a desired behavior because switching back to a high frequency took
> time and slow down the wakeup (in particular with the cpufreq slow
> path), and the power cost was not huge because long idle times would
> power off the power domain, and idle time in WFI would be corrected by
> the next tick. Do I understand correctly that in your case you only
> have WFI, so you never power down domain for long sleep ?
> 
> I'm not sure I get why you want to decrease frequency in WFI instead
> of letting the tick update the utilization. Or Am I missing something
> in your use case ?

I'm specifically worried about WFI + stopped tick, I think the rest
is acceptable.