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@xxxxxxx>
> ---
>   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.