Re: [PATCH v2] cpufreq: schedutil: Serialize start against rate limit updates

[Nirjhar Roy <nirjhar.roy@oss.qualcomm.com>]

On Thu, Oct 08, 2026 at 12:58:20AM +0900, Hui Su wrote:
> rate_limit_us_store() is called with the governor attribute set
> update_lock held, but sugov_start() updates freq_update_delay_ns from
> the same tunable without taking that lock.
> 
> This allows the two paths to interleave as follows:
> 
>         sugov_start()             rate_limit_us_store()
> 
>         read old rate_limit_us
>                                   write new rate_limit_us
>                                   publish the new delay
>         publish the old delay
> 
> The sysfs tunable then contains the new value while
> freq_update_delay_ns contains the old one. Unlike a transient torn
> read, the stale delay remains in effect until another rate limit update
> or governor restart.
> 
> Serialize sugov_start() against sysfs stores by taking the existing
> governor attribute set lock around sugov_update_rate_limit_us(). Use
> gov_attr_set_lock() and gov_attr_set_unlock() to keep locking details
> within the attribute set interface. This keeps the fix off the scheduler
> hot path.
> 
> Fixes: 9bdcb44e391d ("cpufreq: schedutil: New governor based on scheduler utilization data")
> Reviewed-by: Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>
> Signed-off-by: Hui Su <sh_def@163.com>
> ---
> Changes in v2:
> - Wrap governor attribute set locking in helpers as suggested by
>   Zhongqiu Han.
> 
> v1: https://lore.kernel.org/lkml/20260929103635.3780346-1-sh_def@163.com/
> 
>  include/linux/cpufreq.h          | 10 ++++++++++
>  kernel/sched/cpufreq_schedutil.c |  4 ++++
>  2 files changed, 14 insertions(+)
> 
> diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
> index 35ce665edfd8..5448eb0a0885 100644
> --- a/include/linux/cpufreq.h
> +++ b/include/linux/cpufreq.h
> @@ -694,6 +694,16 @@ static inline struct gov_attr_set *to_gov_attr_set(struct kobject *kobj)
>  	return container_of(kobj, struct gov_attr_set, kobj);
>  }
>  
> +static inline void gov_attr_set_lock(struct gov_attr_set *attr_set)
> +{
> +	mutex_lock(&attr_set->update_lock);
> +}
> +
> +static inline void gov_attr_set_unlock(struct gov_attr_set *attr_set)
> +{
> +	mutex_unlock(&attr_set->update_lock);
> +}
> +

Can use the above wrappers in governor_store()? Right now it directly calls mutex_{lock, unlock}(&attr_set->update_lock) ?
--NR
 

>  void gov_attr_set_init(struct gov_attr_set *attr_set, struct list_head *list_node);
>  void gov_attr_set_get(struct gov_attr_set *attr_set, struct list_head *list_node);
>  unsigned int gov_attr_set_put(struct gov_attr_set *attr_set, struct list_head *list_node);
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 49ccd6f1c185..b847311c307d 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -861,7 +861,11 @@ static int sugov_start(struct cpufreq_policy *policy)
>  	void (*uu)(struct update_util_data *data, u64 time, unsigned int flags);
>  	unsigned int cpu;
>  
> +	/* Serialize against rate_limit_us_store(). */
> +	gov_attr_set_lock(&sg_policy->tunables->attr_set);
>  	sugov_update_rate_limit_us(sg_policy);
> +	gov_attr_set_unlock(&sg_policy->tunables->attr_set);
> +
>  	sg_policy->last_freq_update_time	= 0;
>  	sg_policy->next_freq			= 0;
>  	sg_policy->work_in_progress		= false;
> 
> base-commit: 63de2367f6d280904a9c5e5bbc7c75bb91c5b3d8
>