linux-kernel - Re: [PATCH v2 2/2] sched/uclamp: Add uclamp_is

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Message-ID: <5ff60151-cb77-46b7-ad52-813981c99d26@arm.com>
Date: Wed, 19 Feb 2025 15:59:30 +0000
From: Christian Loehle <christian.loehle@....com>
To: Xuewen Yan <xuewen.yan@...soc.com>, vincent.guittot@...aro.org,
 mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com
Cc: dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, vschneid@...hat.com, hongyan.xia2@....com,
 ke.wang@...soc.com, di.shen@...soc.com, xuewen.yan94@...il.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] sched/uclamp: Add uclamp_is_used() check before
 enable it

On 2/19/25 09:37, Xuewen Yan wrote:
> Because the static_branch_enable() would get the cpus_read_lock(),
> and sometimes users may frequently set the uclamp value of tasks,
> and this operation would call the static_branch_enable()
> frequently, so add the uclamp_is_used() check to prevent calling
> the cpus_read_lock() frequently.
> And to make the code more concise, add a helper function to encapsulate
> this and use it everywhere we enable sched_uclamp_used.
> 
> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> ---
> V2:
> - change some commit message;
> - Add a helper function and use it everywhere when enable uclamp (Vincent)
> ---
> ---
>  kernel/sched/core.c     |  6 +++---
>  kernel/sched/sched.h    | 14 ++++++++++++++
>  kernel/sched/syscalls.c |  2 +-
>  3 files changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 38a7192bfc19..0466a2f61b99 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1942,12 +1942,12 @@ static int sysctl_sched_uclamp_handler(const struct ctl_table *table, int write,
>  	}
>  
>  	if (update_root_tg) {
> -		static_branch_enable(&sched_uclamp_used);
> +		sched_uclamp_enable();
>  		uclamp_update_root_tg();
>  	}
>  
>  	if (old_min_rt != sysctl_sched_uclamp_util_min_rt_default) {
> -		static_branch_enable(&sched_uclamp_used);
> +		sched_uclamp_enable();
>  		uclamp_sync_util_min_rt_default();
>  	}
>  
> @@ -9295,7 +9295,7 @@ static ssize_t cpu_uclamp_write(struct kernfs_open_file *of, char *buf,
>  	if (req.ret)
>  		return req.ret;
>  
> -	static_branch_enable(&sched_uclamp_used);
> +	sched_uclamp_enable();
>  
>  	guard(mutex)(&uclamp_mutex);
>  	guard(rcu)();
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 3624fdce5536..bd22af347d0b 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -3407,6 +3407,18 @@ static inline bool uclamp_is_used(void)
>  	return static_branch_likely(&sched_uclamp_used);
>  }
>  
> +/*
> + * Enabling static branches would get the cpus_read_lock(),
> + * check whether uclamp_is_used before enable it to avoid always
> + * calling cpus_read_lock(). Because we never disable this
> + * static key once enable it.
> + */
> +static inline void sched_uclamp_enable(void)
> +{
> +	if (!uclamp_is_used())
> +		static_branch_enable(&sched_uclamp_used);
> +}
> +
Reviewed-by 
>  static inline unsigned long uclamp_rq_get(struct rq *rq,
>  					  enum uclamp_id clamp_id)
>  {
> @@ -3486,6 +3498,8 @@ static inline bool uclamp_is_used(void)
>  	return false;
>  }
>  
> +static inline void sched_uclamp_enable(void) {}
> +
>  static inline unsigned long
>  uclamp_rq_get(struct rq *rq, enum uclamp_id clamp_id)
>  {
> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> index 456d339be98f..9100a77e9d79 100644
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -368,7 +368,7 @@ static int uclamp_validate(struct task_struct *p,
>  	 * blocking operation which obviously cannot be done while holding
>  	 * scheduler locks.
>  	 */
> -	static_branch_enable(&sched_uclamp_used);
> +	sched_uclamp_enable();
>  
>  	return 0;
>  }

Reviewed-by: Christian Loehle <christian.loehle@....com>