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

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Message-ID: <b9ab5474-0b43-4200-8217-5cd95b365cad@arm.com>
Date: Thu, 13 Feb 2025 14:51:30 +0000
From: Christian Loehle <christian.loehle@....com>
To: Xuewen Yan <xuewen.yan@...soc.com>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org
Cc: dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, vschneid@...hat.com, qyousef@...alina.io,
 ke.wang@...soc.com, di.shen@...soc.com, xuewen.yan94@...il.com,
 linux-kernel@...r.kernel.org, Hongyan Xia <Hongyan.Xia2@....com>
Subject: Re: [PATCH 2/2] sched/uclamp: Add uclamp_is_used() check before
 enable it

On 2/13/25 09:15, 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 the uclamp_validate() would call the static_branch_enable()
> frequently, so add the uclamp_is_used() check to prevent calling
> the cpus_read_lock() frequently.
> 
> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> ---
>  kernel/sched/syscalls.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> index 456d339be98f..d718fddadb03 100644
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -368,7 +368,8 @@ 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);
> +	if (!uclamp_is_used())
> +		static_branch_enable(&sched_uclamp_used);
>  
>  	return 0;
>  }

Given that we never disable sched_uclamp_used once active this
is fine.
Reviewed-by: Christian Loehle <christian.loehle@....com>