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

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Message-ID: <CAKfTPtAgsn0=_LjD1HovuN__fXanOOrOawatx5EhUmMn3KDYig@mail.gmail.com>
Date: Tue, 18 Feb 2025 18:24:14 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Xuewen Yan <xuewen.yan@...soc.com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com, 
	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
Subject: Re: [PATCH 2/2] sched/uclamp: Add uclamp_is_used() check before
 enable it

On Thu, 13 Feb 2025 at 10:18, Xuewen Yan <xuewen.yan@...soc.com> 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.

You should extended this to all calls to
static_branch_enable(&sched_uclamp_used) which are cgroups and system
wide uclamp
Android likes to play with task's uclamp but the same could happen with others

>
> 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);

may be use a helper function to encapsulate this and use it everywhere
we enable sched_uclamp_used

>
>         return 0;
>  }
> --
> 2.25.1
>