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

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Message-ID: <20250222233627.3yx55ks5lnq3elby@airbuntu>
Date: Sat, 22 Feb 2025 23:36:27 +0000
From: Qais Yousef <qyousef@...alina.io>
To: Xuewen Yan <xuewen.yan@...soc.com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, 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 02/13/25 17:15, 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.

FWIW original patch was doing such check but it was taken out after review
comments.

Is something like below completely broken instead? I think uclamp usage isn't
unique but haven't really audited the code to see if there are similar users.

I think it is a valid pattern to allow and the expectation was there shouldn't
be side effect of calling this repeatedly.

Good catch by the way.

--->8---

diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index d9c822bbffb8..17583c98c447 100644
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -214,6 +214,13 @@ EXPORT_SYMBOL_GPL(static_key_enable_cpuslocked);

 void static_key_enable(struct static_key *key)
 {
+       STATIC_KEY_CHECK_USE(key);
+
+       if (atomic_read(&key->enabled) > 0) {
+               WARN_ON_ONCE(atomic_read(&key->enabled) != 1);
+               return;
+       }
+
        cpus_read_lock();
        static_key_enable_cpuslocked(key);
        cpus_read_unlock();
@@ -239,6 +246,13 @@ EXPORT_SYMBOL_GPL(static_key_disable_cpuslocked);

 void static_key_disable(struct static_key *key)
 {
+       STATIC_KEY_CHECK_USE(key);
+
+       if (atomic_read(&key->enabled) > 0) {
+               WARN_ON_ONCE(atomic_read(&key->enabled) != 1);
+               return;
+       }
+
        cpus_read_lock();
        static_key_disable_cpuslocked(key);
        cpus_read_unlock();

--->8---

> 
> 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;
>  }
> -- 
> 2.25.1
>