linux-kernel - Re: [PATCH v3 2/2] sched/uclamp: Add uclamp_is

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Message-ID: <CAB8ipk_NOi0rZQR0X7zveyyL-E7mJVLX92sKVO0=C0TmkcvDOQ@mail.gmail.com>
Date: Tue, 25 Feb 2025 14:23:14 +0800
From: Xuewen Yan <xuewen.yan94@...il.com>
To: Qais Yousef <qyousef@...alina.io>
Cc: Xuewen Yan <xuewen.yan@...soc.com>, vincent.guittot@...aro.org, mingo@...hat.com, 
	peterz@...radead.org, juri.lelli@...hat.com, christian.loehle@....com, 
	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, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] sched/uclamp: Add uclamp_is_used() check before
 enable it

On Tue, Feb 25, 2025 at 7:45 AM Qais Yousef <qyousef@...alina.io> wrote:
>
> On 02/20/25 13:59, 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>
> > Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
> > Reviewed-by: Christian Loehle <christian.loehle@....com>
> > ---
>
> [...]
>
> > +/*
> > + * Enabling static branches would get the cpus_read_lock(),
> > + * check uclamp_is_used before enabling it. There is no race
> > + * issue because we never disable this static key once enabled.
> > + */
> > +static inline void sched_uclamp_enable(void)
> > +{
> > +     if (!uclamp_is_used())
> > +             static_branch_enable(&sched_uclamp_used);
> > +}
> > +
>
> As I indicated in [1] I think the pattern of repeatedly enable is actually sane
> and what we probably should be doing is modify the static_key_enable() logic to
> do the early bail out logic outside of the lock. I had this code this way FWIW
> initially and Peter asked for it to be called unconditionally instead.
>
> I think repeated calls to static_key_enable/disable() should be made cheap and
> I don't see a side effect of _replicating_ the early bail out logic outside of
> the lock so that if we have already enabled/disabled we just return immediately
> without any side effect (holding the lock in this case). I agree the hotplug

Because of the jump_lable_lock(), early bailout could indeed be a good idea.

> lock is ugly and if we can avoid touching it when we don't really need to that
> would be better.
>
> --->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;
> +       }

Maybe here should be just check whether == 0, because when enabling
the static key, the enable may occur to be -1.

+       if (atomic_read(&key->enabled) == 0)
+               return;

> +
>         cpus_read_lock();
>         static_key_disable_cpuslocked(key);
>         cpus_read_unlock();
>
> --->8---
>
> [1] https://lore.kernel.org/all/20250222233627.3yx55ks5lnq3elby@airbuntu/

BR