linux-kernel - Re: [PATCH 2/2] sched/uclamp: Protect uclamp fast path code with static key

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Date:   Fri, 19 Jun 2020 18:53:00 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Chris Redpath <chrid.redpath@....com>,
        Lukasz Luba <lukasz.luba@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] sched/uclamp: Protect uclamp fast path code with
 static key

On 06/19/20 19:45, Peter Zijlstra wrote:
> On Thu, Jun 18, 2020 at 08:55:25PM +0100, Qais Yousef wrote:
> 
> > +/*
> > + * This static key is used to reduce the uclamp overhead in the fast path. It
> > + * only disables the call to uclamp_rq_{inc, dec}() in enqueue/dequeue_task().
> > + *
> > + * This allows users to continue to enable uclamp in their kernel config with
> > + * minimum uclamp overhead in the fast path.
> > + *
> > + * As soon as userspace modifies any of the uclamp knobs, the static key is
> > + * disabled, since we have an actual users that make use of uclamp
> > + * functionality.
> > + *
> > + * The knobs that would disable this static key are:
> > + *
> > + *   * A task modifying its uclamp value with sched_setattr().
> > + *   * An admin modifying the sysctl_sched_uclamp_{min, max} via procfs.
> > + *   * An admin modifying the cgroup cpu.uclamp.{min, max}
> > + */
> > +DEFINE_STATIC_KEY_TRUE(sched_uclamp_unused);
> 
> Maybe call the thing: 'sched_uclamp_users', instead?
> 
> 
> > +		if (static_branch_unlikely(&sched_uclamp_unused))
> > +			static_branch_disable(&sched_uclamp_unused);
> 
> 
> > +	if (static_branch_unlikely(&sched_uclamp_unused))
> > +		static_branch_disable(&sched_uclamp_unused);
> 
> 
> > +	if (static_branch_unlikely(&sched_uclamp_unused))
> > +		static_branch_disable(&sched_uclamp_unused);
> 
> That's an anti-pattern... just static_branch_disable(), or _enable()
> with a 'better' name is sufficient.

I misread the code. I saw there's a WAN_ON_ONCE() but that only triggers if the
atomic variable has a value that is ! in (0, 1) range.

So yes we can call it unconditionally.

Thanks

--
Qais Yousef