Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 19 Jun 2020 19:45:10 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Qais Yousef <qais.yousef@arm.com>
Cc:     Ingo Molnar <mingo@redhat.com>, Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Patrick Bellasi <patrick.bellasi@matbug.net>,
        Chris Redpath <chrid.redpath@arm.com>,
        Lukasz Luba <lukasz.luba@arm.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] sched/uclamp: Protect uclamp fast path code with
 static key
Message-ID: <20200619174510.GB576888@hirez.programming.kicks-ass.net>
References: <20200618195525.7889-1-qais.yousef@arm.com>
 <20200618195525.7889-3-qais.yousef@arm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200618195525.7889-3-qais.yousef@arm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.