Re: [RFC v3 1/5] sched/core: add capacity constraints to CPU controller

From: "Joel Fernandes (Google)" <joel.opensrc@gmail.com>
To: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-pm@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>, Tejun Heo <tj@kernel.org>,
	Joel Fernandes <joelaf@google.com>
Subject: Re: [RFC v3 1/5] sched/core: add capacity constraints to CPU controller
Date: Mon, 13 Mar 2017 03:46:13 -0700	[thread overview]
Message-ID: <CAEi0qNkAK_3a_2x9BgvyEC+bFvckGFoQX1fjFba6boC1ws_R5w@mail.gmail.com> (raw)
In-Reply-To: <1488292722-19410-2-git-send-email-patrick.bellasi@arm.com>

On Tue, Feb 28, 2017 at 6:38 AM, Patrick Bellasi
<patrick.bellasi@arm.com> wrote:
> The CPU CGroup controller allows to assign a specified (maximum)
> bandwidth to tasks within a group, however it does not enforce any
> constraint on how such bandwidth can be consumed.
> With the integration of schedutil, the scheduler has now the proper
> information about a task to select  the most suitable frequency to
> satisfy tasks needs.
[..]

> +static u64 cpu_capacity_min_read_u64(struct cgroup_subsys_state *css,
> +                                    struct cftype *cft)
> +{
> +       struct task_group *tg;
> +       u64 min_capacity;
> +
> +       rcu_read_lock();
> +       tg = css_tg(css);
> +       min_capacity = tg->cap_clamp[CAP_CLAMP_MIN];

Shouldn't the cap_clamp be accessed with READ_ONCE (and WRITE_ONCE in
the write path) to avoid load-tearing?

Thanks,
Joel