linux-kernel - Re: [PATCH]cpuset: add new API to change cpuset top group's cpus

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Message-ID: <6599ad830905191255r13b6166fs105213faf30f229f@mail.gmail.com>
Date:	Tue, 19 May 2009 12:55:18 -0700
From:	Paul Menage <menage@...gle.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
	lenb@...nel.org
Subject: Re: [PATCH]cpuset: add new API to change cpuset top group's cpus

On Tue, May 19, 2009 at 12:39 AM, Shaohua Li <shaohua.li@...el.com> wrote:
>
> This patch adds one API to change cpuset top group's cpus. If we want to
> make one cpu idle, simply remove the cpu from cpuset top group's cpu list,
> then all tasks will be migrate to other cpus, and other tasks will not be
> migrated to this cpu again. No functional changes.

>
> +int cpuset_change_top_cpumask(const char *buf)
> +{
> +       int retval = 0;
> +       struct cpuset *cs = &top_cpuset;
> +       struct cpuset *trialcs;
> +
> +       if (!cgroup_lock_live_group(cs->css.cgroup))
> +               return -ENODEV;

top_cpuset can't possibly be dead, so a plain cgroup_lock() would be fine here.

> +
> +       trialcs = alloc_trial_cpuset(cs);
> +       if (!trialcs)
> +               return -ENOMEM;

You returned without doing a cgroup_unlock()

> +
> +       retval = update_cpumask(cs, trialcs, buf, true);

This will fail if any child cpuset is using any cpu not in the new
cpumask, since a child's cpumask must be a subset of its parent's.

So this can't work without co-ordination with userspace regarding
child cpusets. Given that, it seems simpler to do the whole thing in
userspace, or just use the existing hotplug infrastructure.

Paul
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