linux-kernel - Re: bind() call in cgroup's css structure

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Date:	Mon, 9 Apr 2012 11:09:32 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Glauber Costa <glommer@...allels.com>
Cc:	Li Zefan <lizefan@...wei.com>, Paul Turner <pjt@...gle.com>,
	cgroups@...r.kernel.org, "devel@...nvz.org" <devel@...nvz.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: bind() call in cgroup's css structure

Hello, Glauber.

On Mon, Apr 09, 2012 at 10:59:56AM -0300, Glauber Costa wrote:
> During your cgroup refactor, I was wondering if you have any plans
> to get rid of the bind() callback that is called when hierarchies
> are moved?
>
> At least in tree, there seems to be no users for that.

I don't have any current plan for the callback but if it doesn't have
in-kernel user, I'd prefer to remove it.

> I actually planned to use it myself, to start or remove a jump label
> when cpuacct and cpu cgroups were comounted.

I see.

> Problem is, because we have some calls in the cpuset cgroup from
> inside the cpu hotplug handler, we end up taking the almighty
> cgroup_mutex from inside the cpu_hotplug.lock.

Yeah, those two are pretty big locks.

> jump labels take it in most arches through the get_online_cpus()
> function call. This means we effectively can't apply jump labels
> with the cgroup_mutex held, which is the case throughout the whole
> bind() call.
> 
> All that explained, I figured I might as well ask before I attempted
> a solution to that myself: as much as populate(), bind seems to be
> one of the overly complicated callbacks, designed for a scenario in
> which everything can come and go at will, which is something we're
> trying to fix.

I haven't read the code so this could be completely off but if this is
jump label optimization which can be made to work w/o it immediately
applied, maybe just punt it to a work item from the callback?  Note
that if cancellation is necessary for e.g. unbinding, it may
re-introduce locking dependency through flushing.

Thanks.

-- 
tejun
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