linux-kernel - Re: [RFC PATCH 4/4] cgroups: Add an rlimit subsystem

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Message-ID: <BANLkTimiQBkDZ=L9UR-yGMATw9o-3JVtMEaMP36zey=_9pxL9Q@mail.gmail.com>
Date:	Fri, 24 Jun 2011 15:22:11 -0700
From:	Paul Menage <menage@...gle.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Li Zefan <lizf@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 4/4] cgroups: Add an rlimit subsystem

On Thu, Jun 23, 2011 at 6:48 AM, Frederic Weisbecker <fweisbec@...il.com> wrote:
>> Can't this fail spuriously in the presence of hierarchies?
>>
>> E.g. if cgroup A has children, and A is at its limit, then moving
>> tasks around between A and its children, or between different children
>> of A, seems like it would fail due to the temporary double counting.
>
> Good point. Probably I should first uncharge the old cgroup and its parents.
>

That can fail too - if a new task gets created between the uncharge
and the charge.

What we need is a res_counter_move_charge(A, B, amount) function which will:

- locate C, the nearest common ancestor of A and B
- lock up the chain from B up to but not including C, adding the new charge
- unlock up the chain from B to C
- uncharge along the chain from A up to but not including C (not sure
how much locking is needed there since there's no need for roll back).

Paul
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