linux-kernel - Re: [PATCH 0/2] CGroups: cgroup member list enhancement/fix

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Date:	Tue, 14 Jul 2009 13:34:10 -0400
From:	Benjamin Blum <bblum@...gle.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	Paul Menage <menage@...gle.com>, lizf@...fujitzu.com,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org,
	libcg-devel <libcg-devel@...ts.sourceforge.net>,
	akpm@...ux-foundation.org
Subject: Re: [PATCH 0/2] CGroups: cgroup member list enhancement/fix

On Tue, Jul 14, 2009 at 3:16 AM, Balbir Singh<balbir@...ux.vnet.ibm.com> wrote:
> * menage@...gle.com <menage@...gle.com> [2009-07-13 23:49:16]:
>> As a first cut, we were planning to add an rwsem that gets taken for
>> read in cgroup_fork(), released in cgroup_post_fork(), and taken for
>> write when moving an entire process to a new cgroup; not ideal
>> performance-wise, but safe.
>>
>> If adding a field to task_struct is an option, then the rwsem could be
>> per thread-group leader, which would reduce contention.

That would indeed help, but would only improve system-wide performance
in the case with write-contention (no improvement at all if, say, the
threadgroup is the only one forking, or when nobody is writing to the
procs file). While that's preferable to a global lock, if we can add a
field to task_struct, a (lockless) flag-based approach might be
possible.

> We should also document that moving large processes with several
> threads can be expensive.

Indeed. Or more specifically, that moving fast-growing threadgroups
can be expensive.
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