linux-kernel - Re: [PATCH 1/3] cgroup: convert open-coded mutex_lock(&cgroup_mutex) calls into cgroup

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Date:	Sun, 18 Jan 2009 17:37:21 -0800
From:	Paul Menage <menage@...gle.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	miaox@...fujitsu.com, maxk@...lcomm.com,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 1/3] cgroup: convert open-coded mutex_lock(&cgroup_mutex) 
	calls into cgroup_lock() calls

On Sun, Jan 18, 2009 at 1:10 AM, Ingo Molnar <mingo@...e.hu> wrote:
> this just changes over a clean mutex call to a wrapped lock/unlock
> sequence that has higher overhead in the common case.
>
> We should do the exact opposite, we should change this opaque API:
>
>  void cgroup_lock(void)
>  {
>         mutex_lock(&cgroup_mutex);
>  }
>
> To something more explicit (and more maintainable) like:

I disagree - cgroup_mutex is a very coarse lock that can be held for
pretty long periods of time by the cgroups framework, and should never
be part of any fastpath code. So the overhead of a function call
should be irrelevant.

The change that you're proposing would send the message that
cgroup_mutex_lock(&cgroup_mutex) is appropriate to use in a
performance-sensitive function, when in fact we want to discourage
such code from taking this lock and instead use more appropriately
fine-grained locks.

Paul
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