linux-kernel - Re: [PATCH 2/6] cgroup: improve old cgroup handling in cgroup_attach

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Message-ID: <CALdu-PAj1ZUmB2ixxA6yeppB8MerBGk1cSeQadobH0H4cRSe7Q@mail.gmail.com>
Date:	Thu, 25 Aug 2011 01:51:39 -0700
From:	Paul Menage <paul@...lmenage.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	rjw@...k.pl, lizf@...fujitsu.com,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org
Subject: Re: [PATCH 2/6] cgroup: improve old cgroup handling in cgroup_attach_proc()

On Tue, Aug 23, 2011 at 3:19 PM, Tejun Heo <tj@...nel.org> wrote:
> cgroup_attach_proc() behaves differently from cgroup_attach_task() in
> the following aspects.
>
> * All hooks are invoked even if no task is actually being moved.
>
> * ->can_attach_task() is called for all tasks in the group whether the
>  new cgrp is different from the current cgrp or not; however,
>  ->attach_task() is skipped if new equals new.  This makes the calls
>  asymmetric.
>
> This patch improves old cgroup handling in cgroup_attach_proc() by
> looking up the current cgroup at the head, recording it in the flex
> array along with the task itself, and using it to remove the above two
> differences.  This will also ease further changes.

While I'm all in favour of making things more consistent, do we need
such a big change?

In particular, making the group flex-array entries contain both a task
and a cgroup appears to be only necessary in order to skip tasks where
new_cgroup == old_cgroup. Can't we get the same effect by simply
leaving all such tasks out of the flex-array in the first place?

Paul
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