linux-kernel - Re: [RFC][PATCH] cgroup: Fix cgroup

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Message-ID: <AANLkTimF+8XAZxF4psrXSKx1CHFxin6AW=tu5OLGUgNi@mail.gmail.com>
Date:	Mon, 7 Feb 2011 11:28:59 -0800
From:	Paul Menage <menage@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	balbir@...ux.vnet.ibm.com, eranian@...gle.com,
	linux-kernel@...r.kernel.org, mingo@...e.hu, paulus@...ba.org,
	davem@...emloft.net, fweisbec@...il.com,
	perfmon2-devel@...ts.sf.net, eranian@...il.com,
	robert.richter@....com, acme@...hat.com, lizf@...fujitsu.com
Subject: Re: [RFC][PATCH] cgroup: Fix cgroup_subsys::exit callback

On Mon, Feb 7, 2011 at 8:10 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>
> Make the ::exit method act like ::attach, it is after all very nearly
> the same thing.

The major difference between attach and exit is that the former is
only triggered in response to user cgroup-management action, whereas
the latter is triggered whenever a task exits, even if cgroups aren't
set up.


>  void cgroup_exit(struct task_struct *tsk, int run_callbacks)
>  {
> -       int i;
>        struct css_set *cg;
> +       int i;
>
> -       if (run_callbacks && need_forkexit_callback) {
> -               /*
> -                * modular subsystems can't use callbacks, so no need to lock
> -                * the subsys array
> -                */
> -               for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
> -                       struct cgroup_subsys *ss = subsys[i];
> -                       if (ss->exit)
> -                               ss->exit(ss, tsk);
> -               }
> -       }
> +       mutex_lock(&cgroup_mutex);

NACK - cgroup_mutex is way too heavy to take in the task exit path.
We'll need to find some other way to fix this if it's really needed.
task->alloc_lock is also normally a valid thing to synchronize against
cgroup moves, but I'd have to look at the exit path to see if it's
still valid there.

Paul
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