linux-kernel - Re: query: [PATCH 2/2] cgroup: Remove call to synchronize_rcu in cgroup_attach

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Message-ID: <1302713819.7448.22.camel@marge.simson.net>
Date:	Wed, 13 Apr 2011 18:56:59 +0200
From:	Mike Galbraith <efault@....de>
To:	Paul Menage <menage@...gle.com>
Cc:	Li Zefan <lizf@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Colin Cross <ccross@...roid.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: query: [PATCH 2/2] cgroup: Remove call to synchronize_rcu in
 cgroup_attach_task

On Wed, 2011-04-13 at 15:16 +0200, Paul Menage wrote:
> On Wed, Apr 13, 2011 at 5:11 AM, Mike Galbraith <efault@....de> wrote:
> > If the user _does_ that rmdir(), it's more or less back to square one.
> > RCU grace periods should not impact userland, but if you try to do
> > create/attach/detach/destroy, you run into the same bottleneck, as does
> > any asynchronous GC, though that's not such a poke in the eye.  I tried
> > a straight forward move to schedule_work(), and it seems to work just
> > fine.  rmdir() no longer takes ~30ms on my box, but closer to 20us.
> 
> > +       /*
> > +        * Release the subsystem state objects.
> > +        */
> > +       for_each_subsys(cgrp->root, ss)
> > +               ss->destroy(ss, cgrp);
> > +
> > +       cgrp->root->number_of_cgroups--;
> > +       mutex_unlock(&cgroup_mutex);
> > +
> > +       /*
> > +        * Drop the active superblock reference that we took when we
> > +        * created the cgroup
> > +        */
> > +       deactivate_super(cgrp->root->sb);
> > +
> > +       /*
> > +        * if we're getting rid of the cgroup, refcount should ensure
> > +        * that there are no pidlists left.
> > +        */
> > +       BUG_ON(!list_empty(&cgrp->pidlists));
> > +
> > +       kfree(cgrp);
> 
> We might want to punt this through RCU again, in case the subsystem
> destroy() callbacks left anything around that was previously depending
> on the RCU barrier.
> 
> Also, I'd be concerned that subsystems might get confused by the fact
> that a new group called 'foo' could be created before the old 'foo'
> has been cleaned up? (And do any subsystems rely on being able to
> access the cgroup dentry up until the point when destroy() is called?

Yeah, I already have head scratching sessions planned for these, why I
used 'seems' to work fine, and Not-signed-off-by: :)

	-Mike

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