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

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Message-ID: <1302713529.7448.18.camel@marge.simson.net>
Date:	Wed, 13 Apr 2011 18:52:09 +0200
From:	Mike Galbraith <efault@....de>
To:	Paul Menage <menage@...gle.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Colin Cross <ccross@...roid.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, Li Zefan <lizf@...fujitsu.com>
Subject: Re: query: [PATCH 2/2] cgroup: Remove call to synchronize_rcu in
 cgroup_attach_task

On Wed, 2011-04-13 at 15:10 +0200, Paul Menage wrote:
> On Thu, Apr 7, 2011 at 11:55 AM, Mike Galbraith <efault@....de> wrote:
> > Greetings,
> >
> > Wrt these patches:
> >
> > https://lkml.org/lkml/2010/11/24/14 [PATCH 1/2] cgroup: Set CGRP_RELEASABLE when adding to a cgroup
> > https://lkml.org/lkml/2010/11/24/15 [PATCH 2/2] cgroup: Remove call to synchronize_rcu in cgroup_attach_task
> >
> > I received a query regarding 2/2 because a large database company is
> > apparently moving tasks between cgroups frequently enough that their
> > database initialization time dropped from ~11 hours to ~4 hours when
> > they applied this patch.
> 
> That sounds like a problem in their user-space code too, although I
> agree that making cgroup moves faster is a good thing.

I suspect they could avoid the issue, but don't have details.

> > Curious why these got no traction.
> >
> 
> Apart from just my chronic lack of time to work on cgroups, there were
> a couple of issues:
> 
> 1) we had trouble getting the semantics right for the release_agent
> notifications. Not that this is something that I suspect many people
> care about, but it has been part of the API since the cpuset days. I
> spent a while trying to juggle the way that release notifications were
> done (via an event counter rather than a simple flag) but never got
> them finished.
> 
> 2) I have this nagging feeling that the synchronize_rcu() call in
> cgroup_attach_task() was protecting more than is obvious. Certainly
> when cgroups first went in, that synchronize_rcu() call meant that
> cgroup_rmdir() could guarantee that if the cgroup was empty, there
> were no threads in RCU-read sections accessing their old cgroup via
> their RCU-proected current->cgroups pointers, so objects could just be
> deleted at that point. A year or two ago we RCU-ified most/all of the
> cgroup deletion path, so this shouldn't be an issue now, but I'm still
> a bit worried that we missed something. I'm probably being
> over-paranoid though.

Thanks for the info.

> We're looking at testing these patches at Google, which will give a
> little more confidence.

Goody.  I'll be doing some small scale beating too fwiw.

> There's a conflicting patchset (allowing moving entire processes by
> writing to cgroup.procs) that Ben Blum has been trying to get in for
> ages, and which has just gone in to -mm - the RCU change patches will
> likely need a bit of merge love.

Yeah, saw those.

	-Mike

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