linux-kernel - Re: [RFC 3/3] memcg: get rid of mm

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Message-ID: <20150528210742.GF27479@htj.duckdns.org>
Date:	Thu, 28 May 2015 17:07:42 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	Oleg Nesterov <oleg@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 3/3] memcg: get rid of mm_struct::owner

Hello, Johannes, Michal.

On Tue, May 26, 2015 at 10:10:11AM -0400, Johannes Weiner wrote:
> On Tue, May 26, 2015 at 01:50:06PM +0200, Michal Hocko wrote:
> > Please note that this patch introduces a USER VISIBLE CHANGE OF BEHAVIOR.
> > Without mm->owner _all_ tasks associated with the mm_struct would
> > initiate memcg migration while previously only owner of the mm_struct
> > could do that. The original behavior was awkward though because the user
> > task didn't have any means to find out the current owner (esp. after
> > mm_update_next_owner) so the migration behavior was not well defined
> > in general.
> > New cgroup API (unified hierarchy) will discontinue tasks file which
> > means that migrating threads will no longer be possible. In such a case
> > having CLONE_VM without CLONE_THREAD could emulate the thread behavior
> > but this patch prevents from isolating memcg controllers from others.
> > Nevertheless I am not convinced such a use case would really deserve
> > complications on the memcg code side.
> 
> I think such a change is okay.  The memcg semantics of moving threads
> with the same mm into separate groups have always been arbitrary.  No
> reasonable behavior can be expected of this, so what sane real life
> usecase would rely on it?

I suppose that making mm always follow the threadgroup leader should
be fine, right?  While this wouldn't make any difference in the
unified hierarchy, I think this would make more sense for traditional
hierarchies.

Thanks.

-- 
tejun
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