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Date:	Mon, 16 Jun 2014 16:04:48 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Greg Thelen <gthelen@...gle.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Michel Lespinasse <walken@...gle.com>,
	Roman Gushchin <klamm@...dex-team.ru>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	Li Zefan <lizefan@...wei.com>
Subject: Re: [PATCH 2/2] memcg: Allow guarantee reclaim

On Mon 16-06-14 09:57:41, Tejun Heo wrote:
> Hello, Michal.
> 
> On Mon, Jun 16, 2014 at 02:59:15PM +0200, Michal Hocko wrote:
> > > There sure is a question of how fast userland will move to the new
> > > interface. 
> > 
> > Yeah, I was mostly thinking about those who would need to to bigger
> > changes. AFAIR threads will no longer be distributable between groups.
> 
> Thread-level granularity should go away no matter what, but this is
> completely irrelevant to memcg which can't do per-thread anyway.

Yes, I wasn't afraid about memcg. It was a setup which requires more
controllers that I was worried about.

> For whatever reason, a user is stuck with thread-level granularity for
> controllers which work that way, the user can use the old hierarchies
> for them for the time being.

So he can mount memcg with new cgroup API and others with old?

> > > is used but I don't think there's any chance of removing the knob.
> > > There's a reason why we're introducing a new version of the whole
> > > cgroup interface which can co-exist with the existing one after all.
> > > If you wanna version memcg interface separately, maybe that'd work but
> > > it sounds like a lot of extra hassle for not much gain.
> > 
> > No, I didn't mean to version the interface. I just wanted to have
> > gradual transition for potential soft_limit users.
> > 
> > Maybe I am misunderstanding something but I thought that new version of
> > API will contain all knobs which are not marked .flags = CFTYPE_INSANE
> > while the old API will contain all of them.
> 
> Nope, some changes don't fit that model.  CFTYPE_ON_ON_DFL is the
> opposite. 

OK, I wasn't aware of this. On which branch I find this?

> Knobs marked with the flag only appear on the default
> hierarchy (cgroup core internally calls it the default hierarchy as
> this is the tree all the controllers are attached to by default).

I am not sure I understand. So they are visible only in the hierarchy
mounted with the new cgroup API (sane or how is it called)?
-- 
Michal Hocko
SUSE Labs
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