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Date:	Mon, 16 Jun 2014 14:59:15 +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 Thu 12-06-14 12:17:33, Tejun Heo wrote:
> Hello, Michal.
> 
> On Thu, Jun 12, 2014 at 04:22:37PM +0200, Michal Hocko wrote:
> > The primary question would be, whether this is is the best transition
> > strategy. I do not know how many users apart from developers are really
> > using unified hierarchy. I would be worried that we merge a feature which
> > will not be used for a long time.
> 
> I'm planning to drop __DEVEL__ mask from the unified hierarchy in a
> cycle, at most two. 

OK, I am obviously behind the current cgroup core changes. I thought
that unified hierarchy will be for development only for much more time.

> The biggest hold up at the moment is
> straightening out the interfaces and interaction between memcg and
> blkcg because I think it'd be silly to have to go through another
> round of interface versioning effort right after transitioning to
> unified hierarchy.  I'm not too confident whether it'd be possible to
> get blkcg completely in shape by that time, but, if that takes too
> long, I'll just leave blkcg behind temporarily.  So, at least from
> kernel side, it's not gonna be too long.
> 
> 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.

> Some are already playing with unified hierarchy and
> planning to migrate as soon as possible but there sure will be others
> who will take more time.  Can't tell for sure, but the thing is that
> migration to min/low/high/max scheme is a signficant migration effort
> too, so I'm not sure how much we'd gain by doing that separately.
> It'd be an extra transition step for userland (optional but still),
> more combinations of configration to handle for memcg, and it's not
> like unified hierarchy is that difficult to transition to.
> 
> > Moreover, if somebody wants to transition from soft limit then it would
> > be really hard because switching to unified hierarchy might be a no-go.
> 
> Why would that be a no-go? 

I remember discussions about per-thread distributions and some other
things missing from the new API.

> Its usage is mostly similar with
> tranditional hierarchies and can be used with other hierarchies, so
> while it'd take some adaptation, in most cases gradual transition
> shouldn't be a big problem.

OK

> > I think that it is clear that we should deprecate soft_limit ASAP. I
> > also think it wont't hurt to have min, low, high in both old and unified
> > API and strongly warn if somebody tries to use soft_limit along with any
> > of the new APIs in the first step. Later we can even forbid any
> > combination by a hard failure.
> 
> I don't quite understand how you plan to deprecate it.  Sure you can
> fail with -EINVAL or whatnot when the wrong combination

Yes, I was thinking that direction. First warn and then EINVAL later.

> 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.
-- 
Michal Hocko
SUSE Labs
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