linux-kernel - Re: [PATCH 1/5] memcg: export struct mem

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Message-ID: <20150720112356.GF1211@dhcp22.suse.cz>
Date:	Mon, 20 Jul 2015 13:23:56 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Tejun Heo <tj@...nel.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] memcg: export struct mem_cgroup

On Fri 17-07-15 08:28:19, Johannes Weiner wrote:
> On Thu, Jul 16, 2015 at 04:03:58PM -0700, Andrew Morton wrote:
> > On Thu, 16 Jul 2015 18:56:39 -0400 Johannes Weiner <hannes@...xchg.org> wrote:
> > 
> > > On Thu, Jul 16, 2015 at 02:34:33PM -0700, Andrew Morton wrote:
> > > > On Thu, 16 Jul 2015 09:19:49 +0200 Michal Hocko <mhocko@...nel.org> wrote:
> > > > > I agree with Johannes who originally suggested to expose mem_cgroup that
> > > > > it will allow for a better code later.
> > > > 
> > > > Sure, but how *much* better?  Are there a significant number of
> > > > fastpath functions involved?
> > > > 
> > > > From a maintainability/readability point of view, this is quite a bad
> > > > patch.  It exposes a *lot* of stuff to the whole world.  We need to get
> > > > a pretty good runtime benefit from doing this to ourselves.  I don't
> > > > think that saving 376 bytes on a fatconfig build is sufficient
> > > > justification?
> > > 
> > > It's not a performance issue for me.  Some stuff is hard to read when
> > > you have memcg functions with klunky names interrupting the code flow
> > > to do something trivial to a struct mem_cgroup member, like
> > > mem_cgroup_lruvec_online() and mem_cgroup_get_lru_size().
> > > 
> > > Maybe we can keep thresholds private and encapsulate the softlimit
> > > tree stuff in mem_cgroup_per_zone into something private as well, as
> > > this is not used - and unlikely to be used - outside of memcg proper.
> > > 
> > > But otherwise, I think struct mem_cgroup should have mm-scope.
> > 
> > Meaning a new mm/memcontrol.h?  That's a bit better I suppose.
> 
> I meant as opposed to being private to memcontrol.c.  I'm not sure I
> quite see the problem of having these definitions in include/linux, as
> long as we keep the stuff that is genuinely only used in memcontrol.c
> private to that file. 

Completely agreed

> But mm/memcontrol.h would probably work too.

I am not sure this is a good idea. There is a code outside of mm which
is using memcg functionality. I do not think we want two sets of header
files - one for mm and other for other external users.
-- 
Michal Hocko
SUSE Labs
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