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

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Message-ID: <20150716225639.GA11131@cmpxchg.org>
Date:	Thu, 16 Jul 2015 18:56:39 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Michal Hocko <mhocko@...nel.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 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.
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