linux-kernel - Re: [PATCH 16/17] mem/memcg: cache rightmost node

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Message-ID: <20170727070608.GF20970@dhcp22.suse.cz>
Date:   Thu, 27 Jul 2017 09:06:08 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Davidlohr Bueso <dave@...olabs.net>, mingo@...nel.org,
        peterz@...radead.org, jack@...e.cz, torvalds@...ux-foundation.org,
        kirill.shutemov@...ux.intel.com, hch@...radead.org,
        ldufour@...ux.vnet.ibm.com, mgorman@...hsingularity.net,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>
Subject: Re: [PATCH 16/17] mem/memcg: cache rightmost node

On Wed 26-07-17 14:09:27, Andrew Morton wrote:
> On Wed, 19 Jul 2017 09:50:36 +0200 Michal Hocko <mhocko@...nel.org> wrote:
> 
> > [CC Johannes and Vladimir - the whole series is
> > http://lkml.kernel.org/r/20170719014603.19029-1-dave@stgolabs.net]
> > 
> > On Tue 18-07-17 18:46:02, Davidlohr Bueso wrote:
> > > Such that we can optimize __mem_cgroup_largest_soft_limit_node().
> > > The only overhead is the extra footprint for the cached pointer,
> > > but this should not be an issue for mem_cgroup_tree_per_node.
> > 
> > The soft limit reclaim and the associated tree manipulation is not worth
> > touching/optimizing IMHO. We strongly discourage anybody configuring
> > soft limit because of the way how it is implemented and disruptive.
> 
> I'm inclined to merge this.  Unless we plan to actually remove the code
> "soon",

this is not going to happen. It is a user visible interface so we will
have to maintain it as long as cgroup v1 interface is available

> I think it's best to continue to improve it.  Improving
> performance may never matter to anyone, but there is benefit in keeping
> up to date with the current interfaces and best practices.
 
 Well, I am not opposing the change I just think it is not worth
 bothering. Soft limit reclaim tends to be so expensive (direct limit
 down to the soft limit) that a tiny otimization has hard times to help.

-- 
Michal Hocko
SUSE Labs