linux-kernel - Re: [PATCH v2 10/28] mm: memcg: introduce mod_lruvec_memcg

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Date:   Mon, 3 Feb 2020 12:39:58 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Roman Gushchin <guro@...com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Bharata B Rao <bharata@...ux.ibm.com>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH v2 10/28] mm: memcg: introduce mod_lruvec_memcg_state()

On Mon, Jan 27, 2020 at 09:34:35AM -0800, Roman Gushchin wrote:
> To prepare for per-object accounting of slab objects, let's introduce
> __mod_lruvec_memcg_state() and mod_lruvec_memcg_state() helpers,
> which are similar to mod_lruvec_state(), but do not update global
> node counters, only lruvec and per-cgroup.
> 
> It's necessary because soon node slab counters will be used for
> accounting of all memory used by slab pages, however on memcg level
> only the actually used memory will be counted. The free space will be
> shared between all cgroups, so it can't be accounted to any
> specific cgroup.

Makes perfect sense. However, I think the existing mod_lruvec_state()
has a bad and misleading name, and adding to it in the same style
makes things worse.

Can we instead rename lruvec_state to node_memcg_state to capture that
it changes all levels. And then do the following, clean API?

- node_state for node only

- memcg_state for memcg only

- lruvec_state for lruvec only

- node_memcg_state convenience wrapper to change node, memcg, lruvec counters

You can then open-code the disjunct node and memcg+lruvec counters.

[ Granted, lruvec counters are never modified on their own - always in
  conjunction with the memcg counters. And frankly, the only memcg
  counters that are modified *without* the lruvec counter-part are the
  special-case MEMCG_ counters.

  It would be nice to have 1) a completely separate API for the MEMCG_
  counters; and then 2) the node API for node and 3) a cgroup API for
  memcg+lruvec VM stat counters that allow you to easily do the
  disjunct accounting for slab memory.

  But I can't think of poignant names for these. At least nothing that
  would be better than separate memcg_state and lruvec_state calls. ]