linux-kernel - Re: [PATCH v2 21/28] mm: memcg/slab: use a single set of kmem

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Message-ID: <20200203195048.GA4396@cmpxchg.org>
Date:   Mon, 3 Feb 2020 14:50:48 -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 21/28] mm: memcg/slab: use a single set of kmem_caches
 for all memory cgroups

On Mon, Jan 27, 2020 at 09:34:46AM -0800, Roman Gushchin wrote:
> This is fairly big but mostly red patch, which makes all non-root
> slab allocations use a single set of kmem_caches instead of
> creating a separate set for each memory cgroup.
> 
> Because the number of non-root kmem_caches is now capped by the number
> of root kmem_caches, there is no need to shrink or destroy them
> prematurely. They can be perfectly destroyed together with their
> root counterparts. This allows to dramatically simplify the
> management of non-root kmem_caches and delete a ton of code.

This is definitely going in the right direction. But it doesn't quite
explain why we still need two sets of kmem_caches?

In the old scheme, we had completely separate per-cgroup caches with
separate slab pages. If a cgrouped process wanted to allocate a slab
object, we'd go to the root cache and used the cgroup id to look up
the right cgroup cache. On slab free we'd use page->slab_cache.

Now we have slab pages that have a page->objcg array. Why can't all
allocations go through a single set of kmem caches? If an allocation
is coming from a cgroup and the slab page the allocator wants to use
doesn't have an objcg array yet, we can allocate it on the fly, no?