Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 3 Feb 2020 14:50:48 -0500
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        linux-kernel@vger.kernel.org, kernel-team@fb.com,
        Bharata B Rao <bharata@linux.ibm.com>,
        Yafang Shao <laoar.shao@gmail.com>
Subject: Re: [PATCH v2 21/28] mm: memcg/slab: use a single set of kmem_caches
 for all memory cgroups
Message-ID: <20200203195048.GA4396@cmpxchg.org>
References: <20200127173453.2089565-1-guro@fb.com>
 <20200127173453.2089565-22-guro@fb.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200127173453.2089565-22-guro@fb.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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?