linux-kernel - Re: [PATCH v2] memcg: add per-memcg total kernel memory stat

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Date:   Fri, 4 Feb 2022 14:22:40 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Muchun Song <songmuchun@...edance.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v2] memcg: add per-memcg total kernel memory stat

On Thu, Feb 03, 2022 at 07:38:56PM +0000, Yosry Ahmed wrote:
> Currently memcg stats show several types of kernel memory:
> kernel stack, page tables, sock, vmalloc, and slab.
> However, there are other allocations with __GFP_ACCOUNT
> (or supersets such as GFP_KERNEL_ACCOUNT) that are not accounted
> in any of those stats, a few examples are:
> - various kvm allocations (e.g. allocated pages to create vcpus)
> - io_uring
> - tmp_page in pipes during pipe_write()
> - bpf ringbuffers
> - unix sockets
> 
> Keeping track of the total kernel memory is essential for the ease of
> migration from cgroup v1 to v2 as there are large discrepancies between
> v1's kmem.usage_in_bytes and the sum of the available kernel memory stats
> in v2. Adding separate memcg stats for all __GFP_ACCOUNT kernel
> allocations is an impractical maintenance burden as there a lot of those
> all over the kernel code, with more use cases likely to show up in the
> future.
> 
> Therefore, add a "kernel" memcg stat that is analogous to kmem
> page counter, with added benefits such as using rstat infrastructure
> which aggregates stats more efficiently. Additionally, this provides a
> lighter alternative in case the legacy kmem is deprecated in the future
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> Acked-by: Shakeel Butt <shakeelb@...gle.com>
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> Reported-by: kernel test robot <lkp@...el.com>

Looks good, thanks Yosry!