linux-kernel - Re: [PATCH v2 0/2] memcg: reading memcg stats more efficiently

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Message-ID: <2fa573e6-bd9a-46b9-a2a6-bfb233d0389a@gmail.com>
Date: Thu, 16 Oct 2025 13:26:44 -0700
From: JP Kobryn <inwardvessel@...il.com>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: Shakeel Butt <shakeel.butt@...ux.dev>, andrii@...nel.org, ast@...nel.org,
 mkoutny@...e.com, yosryahmed@...gle.com, hannes@...xchg.org, tj@...nel.org,
 akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
 cgroups@...r.kernel.org, linux-mm@...ck.org, bpf@...r.kernel.org,
 kernel-team@...a.com, mhocko@...nel.org, muchun.song@...ux.dev
Subject: Re: [PATCH v2 0/2] memcg: reading memcg stats more efficiently

On 10/15/25 6:10 PM, Roman Gushchin wrote:
> JP Kobryn <inwardvessel@...il.com> writes:
> 
>> On 10/15/25 1:46 PM, Shakeel Butt wrote:
>>> Cc memcg maintainers.
>>> On Wed, Oct 15, 2025 at 12:08:11PM -0700, JP Kobryn wrote:
>>>> When reading cgroup memory.stat files there is significant kernel overhead
>>>> in the formatting and encoding of numeric data into a string buffer. Beyond
>>>> that, the given user mode program must decode this data and possibly
>>>> perform filtering to obtain the desired stats. This process can be
>>>> expensive for programs that periodically sample this data over a large
>>>> enough fleet.
>>>>
>>>> As an alternative to reading memory.stat, introduce new kfuncs that allow
>>>> fetching specific memcg stats from within cgroup iterator based bpf
>>>> programs. This approach allows for numeric values to be transferred
>>>> directly from the kernel to user mode via the mapped memory of the bpf
>>>> program's elf data section. Reading stats this way effectively eliminates
>>>> the numeric conversion work needed to be performed in both kernel and user
>>>> mode. It also eliminates the need for filtering in a user mode program.
>>>> i.e. where reading memory.stat returns all stats, this new approach allows
>>>> returning only select stats.
> 
> It seems like I've most of these functions implemented as part of
> bpfoom: https://lkml.org/lkml/2025/8/18/1403
> 
> So I definitely find them useful. Would be nice to merge our efforts.

Sounds great. I see in your series that you allow the kfuncs to accept
integers as item numbers. Would my approach of using typed enums work
for you? I wanted to take advantage of libbpf core so that the bpf
program could gracefully handle cases where a given enumerator is not
present in a given kernel version. I made use of this in the selftests.

I'm planning on sending out a v3 so let me know if you would like to see
any alterations that would align with bpfoom.