linux-kernel - Re: [RFC PATCH v2 0/3] Memory Controller eBPF support

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Message-ID: <enlefo5mmoha2htsrvv76tdmj6yum4jan6hgym76adtpxuhvrp@aug6qh3ocde5>
Date: Tue, 30 Dec 2025 10:49:25 +0100
From: Michal Koutný <mkoutny@...e.com>
To: Hui Zhu <hui.zhu@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Shakeel Butt <shakeel.butt@...ux.dev>, 
	Muchun Song <muchun.song@...ux.dev>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, 
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, Shuah Khan <shuah@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Miguel Ojeda <ojeda@...nel.org>, 
	Nathan Chancellor <nathan@...nel.org>, Kees Cook <kees@...nel.org>, Tejun Heo <tj@...nel.org>, 
	Jeff Xu <jeffxu@...omium.org>, Jan Hendrik Farr <kernel@...rr.cc>, 
	Christian Brauner <brauner@...nel.org>, Randy Dunlap <rdunlap@...radead.org>, 
	Brian Gerst <brgerst@...il.com>, Masahiro Yamada <masahiroy@...nel.org>, davem@...emloft.net, 
	Jakub Kicinski <kuba@...nel.org>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, cgroups@...r.kernel.org, bpf@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, Hui Zhu <zhuhui@...inos.cn>
Subject: Re: [RFC PATCH v2 0/3] Memory Controller eBPF support

Hi Hui.

On Tue, Dec 30, 2025 at 11:01:58AM +0800, Hui Zhu <hui.zhu@...ux.dev> wrote:
> This allows administrators to suppress low-priority cgroups' memory
> usage based on custom policies implemented in BPF programs.

BTW memory.low was conceived as a work-conserving mechanism for
prioritization of different workloads. Have you tried that? No need to
go directly to (high) limits. (<- Main question, below are some
secondary implementation questions/remarks.)

...
> This series introduces a BPF hook that allows reporting
> additional "pages over high" for specific cgroups, effectively
> increasing memory pressure and throttling for lower-priority
> workloads when higher-priority cgroups need resources.

Have you considered hooking into calculate_high_delay() instead? (That
function has undergone some evolution so it'd seem like the candidate
for BPFication.)

...
> 3. Cgroup hierarchy management (inheritance during online/offline)

I see you're copying the program upon memcg creation.
Configuration copies aren't such a good way to properly handle
hierarchical behavior.
I wonder if this could follow the more generic pattern of how BPF progs
are evaluated in hierarchies, see BPF_F_ALLOW_OVERRIDE and
BPF_F_ALLOW_MULTI.


> Example Results
...
> Results show the low-priority cgroup (/sys/fs/cgroup/low) was
> significantly throttled:
> - High-priority cgroup: 21,033,377 bogo ops at 347,825 ops/s
> - Low-priority cgroup: 11,568 bogo ops at 177 ops/s
> 
> The stress-ng process in the low-priority cgroup experienced a
> ~99.9% slowdown in memory operations compared to the
> high-priority cgroup, demonstrating effective priority
> enforcement through BPF-controlled memory pressure.

As a demonstrator, it'd be good to compare this with a baseline without
any extra progs, e.g. show that high-prio performed better and low-prio
wasn't throttled for nothing.

Thanks,
Michal

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