Message-ID: <aSWdSlhU3acQ9Rq1@tiehlicka>
Date: Tue, 25 Nov 2025 13:12:58 +0100
From: Michal Hocko <mhocko@...e.com>
To: hui.zhu@...ux.dev
Cc: Roman Gushchin <roman.gushchin@...ux.dev>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	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>,
	mkoutny@...e.com, 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>,
	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 0/3] Memory Controller eBPF support

On Fri 21-11-25 02:46:31, hui.zhu@...ux.dev wrote:
> 2025年11月21日 03:20, "Michal Hocko" <mhocko@...e.com mailto:mhocko@...e.com?to=%22Michal%20Hocko%22%20%3Cmhocko%40suse.com%3E > 写到:
> 
> 
> > 
> > On Thu 20-11-25 09:29:52, hui.zhu@...ux.dev wrote:
> > [...]
> > 
> > > 
> > > I generally agree with an idea to use BPF for various memcg-related
> > >  policies, but I'm not sure how specific callbacks can be used in
> > >  practice.
> > >  
> > >  Hi Roman,
> > >  
> > >  Following are some ideas that can use ebpf memcg:
> > >  
> > >  Priority‑Based Reclaim and Limits in Multi‑Tenant Environments:
> > >  On a single machine with multiple tenants / namespaces / containers,
> > >  under memory pressure it’s hard to decide “who should be squeezed first”
> > >  with static policies baked into the kernel.
> > >  Assign a BPF profile to each tenant’s memcg:
> > >  Under high global pressure, BPF can decide:
> > >  Which memcgs’ memory.high should be raised (delaying reclaim),
> > >  Which memcgs should be scanned and reclaimed more aggressively.
> > >  
> > >  Online Profiling / Diagnosing Memory Hotspots:
> > >  A cgroup’s memory keeps growing, but without patching the kernel it’s
> > >  difficult to obtain fine‑grained information.
> > >  Attach BPF to the memcg charge/uncharge path:
> > >  Record large allocations (greater than N KB) with call stacks and
> > >  owning file/module, and send them to user space via a BPF ring buffer.
> > >  Based on sampled data, generate:
> > >  “Top N memory allocation stacks in this container over the last 10 minutes,”
> > >  Reports of which objects / call paths are growing fastest.
> > >  This makes it possible to pinpoint the root cause of host memory
> > >  anomalies without changing application code, which is very useful
> > >  in operations/ops scenarios.
> > >  
> > >  SLO‑Driven Auto Throttling / Scale‑In/Out Signals:
> > >  Use eBPF to observe memory usage slope, frequent reclaim,
> > >  or near‑OOM behavior within a memcg.
> > >  When it decides “OOM is imminent,” instead of just killing/raising
> > >  limits, it can emit a signal to a control‑plane component.
> > >  For example, send an event to a user‑space agent to trigger
> > >  automatic scaling, QPS adjustment, or throttling.
> > >  
> > >  Prevent a cgroup from launching a large‑scale fork+malloc attack:
> > >  BPF checks per‑uid or per‑cgroup allocation behavior over the
> > >  last few seconds during memcg charge.
> > > 
> > AFAIU, these are just very high level ideas rather than anything you are
> > trying to target with this patch series, right?
> > 
> > All I can see is that you add a reclaim hook but it is not really clear
> > to me how feasible it is to actually implement a real memory reclaim
> > strategy this way.
> > 
> > In prinicipal I am not really opposed but the memory reclaim process is
> > rather involved process and I would really like to see there is
> > something real to be done without exporting all the MM code to BPF for
> > any practical use. Is there any POC out there?
> 
> Hi Michal,
> 
> I apologize for not delivering a more substantial POC.
> 
> I was hesitant to add extensive eBPF support to memcg
> because I wasn't certain it aligned with the community's
> vision—and such support would require introducing many
> eBPF hooks into memcg.
> 
> I will add more eBPF hook to memcg and provide a more
> meaningful POC in the next version.

Just to make sure we are on the same page. I am not suggesting we need
more of those hooks. I just want to see how many do we really need in
order to have a sensible eBPF driven reclaim policy which seems to be
the main usecase you want to puruse, right?
-- 
Michal Hocko
SUSE Labs

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