Re: [RFC PATCH 0/3] Memory Controller eBPF support

Michal Hocko
2025-11-20T19:20:21Z

On Thu 20-11-25 09:29:52, hui.zhu@linux.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?
--
Michal Hocko
SUSE Labs