linux-kernel - Re: [PATCH] memcg: skip cgroup_file_notify if spinning is not allowed

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Message-ID: <87ecska85y.fsf@linux.dev>
Date: Fri, 05 Sep 2025 14:42:01 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>,  Tejun Heo <tj@...nel.org>,
  Johannes Weiner <hannes@...xchg.org>,  Michal Hocko <mhocko@...nel.org>,
  Muchun Song <muchun.song@...ux.dev>,  Alexei Starovoitov
 <ast@...nel.org>,  Peilin Ye <yepeilin@...gle.com>,  Kumar Kartikeya
 Dwivedi <memxor@...il.com>,  bpf@...r.kernel.org,  linux-mm@...ck.org,
  cgroups@...r.kernel.org,  linux-kernel@...r.kernel.org,  Meta kernel team
 <kernel-team@...a.com>
Subject: Re: [PATCH] memcg: skip cgroup_file_notify if spinning is not allowed

Shakeel Butt <shakeel.butt@...ux.dev> writes:

> On Fri, Sep 05, 2025 at 02:20:46PM -0700, Roman Gushchin wrote:
>> Shakeel Butt <shakeel.butt@...ux.dev> writes:
>> 
>> > Generally memcg charging is allowed from all the contexts including NMI
>> > where even spinning on spinlock can cause locking issues. However one
>> > call chain was missed during the addition of memcg charging from any
>> > context support. That is try_charge_memcg() -> memcg_memory_event() ->
>> > cgroup_file_notify().
>> >
>> > The possible function call tree under cgroup_file_notify() can acquire
>> > many different spin locks in spinning mode. Some of them are
>> > cgroup_file_kn_lock, kernfs_notify_lock, pool_workqeue's lock. So, let's
>> > just skip cgroup_file_notify() from memcg charging if the context does
>> > not allow spinning.
>> 
>> Hmm, what about OOM events? Losing something like MEMCG_LOW doesn't look
>> like a bit deal, but OOM events can be way more important.
>> 
>> Should we instead preserve the event (e.g. as a pending_event_mask) and
>> raise it on the next occasion / from a different context?
>>
>
> Thanks for the review. For now only MAX can happen in non-spinning
> context. All others only happen in process context. Maybe with BPF OOM,
> OOM might be possible in a different context (is that what you are
> thinking?). I think we can add the complexity of preserving the event
> when the actual need arise.

No, I haven't thought about any particular use case, just a bit
worried about silently dropping some events. It might be not an issue
now, but might be easy to miss a moment when it becomes a problem.

So in my opinion using some delayed delivery mechanism is better
than just dropping these events.