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

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Message-ID: <87cy848qpf.fsf@linux.dev>
Date: Fri, 05 Sep 2025 15:44:28 -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 02:42:01PM -0700, Roman Gushchin wrote:
>> 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.
>> 
>
> Only the notification can be dropped and not the event (i.e. we are
> still incrementing the counters). Also for MAX only but I got your
> point.
>
>> So in my opinion using some delayed delivery mechanism is better
>> than just dropping these events.
>
> Let me see how doing this irq_work looks like and will update here.

Thanks!

If it won't work out for some reason, maybe at least explicitly
narrow it down to the MEMCG_MAX events.