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

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Message-ID: <6bcjnhdsbyfmlua2x7olz6w3gheejfatnrtn5qu7ls5svegrok@zeatti7whrnq>
Date: Fri, 5 Sep 2025 14:50:17 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Roman Gushchin <roman.gushchin@...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

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.