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

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Message-ID: <eba3aiglp2hmj65sd4vsmav26o45orrlog2ifexd44yovygcdg@43wfk6dbgqda>
Date: Fri, 19 Sep 2025 21:31:30 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Tejun Heo <tj@...nel.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, 
	Alexei Starovoitov <ast@...nel.org>, Peilin Ye <yepeilin@...gle.com>, 
	Kumar Kartikeya Dwivedi <memxor@...il.com>, bpf <bpf@...r.kernel.org>, linux-mm <linux-mm@...ck.org>, 
	"open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>, LKML <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 19, 2025 at 07:47:57PM -0700, Alexei Starovoitov wrote:
> On Thu, Sep 18, 2025 at 7:49 PM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> >
[...]
> > +
> > +static DEFINE_PER_CPU(struct defer_memcg_events, postpone_events) = {
> > +       .memcg_llist = LLIST_HEAD_INIT(memcg_llist),
> > +       .work = IRQ_WORK_INIT(process_deferred_events),
> > +};
> 
> Why all these per cpu stuff ? I don't understand what it helps with.
> To have max_events per-cpu?
> Just one global llist and irq_work will do just fine.
> and global max_events too.

Right, global llist and irq_work will work but max_events has to be
per-memcg. In addition we will need llist_node per-memcg and I think we
will need to do a similar cmpxchg trick I did in css_rstat_updated() to
eliminate the potential race of llist_node of a given memcg i.e.
multiple CPUs trying to add llist_node of a memcg to the global llist.

This can work but I am still debating if this additional complexity is
worth it as compared to the original patch where we skip
cgroup_file_notify() for !allow_spinning condition.