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

[Shakeel Butt]

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@xxxxxxxxx> 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.