Re: [PATCH] mm/percpu, memcontrol: Per-memcg-lruvec percpu accounting

[Yosry Ahmed]

On Mon, Mar 30, 2026 at 7:21 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Mon 30-03-26 07:10:10, Joshua Hahn wrote:
> > On Mon, 30 Mar 2026 14:03:29 +0200 Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > > On Fri 27-03-26 12:19:35, Joshua Hahn wrote:
> > > > Convert MEMCG_PERCPU_B from a memcg_stat_item to a memcg_node_stat_item
> > > > to give visibility into per-node breakdowns for percpu allocations and
> > > > turn it into NR_PERCPU_B.
> > >
> > > Why do we need/want this?
> >
> > Hello Michal,
> >
> > Thank you for reviewing my patch! I hope you are doing well.
> >
> > You're right, I could have done a better job of motivating the patch.
> > My intent with this patch is to give some more visibility into where
> > memory is physically, once you know which memcg it is in.
>
> Please keep in mind that WHY is very often much more important than HOW
> in the patch so you should always start with the intention and
> justification.
>
> > Percpu memory could probably be seen as "trivial" when it comes to figuring
> > out what node it is on, but I'm hoping to make similar transitions to the
> > rest of enum memcg_stat_item as well (you can see my work for the zswap
> > stats in [1]).
> >
> > When all of the memory is moved from being tracked per-memcg to per-lruvec,
> > then the final vision would be able to attribute node placement within
> > each memcg, which can help with diagnosing things like asymmetric node
> > pressure within a memcg, which is currently only partially accurate.
> >
> > Getting per-node breakdowns of percpu memory orthogonal to memcgs also
> > seems like a win to me. While unlikely, I think that we can benefit from
> > some amount of visibility into whether percpu allocations are happening
> > equally across all CPUs.
> >
> > What do you think? Thank you again, I hope you have a great day!
>
> I think that you should have started with this intended outcome first
> rather than slicing it in pieces. Why do we want to shift to per-node
> stats for other/all counters? What is the cost associated comparing to the
> existing accounting (if any)? Please go into details on how do you plan
> to use the data before we commit into a lot of code churn.
>
> TBH I do not see any fundamental reasons why this would be impossible
> but I am not really sure this is worth the work and I also do not see
> potential subtle issues that we might stumble over when getting there.
> So I would appreciate if you could have a look into that deeper and
> provide us with evaluation on how do you want to achieve your end goal
> and what can we expect on the way. It is, of course, impossible to see
> all potential problems without starting implementing the thing but a
> high level evaluation would be really helpful.

You should probably also speak to extra memory overhead to move all
these stats from per-memcg to per-lruvec.