Re: [PATCH 0/8] per-memcg-per-node kmem accounting

[Joshua Hahn]

On Mon, 11 May 2026 22:20:35 +0200 Alexandre Ghiti <alex@xxxxxxxx> wrote:

> This series pursues the work initiated by Joshua [1]. We need kernel  
> memory to be accounted on a per-node basis in order to be able to  
> know the memcg and physical memory association.  
>   
> This series takes advantage of the recent introduction of per-node  
> obj_cgroup [2] and makes those obj_cgroup tied to their numa node.  
>   
> The bulk of the series is percpu per-node accounting: percpu  
> "precharges" the memcg before we know the actual location of the pages  
> it uses, so charging and accounting had to be split. All other kmem 
> users (slab, zswap, __memcg_kmem_charge_page) are straightforward 
> conversions (zswap support is limited in this series because Joshua 
> is working on it in parallel [3]). 
>  
> Thanks Joshua for your early feedbacks! 

Hello Alex,

Thank you for your work!

Overall I think the direction makes sense to me. Pre-overcharging makes sense to
me as an approach, we would much rather overaccount than underaccount and
later have to breach limits.

I do have some concerns on performance, though. Namely, I think there are
some expensive operations that I think would benefit from some performane
benchmarking with this patch added (maybe some simple microbenchmarks that
demonstrates kernel allocation overhead could be useful).

>From what I can tell, there is some additional performance overhead that has
to do with iterating over num_possible_cpus() x pages_per_alloc, which
doesn't seem trivial to me.

Another concern that I see is the stock credit system. Maybe we could be
bypassing the stock check leading to more time spent doing the atomic
operations.

obj_stock caches a single obj_cgroup, which means that if we split the objcg
to be per-node (in patch 6), then the obj_stock basically gets invalidated
every operation since we iterate over more objcgs (even though we are in
the same logical objcg). Maybe I'm missing something?

I haven't taken a deep look at the implementation details but just wanted to
raise some high level items that I noticed. Of course, all of these concerns
are just theoretical, if you can show that the performance delta is not
noticable then all of my concerns don't matter.

I also want to talk more about the local credit system but let's first see
what the numbers are first.

Thanks again, Alex. And I really like patch 2 because it is a solution to
a problem that I ran into in my percpu tracking series that I couldn't think
of before! Thank you for solving my problem too : -)

Have a great day!
Joshua
   
> [1] https://lore.kernel.org/linux-mm/20260404033844.1892595-1-joshua.hahnjy@xxxxxxxxx/  
> [2] https://lore.kernel.org/linux-mm/56c04b1c5d54f75ccdc12896df6c1ca35403ecc3.1772711148.git.zhengqi.arch@xxxxxxxxxxxxx/  
> [3] https://lore.kernel.org/linux-mm/20260311195153.4013476-1-joshua.hahnjy@xxxxxxxxx/
> 
> Alexandre Ghiti (8):
>   mm: memcontrol: propagate NMI slab stats to memcg vmstats
>   mm: percpu: charge obj_exts allocation with __GFP_ACCOUNT
>   mm: percpu: Split memcg charging and kmem accounting
>   mm: memcontrol: track MEMCG_KMEM per NUMA node
>   mm: memcontrol: per-node kmem accounting for page charges
>   mm: slab: per-node kmem accounting for slab
>   mm: percpu: per-node kmem accounting using local credit
>   mm: zswap: per-node kmem accounting for zswap/zsmalloc
> 
>  include/linux/memcontrol.h |  27 +++++--
>  include/linux/mmzone.h     |   1 +
>  include/linux/zsmalloc.h   |   2 +
>  mm/memcontrol.c            | 150 ++++++++++++++++++++++++++++---------
>  mm/percpu-internal.h       |  16 +---
>  mm/percpu.c                |  90 ++++++++++++++++++++--
>  mm/vmstat.c                |   1 +
>  mm/zsmalloc.c              |  11 +++
>  mm/zswap.c                 |   9 ++-
>  9 files changed, 242 insertions(+), 65 deletions(-)
> 
> -- 
> 2.54.0
> 
>