Re: [PATCH v2 0/4] cgroup: nmi safe css_rstat_updated

[JP Kobryn <inwardvessel@gmail.com>]

On 6/11/25 3:15 PM, Shakeel Butt wrote:
> BPF programs can run in nmi context and may trigger memcg charged memory
> allocation in such context. Recently linux added support to nmi safe
> page allocation along with memcg charging of such allocations. However
> the kmalloc/slab support and corresponding memcg charging is still
> lacking,
> 
> To provide nmi safe support for memcg charging for kmalloc/slab
> allocations, we need nmi safe memcg stats because for kernel memory
> charging and stats happen together. At the moment, memcg charging and
> memcg stats are nmi safe and the only thing which is not nmi safe is
> adding the cgroup to the per-cpu rstat update tree. i.e.
> css_rstat_updated() which this series is doing.
> 
> This series made css_rstat_updated by using per-cpu lockless lists whose
> node in embedded in individual struct cgroup_subsys_state and the
> per-cpu head is placed in struct cgroup_subsys. For rstat users without
> cgroup_subsys, a global per-cpu lockless list head is created. The main
> challenge to use lockless in this scenario was the potential multiple
> inserters from the stacked context i.e. process, softirq, hardirq & nmi,
> potentially using the same per-cpu lockless node of a given
> cgroup_subsys_state. The normal lockless list does not protect against
> such scenario.
> 
> The multiple stacked inserters using potentially same lockless node was
> resolved by making one of them succeed on reset the lockless node and the
> winner gets to insert the lockless node in the corresponding lockless
> list. The losers can assume the lockless list insertion will eventually
> succeed and continue their operation.
> 
> Changelog since v2:
> - Add more clear explanation in cover letter and in the comment as
>    suggested by Andrew, Michal & Tejun.
> - Use this_cpu_cmpxchg() instead of try_cmpxchg() as suggested by Tejun.
> - Remove the per-cpu ss locks as they are not needed anymore.
> 
> Changelog since v1:
> - Based on Yosry's suggestion always use llist on the update side and
>    create the update tree on flush side
> 
> [v1] https://lore.kernel.org/cgroups/20250429061211.1295443-1-shakeel.butt@linux.dev/
>   
> 
> 
> Shakeel Butt (4):
>    cgroup: support to enable nmi-safe css_rstat_updated
>    cgroup: make css_rstat_updated nmi safe
>    cgroup: remove per-cpu per-subsystem locks
>    memcg: cgroup: call css_rstat_updated irrespective of in_nmi()
> 
>   include/linux/cgroup-defs.h   |  11 +--
>   include/trace/events/cgroup.h |  47 ----------
>   kernel/cgroup/rstat.c         | 169 +++++++++++++---------------------
>   mm/memcontrol.c               |  10 +-
>   4 files changed, 74 insertions(+), 163 deletions(-)
> 

I tested this series by doing some updates/flushes on a cgroup hierarchy
with four levels. This tag can be added to the patches in this series.

Tested-by: JP Kobryn <inwardvessel@gmail.com>