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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Mon, Jun 16, 2025 at 01:08:49PM -0700, JP Kobryn wrote:
> 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>
> 

Thanks a lot.