Re: [PATCH] cpuset: introduce non-blocking cpuset.mems setting option

[Tejun Heo <tj@kernel.org>]

On Wed, May 21, 2025 at 11:45:27AM +0800, Zhongkun He wrote:
> Setting the cpuset.mems in cgroup v2 can trigger memory
> migrate in cpuset. This behavior is fine for newly created
> cgroups but it can cause issues for the existing cgroups.
> In our scenario, modifying the cpuset.mems setting during
> peak times frequently leads to noticeable service latency
> or stuttering.
> 
> It is important to have a consistent set of behavior for
> both cpus and memory. But it does cause issues at times,
> so we would like to have a flexible option.
> 
> This idea is from the non-blocking limit setting option in
> memory control.
> 
> https://lore.kernel.org/all/20250506232833.3109790-1-shakeel.butt@linux.dev/
> 
> Signed-off-by: Zhongkun He <hezhongkun.hzk@bytedance.com>
> ---
>  Documentation/admin-guide/cgroup-v2.rst | 7 +++++++
>  kernel/cgroup/cpuset-internal.h         | 6 ++++++
>  kernel/cgroup/cpuset.c                  | 7 +++++++
>  3 files changed, 20 insertions(+)
> 
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 1a16ce68a4d7..d9e8e2a770af 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -2408,6 +2408,13 @@ Cpuset Interface Files
>  	a need to change "cpuset.mems" with active tasks, it shouldn't
>  	be done frequently.
>  
> +	If cpuset.mems is opened with O_NONBLOCK then the migration is
> +	bypassed. This is useful for admin processes that need to adjust
> +	the cpuset.mems dynamically without blocking. However, there is
> +	a risk that previously allocated pages are not within the new
> +	cpuset.mems range, which may be altered by move_pages syscall or
> +	numa_balance.

As said in the other thread, nack on this approach.

Thanks.

-- 
tejun