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

[Waiman Long <llong@redhat.com>]

On 5/19/25 11:15 PM, 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 hope 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.c                  | 11 +++++++++++
>   2 files changed, 18 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.
> +
>     cpuset.mems.effective
>   	A read-only multiple values file which exists on all
>   	cpuset-enabled cgroups.
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 24b70ea3e6ce..2a0867e0c6d2 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -3208,7 +3208,18 @@ ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
>   		retval = update_exclusive_cpumask(cs, trialcs, buf);
>   		break;
>   	case FILE_MEMLIST:
> +		bool skip_migrate_once = false;
> +
> +		if ((of->file->f_flags & O_NONBLOCK) &&
> +			is_memory_migrate(cs) &&
> +			!cpuset_update_flag(CS_MEMORY_MIGRATE, cs, 0))
> +			skip_migrate_once = true;
> +
>   		retval = update_nodemask(cs, trialcs, buf);
> +
> +		/* Restore the migrate flag */
> +		if (skip_migrate_once)
> +			cpuset_update_flag(CS_MEMORY_MIGRATE, cs, 1);
>   		break;
>   	default:
>   		retval = -EINVAL;

I would prefer to temporarily make is_memory_migrate() helper return 
false by also checking an internal variable, for example, instead of 
messing with the cpuset flags.

Cheers,
Longman