Re: [PATCH v2] docs: cgroup-v1: Document the cpuset_v2_mode mount option

[Waiman Long <longman@redhat.com>]

On 3/30/20 10:06 AM, Waiman Long wrote:
> The cpuset in cgroup v1 accepts a special "cpuset_v2_mode" mount
> option that make cpuset.cpus and cpuset.mems behave more like those in
> cgroup v2.  Document it to make other people more aware of this feature
> that can be useful in some circumstances.
>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  Documentation/admin-guide/cgroup-v1/cpusets.rst | 11 +++++++++++
>  kernel/cgroup/cpuset.c                          |  8 ++++++--
>  2 files changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/admin-guide/cgroup-v1/cpusets.rst b/Documentation/admin-guide/cgroup-v1/cpusets.rst
> index 86a6ae995d54..7ade3abd342a 100644
> --- a/Documentation/admin-guide/cgroup-v1/cpusets.rst
> +++ b/Documentation/admin-guide/cgroup-v1/cpusets.rst
> @@ -223,6 +223,17 @@ cpu_online_mask using a CPU hotplug notifier, and the mems file
>  automatically tracks the value of node_states[N_MEMORY]--i.e.,
>  nodes with memory--using the cpuset_track_online_nodes() hook.
>  
> +The cpuset.effective_cpus and cpuset.effective_mems files are
> +normally read-only copies of cpuset.cpus and cpuset.mems files
> +respectively.  If the cpuset cgroup filesystem is mounted with the
> +special "cpuset_v2_mode" option, the behavior of these files will become
> +similar to the corresponding files in cpuset v2.  In other words, hotplug
> +events will not change cpuset.cpus and cpuset.mems.  Those events will
> +only affect cpuset.effective_cpus and cpuset.effective_mems which show
> +the actual cpus and memory nodes that are currently used by this cpuset.
> +See Documentation/admin-guide/cgroup-v2.rst for more information about
> +cpuset v2 behavior.
> +
>  
>  1.4 What are exclusive cpusets ?
>  --------------------------------
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 58f5073acff7..729d3a5c772e 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -358,8 +358,12 @@ static DECLARE_WORK(cpuset_hotplug_work, cpuset_hotplug_workfn);
>  static DECLARE_WAIT_QUEUE_HEAD(cpuset_attach_wq);
>  
>  /*
> - * Cgroup v2 behavior is used when on default hierarchy or the
> - * cgroup_v2_mode flag is set.
> + * Cgroup v2 behavior is used on the "cpus" and "mems" control files when
> + * on default hierarchy or when the cpuset_v2_mode flag is set by mounting
> + * the v1 cpuset cgroup filesystem with the "cpuset_v2_mode" mount option.
> + * With v2 behavior, "cpus" and "mems" are always what the users have
> + * requested and won't be changed by hotplug events. Only the effective
> + * cpus or mems will be affected.
>   */
>  static inline bool is_in_v2_mode(void)
>  {

On second thought, I should have clarified the cpuset_v2_mode option in
the cpuset.c file itself also.

Cheers,
Longman