Re: [PATCH v4 4/6] mm: docs: document shrinker debugfs

[Muchun Song <songmuchun@bytedance.com>]

On Wed, May 25, 2022 at 01:25:58PM -0700, Roman Gushchin wrote:
> Add a document describing the shrinker debugfs interface.
> 
> Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
> ---
>  Documentation/admin-guide/mm/index.rst        |   1 +
>  .../admin-guide/mm/shrinker_debugfs.rst       | 100 ++++++++++++++++++
>  2 files changed, 101 insertions(+)
>  create mode 100644 Documentation/admin-guide/mm/shrinker_debugfs.rst
> 
> diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-guide/mm/index.rst
> index c21b5823f126..1bd11118dfb1 100644
> --- a/Documentation/admin-guide/mm/index.rst
> +++ b/Documentation/admin-guide/mm/index.rst
> @@ -36,6 +36,7 @@ the Linux memory management.
>     numa_memory_policy
>     numaperf
>     pagemap
> +   shrinker_debugfs
>     soft-dirty
>     swap_numa
>     transhuge
> diff --git a/Documentation/admin-guide/mm/shrinker_debugfs.rst b/Documentation/admin-guide/mm/shrinker_debugfs.rst
> new file mode 100644
> index 000000000000..2033d696aa59
> --- /dev/null
> +++ b/Documentation/admin-guide/mm/shrinker_debugfs.rst
> @@ -0,0 +1,100 @@
> +.. _shrinker_debugfs:
> +
> +==========================
> +Shrinker Debugfs Interface
> +==========================
> +
> +Shrinker debugfs interface provides a visibility into the kernel memory
> +shrinkers subsystem and allows to get information about individual shrinkers.
> +
> +For each shrinker registered in the system a directory in **<debugfs>/shrinker/**
> +is created. The directory's name is composed from the shrinker's name and an
> +unique id: e.g. *kfree_rcu-0* or *sb-xfs:vda1-36*.
> +
> +Each shrinker directory contains the **count** file, which allows to trigger
> +the *count_objects()* callback for each memcg and numa node (if applicable).
> +
> +Usage:
> +------
> +
> +1. *List registered shrinkers*
> +
> +  ::
> +
> +    $ cd /sys/kernel/debug/shrinker/
> +    $ ls
> +    dqcache-16          sb-hugetlbfs-17  sb-rootfs-2      sb-tmpfs-49
> +    kfree_rcu-0         sb-hugetlbfs-33  sb-securityfs-6  sb-tracefs-13
> +    sb-aio-20           sb-iomem-12      sb-selinuxfs-22  sb-xfs:vda1-36
> +    sb-anon_inodefs-15  sb-mqueue-21     sb-sockfs-8      sb-zsmalloc-19
> +    sb-bdev-3           sb-nsfs-4        sb-sysfs-26      shadow-18
> +    sb-bpf-32           sb-pipefs-14     sb-tmpfs-1       thp_deferred_split-10
> +    sb-btrfs:vda2-24    sb-proc-25       sb-tmpfs-27      thp_zero-9
> +    sb-cgroup2-30       sb-proc-39       sb-tmpfs-29      xfs_buf-vda1-37
> +    sb-configfs-23      sb-proc-41       sb-tmpfs-35      xfs_inodegc-vda1-38
> +    sb-dax-11           sb-proc-45       sb-tmpfs-40      zspool-zram0-34
> +    sb-debugfs-7        sb-proc-46       sb-tmpfs-42
> +    sb-devpts-28        sb-proc-47       sb-tmpfs-43
> +    sb-devtmpfs-5       sb-pstore-31     sb-tmpfs-44
> +
> +2. *Get information about a specific shrinker*
> +
> +  ::
> +
> +    $ cd sb-btrfs\:vda2-24/
> +    $ ls
> +    count
> +
> +3. *Count objects*
> +
> +  Each line in the output has the following format::
> +
> +    <cgroup inode id> <nr of objects on node 0> <nr of objects on node 1> ...
> +    <cgroup inode id> <nr of objects on node 0> <nr of objects on node 1> ...
> +    ...
> +
> +  If there are no objects on all numa nodes, a line is omitted. If there
> +  are no objects at all, the output might be empty.

Should we add the following lines into here?

"
If the shrinker is not memcg-aware or CONFIG_MEMCG is off, 0 is
printed as cgroup inode id. If the shrinker is not numa-aware, 0's are
printed for all nodes except the first one.
"

Thanks.