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

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Date:   Thu, 26 May 2022 13:35:20 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Dave Chinner <dchinner@...hat.com>,
        linux-kernel@...r.kernel.org,
        Kent Overstreet <kent.overstreet@...il.com>,
        Hillf Danton <hdanton@...a.com>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>
Subject: Re: [PATCH v4 4/6] mm: docs: document shrinker debugfs

On Thu, May 26, 2022 at 10:25:47AM +0800, Muchun Song wrote:
> 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@...ux.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.
> "

Yes, it sounds like a great addition.
I hope Andrew can merge this in if there will be no v5. Or I'll
add it by myself otherwise. Thanks!

And thank you for reviewing the patches!