[PATCH rfc 0/5] mm: introduce shrinker sysfs interface

[Roman Gushchin <roman.gushchin@linux.dev>]

There are 50+ different shrinkers in the kernel, many with their own bells and
whistles. Under the memory pressure the kernel applies some pressure on each of
them in the order of which they were created/registered in the system. Some
of them can contain only few objects, some can be quite large. Some can be
effective at reclaiming memory, some not.

The only existing debugging mechanism is a couple of tracepoints in
do_shrink_slab(): mm_shrink_slab_start and mm_shrink_slab_end. They aren't
covering everything though: shrinkers which report 0 objects will never show up,
there is no support for memcg-aware shrinkers. Shrinkers are identified by their
scan function, which is not always enough (e.g. hard to guess which super
block's shrinker it is having only "super_cache_scan"). They are a passive
mechanism: there is no way to call into counting and scanning of an individual
shrinker and profile it.

To provide a better visibility and debug options for memory shrinkers
this patchset introduces a /sys/kernel/shrinker interface, to some extent
similar to /sys/kernel/slab.

For each shrinker registered in the system a folder is created. The folder
contains "count" and "scan" files, which allow to trigger count_objects()
and scan_objects() callbacks. For memcg-aware and numa-aware shrinkers
count_memcg, scan_memcg, count_node, scan_node, count_memcg_node
and scan_memcg_node are additionally provided. They allow to get per-memcg
and/or per-node object count and shrink only a specific memcg/node.

To make debugging more pleasant, the patchset also names all shrinkers,
so that sysfs entries can have more meaningful names.

Usage examples:

1) List registered shrinkers:
  $ cd /sys/kernel/shrinker/
  $ ls
    dqcache-16          sb-cgroup2-30    sb-hugetlbfs-33  sb-proc-41       sb-selinuxfs-22  sb-tmpfs-40    sb-zsmalloc-19
    kfree_rcu-0         sb-configfs-23   sb-iomem-12      sb-proc-44       sb-sockfs-8      sb-tmpfs-42    shadow-18
    sb-aio-20           sb-dax-11        sb-mqueue-21     sb-proc-45       sb-sysfs-26      sb-tmpfs-43    thp_deferred_split-10
    sb-anon_inodefs-15  sb-debugfs-7     sb-nsfs-4        sb-proc-47       sb-tmpfs-1       sb-tmpfs-46    thp_zero-9
    sb-bdev-3           sb-devpts-28     sb-pipefs-14     sb-pstore-31     sb-tmpfs-27      sb-tmpfs-49    xfs_buf-37
    sb-bpf-32           sb-devtmpfs-5    sb-proc-25       sb-rootfs-2      sb-tmpfs-29      sb-tracefs-13  xfs_inodegc-38
    sb-btrfs-24         sb-hugetlbfs-17  sb-proc-39       sb-securityfs-6  sb-tmpfs-35      sb-xfs-36      zspool-34

2) Get information about a specific shrinker:
  $ cd sb-btrfs-24/
  $ ls
    count  count_memcg  count_memcg_node  count_node  scan  scan_memcg  scan_memcg_node  scan_node

3) Count objects on the system/root cgroup level
  $ cat count
    212

4) Count objects on the system/root cgroup level per numa node (on a 2-node machine)
  $ cat count_node
    209 3

5) Count objects for each memcg (output format: cgroup inode, count)
  $ cat count_memcg
    1 212
    20 96
    53 817
    2297 2
    218 13
    581 30
    911 124
    <CUT>

6) Same but with a per-node output
  $ cat count_memcg_node
    1 209 3
    20 96 0
    53 810 7
    2297 2 0
    218 13 0
    581 30 0
    911 124 0
    <CUT>

7) Don't display cgroups with less than 500 attached objects
  $ echo 500 > count_memcg
  $ cat count_memcg
    53 817
    1868 886
    2396 799
    2462 861

8) Don't display cgroups with less than 500 attached objects (sum over all nodes)
  $ echo "500" > count_memcg_node
  $ cat count_memcg_node
    53 810 7
    1868 886 0
    2396 799 0
    2462 861 0

9) Scan system/root shrinker
  $ cat count
    212
  $ echo 100 > scan
  $ cat scan
    97
  $ cat count
    115

10) Scan individual memcg
  $ echo "1868 500" > scan_memcg
  $ cat scan_memcg
    193

11) Scan individual node
  $ echo "1 200" > scan_node
  $ cat scan_node
    2

12) Scan individual memcg and node
  $ echo "1868 0 500" > scan_memcg_node
  $ cat scan_memcg_node
    435

If the output doesn't fit into a single page, "...\n" is printed at the end of
output.


Roman Gushchin (5):
  mm: introduce sysfs interface for debugging kernel shrinker
  mm: memcontrol: introduce mem_cgroup_ino() and
    mem_cgroup_get_from_ino()
  mm: introduce memcg interfaces for shrinker sysfs
  mm: introduce numa interfaces for shrinker sysfs
  mm: provide shrinkers with names

 arch/x86/kvm/mmu/mmu.c                        |   2 +-
 drivers/android/binder_alloc.c                |   2 +-
 drivers/gpu/drm/i915/gem/i915_gem_shrinker.c  |   3 +-
 drivers/gpu/drm/msm/msm_gem_shrinker.c        |   2 +-
 .../gpu/drm/panfrost/panfrost_gem_shrinker.c  |   2 +-
 drivers/gpu/drm/ttm/ttm_pool.c                |   2 +-
 drivers/md/bcache/btree.c                     |   2 +-
 drivers/md/dm-bufio.c                         |   2 +-
 drivers/md/dm-zoned-metadata.c                |   2 +-
 drivers/md/raid5.c                            |   2 +-
 drivers/misc/vmw_balloon.c                    |   2 +-
 drivers/virtio/virtio_balloon.c               |   2 +-
 drivers/xen/xenbus/xenbus_probe_backend.c     |   2 +-
 fs/erofs/utils.c                              |   2 +-
 fs/ext4/extents_status.c                      |   3 +-
 fs/f2fs/super.c                               |   2 +-
 fs/gfs2/glock.c                               |   2 +-
 fs/gfs2/main.c                                |   2 +-
 fs/jbd2/journal.c                             |   2 +-
 fs/mbcache.c                                  |   2 +-
 fs/nfs/nfs42xattr.c                           |   7 +-
 fs/nfs/super.c                                |   2 +-
 fs/nfsd/filecache.c                           |   2 +-
 fs/nfsd/nfscache.c                            |   2 +-
 fs/quota/dquot.c                              |   2 +-
 fs/super.c                                    |   2 +-
 fs/ubifs/super.c                              |   2 +-
 fs/xfs/xfs_buf.c                              |   2 +-
 fs/xfs/xfs_icache.c                           |   2 +-
 fs/xfs/xfs_qm.c                               |   2 +-
 include/linux/memcontrol.h                    |   9 +
 include/linux/shrinker.h                      |  25 +-
 kernel/rcu/tree.c                             |   2 +-
 lib/Kconfig.debug                             |   9 +
 mm/Makefile                                   |   1 +
 mm/huge_memory.c                              |   4 +-
 mm/memcontrol.c                               |  23 +
 mm/shrinker_debug.c                           | 792 ++++++++++++++++++
 mm/vmscan.c                                   |  66 +-
 mm/workingset.c                               |   2 +-
 mm/zsmalloc.c                                 |   2 +-
 net/sunrpc/auth.c                             |   2 +-
 42 files changed, 957 insertions(+), 47 deletions(-)
 create mode 100644 mm/shrinker_debug.c

-- 
2.35.1