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.. _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
and interact with them.
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 **count** and **scan** files, which allow to
trigger *count_objects()* and *scan_objects()* callbacks for each memcg and
numa node (if applicable).
Usage:
------
1. *List registered shrinkers*
::
$ cd /sys/kernel/debug/shrinker/
$ ls
dquota-cache-16 sb-devpts-28 sb-proc-47 sb-tmpfs-42
mm-shadow-18 sb-devtmpfs-5 sb-proc-48 sb-tmpfs-43
mm-zspool:zram0-34 sb-hugetlbfs-17 sb-pstore-31 sb-tmpfs-44
rcu-kfree-0 sb-hugetlbfs-33 sb-rootfs-2 sb-tmpfs-49
sb-aio-20 sb-iomem-12 sb-securityfs-6 sb-tracefs-13
sb-anon_inodefs-15 sb-mqueue-21 sb-selinuxfs-22 sb-xfs:vda1-36
sb-bdev-3 sb-nsfs-4 sb-sockfs-8 sb-zsmalloc-19
sb-bpf-32 sb-pipefs-14 sb-sysfs-26 thp-deferred_split-10
sb-btrfs:vda2-24 sb-proc-25 sb-tmpfs-1 thp-zero-9
sb-cgroup2-30 sb-proc-39 sb-tmpfs-27 xfs-buf:vda1-37
sb-configfs-23 sb-proc-41 sb-tmpfs-29 xfs-inodegc:vda1-38
sb-dax-11 sb-proc-45 sb-tmpfs-35
sb-debugfs-7 sb-proc-46 sb-tmpfs-40
2. *Get information about a specific shrinker*
::
$ cd sb-btrfs\:vda2-24/
$ ls
count scan
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.
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.
::
$ cat count
1 224 2
21 98 0
55 818 10
2367 2 0
2401 30 0
225 13 0
599 35 0
939 124 0
1041 3 0
1075 1 0
1109 1 0
1279 60 0
1313 7 0
1347 39 0
1381 3 0
1449 14 0
1483 63 0
1517 53 0
1551 6 0
1585 1 0
1619 6 0
1653 40 0
1687 11 0
1721 8 0
1755 4 0
1789 52 0
1823 888 0
1857 1 0
1925 2 0
1959 32 0
2027 22 0
2061 9 0
2469 799 0
2537 861 0
2639 1 0
2707 70 0
2775 4 0
2877 84 0
293 1 0
735 8 0
4. *Scan objects*
The expected input format::
<cgroup inode id> <numa id> <number of objects to scan>
For a non-memcg-aware shrinker or on a system with no memory
cgrups **0** should be passed as cgroup id.
::
$ cd /sys/kernel/debug/shrinker/
$ cd sb-btrfs\:vda2-24/
$ cat count | head -n 5
1 212 0
21 97 0
55 802 5
2367 2 0
225 13 0
$ echo "55 0 200" > scan
$ cat count | head -n 5
1 212 0
21 96 0
55 752 5
2367 2 0
225 13 0
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