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Date: Sun, 22 May 2022 19:35:59 +0800
From: Muchun Song <songmuchun@...edance.com>
To: Roman Gushchin <roman.gushchin@...ux.dev>
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 v3 6/6] mm: shrinkers: add scan interface for shrinker
debugfs
On Mon, May 09, 2022 at 11:38:20AM -0700, Roman Gushchin wrote:
> Add a scan interface which allows to trigger scanning of a particular
> shrinker and specify memcg and numa node. It's useful for testing,
> debugging and profiling of a specific scan_objects() callback.
> Unlike alternatives (creating a real memory pressure and dropping
> caches via /proc/sys/vm/drop_caches) this interface allows to interact
> with only one shrinker at once. Also, if a shrinker is misreporting
> the number of objects (as some do), it doesn't affect scanning.
>
> Signed-off-by: Roman Gushchin <roman.gushchin@...ux.dev>
> ---
> .../admin-guide/mm/shrinker_debugfs.rst | 39 +++++++++-
> mm/shrinker_debug.c | 73 +++++++++++++++++++
> 2 files changed, 108 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/shrinker_debugfs.rst b/Documentation/admin-guide/mm/shrinker_debugfs.rst
> index 6783f8190e63..8fecf81d60ee 100644
> --- a/Documentation/admin-guide/mm/shrinker_debugfs.rst
> +++ b/Documentation/admin-guide/mm/shrinker_debugfs.rst
> @@ -5,14 +5,16 @@ Shrinker Debugfs Interface
> ==========================
>
> Shrinker debugfs interface provides a visibility into the kernel memory
> -shrinkers subsystem and allows to get information about individual shrinkers.
> +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 the **count** file, which allows to trigger
> -the *count_objects()* callback for each memcg and numa node (if applicable).
> +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:
> ------
> @@ -43,7 +45,7 @@ Usage:
>
> $ cd sb-btrfs\:vda2-24/
> $ ls
> - count
> + count scan
>
> 3. *Count objects*
>
> @@ -98,3 +100,32 @@ Usage:
> 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
> diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
> index 28b1c1ab60ef..8f67fef5a643 100644
> --- a/mm/shrinker_debug.c
> +++ b/mm/shrinker_debug.c
> @@ -101,6 +101,77 @@ static int shrinker_debugfs_count_show(struct seq_file *m, void *v)
> }
> DEFINE_SHOW_ATTRIBUTE(shrinker_debugfs_count);
>
> +static int shrinker_debugfs_scan_open(struct inode *inode, struct file *file)
> +{
> + file->private_data = inode->i_private;
> + return nonseekable_open(inode, file);
> +}
> +
> +static ssize_t shrinker_debugfs_scan_write(struct file *file,
> + const char __user *buf,
> + size_t size, loff_t *pos)
> +{
> + struct shrinker *shrinker = (struct shrinker *)file->private_data;
Seems we could drop the cast since ->private_data is void * type.
> + unsigned long nr_to_scan = 0, ino;
> + struct shrink_control sc = {
> + .gfp_mask = GFP_KERNEL,
> + };
> + struct mem_cgroup *memcg = NULL;
> + int nid;
> + char kbuf[72];
> + int read_len = size < (sizeof(kbuf) - 1) ? size : (sizeof(kbuf) - 1);
> + ssize_t ret;
> +
> + if (copy_from_user(kbuf, buf, read_len))
> + return -EFAULT;
> + kbuf[read_len] = '\0';
> +
> + if (sscanf(kbuf, "%lu %d %lu", &ino, &nid, &nr_to_scan) < 2)
> + return -EINVAL;
> +
> + if (nid < 0 || nid >= nr_node_ids)
> + return -EINVAL;
> +
Should we break here if nr_to_scan is zero?
> + if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
> + memcg = mem_cgroup_get_from_ino(ino);
> + if (!memcg || IS_ERR(memcg))
Should we drop the check of "!memcg" since mem_cgroup_get_from_ino
cannot return NULL?
> + return -ENOENT;
> +
> + if (!mem_cgroup_online(memcg)) {
> + mem_cgroup_put(memcg);
> + return -ENOENT;
> + }
> + } else {
> + if (ino != 0)
> + return -EINVAL;
> + memcg = NULL;
IIUC, memcg is already NULL if we reach here, right? Then the
assignment is not necessary. Or we cound remove the initialization
of 'memcg' where it is definned.
> + }
> +
> + ret = down_read_killable(&shrinker_rwsem);
> + if (ret) {
> + mem_cgroup_put(memcg);
> + return ret;
> + }
> +
> + sc.nid = nid;
> + sc.memcg = memcg;
> + sc.nr_to_scan = nr_to_scan;
> + sc.nr_scanned = nr_to_scan;
> +
> + shrinker->scan_objects(shrinker, &sc);
> +
> + up_read(&shrinker_rwsem);
> + mem_cgroup_put(memcg);
> +
> + return ret ? ret : size;
Seems "ret" is always equal to 0 here, should we simplify this
to "return size"?
Thanks.
> +}
> +
> +static const struct file_operations shrinker_debugfs_scan_fops = {
> + .owner = THIS_MODULE,
> + .open = shrinker_debugfs_scan_open,
> + .write = shrinker_debugfs_scan_write,
> +};
> +
> int shrinker_debugfs_add(struct shrinker *shrinker)
> {
> struct dentry *entry;
> @@ -130,6 +201,8 @@ int shrinker_debugfs_add(struct shrinker *shrinker)
>
> debugfs_create_file("count", 0220, entry, shrinker,
> &shrinker_debugfs_count_fops);
> + debugfs_create_file("scan", 0440, entry, shrinker,
> + &shrinker_debugfs_scan_fops);
> return 0;
> }
>
> --
> 2.35.3
>
>
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