[PATCH] mm, memcg: Add a memcg_slabinfo debugfs file

[PATCH] mm, memcg: Add a memcg_slabinfo debugfs file

[Waiman Long]

There are concerns about memory leaks from extensive use of memory
cgroups as each memory cgroup creates its own set of kmem caches. There
is a possiblity that the memcg kmem caches may remain even after the
memory cgroup removal. Therefore, it will be useful to show how many
memcg caches are present for each of the kmem caches.

This patch introduces a new <debugfs>/memcg_slabinfo file which is
somewhat similar to /proc/slabinfo in format, but lists only slabs that
are in memcg kmem caches. Information available in /proc/slabinfo are
not repeated in memcg_slabinfo.

A portion of a sample output of the file was:

  # <name> <active_objs> <num_objs> <active_slabs> <num_slabs> <num_caches> <num_empty_caches>
  rpc_inode_cache        0      0      0      0   1   1
  xfs_inode           6342   7888    232    232  59  13
  RAWv6                  0      0      0      0   2   2
  UDPv6                100    100      4      4   5   3
  TCPv6                  0      0      0      0   1   1
  UNIX                4864   4864    152    152  53  35
  RAW                    0      0      0      0   1   1
  TCP                   14     14      1      1   2   1

Besides the number of objects and slabs in the memcg kmem caches only,
it also shows the total number of memcg kmem caches associated with each
root kmem cache as well as the number memcg kmem caches that are empty.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 mm/slab_common.c | 53 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 58251ba63e4a..63fb18f4f811 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -17,6 +17,7 @@
 #include <linux/uaccess.h>
 #include <linux/seq_file.h>
 #include <linux/proc_fs.h>
+#include <linux/debugfs.h>
 #include <asm/cacheflush.h>
 #include <asm/tlbflush.h>
 #include <asm/page.h>
@@ -1498,6 +1499,58 @@ static int __init slab_proc_init(void)
 	return 0;
 }
 module_init(slab_proc_init);
+
+#if defined(CONFIG_DEBUG_FS) && defined(CONFIG_MEMCG)
+/*
+ * Display information about slabs that are in memcg kmem caches, but not
+ * in the root kmem caches.
+ */
+static int memcg_slabinfo_show(struct seq_file *m, void *unused)
+{
+	struct kmem_cache *s, *c;
+	struct slabinfo sinfo, cinfo;
+
+	mutex_lock(&slab_mutex);
+	seq_puts(m, "# <name> <active_objs> <num_objs> <active_slabs>");
+	seq_puts(m, " <num_slabs> <num_caches> <num_empty_caches>\n");
+	memset(&sinfo, 0, sizeof(sinfo));
+	list_for_each_entry(s, &slab_root_caches, root_caches_node) {
+		int scnt = 0;	/* memcg kmem cache count */
+		int ecnt = 0;	/* # of empty kmem caches */
+
+		for_each_memcg_cache(c, s) {
+			memset(&cinfo, 0, sizeof(cinfo));
+			get_slabinfo(c, &cinfo);
+
+			sinfo.active_slabs += cinfo.active_slabs;
+			sinfo.num_slabs += cinfo.num_slabs;
+			sinfo.active_objs += cinfo.active_objs;
+			sinfo.num_objs += cinfo.num_objs;
+			scnt++;
+			if (!cinfo.num_slabs)
+				ecnt++;
+		}
+		if (!scnt)
+			continue;
+		seq_printf(m, "%-17s %6lu %6lu %6lu %6lu %3d %3d\n",
+			   cache_name(s), sinfo.active_objs, sinfo.num_objs,
+			   sinfo.active_slabs, sinfo.num_slabs, scnt, ecnt);
+		memset(&sinfo, 0, sizeof(sinfo));
+	}
+	mutex_unlock(&slab_mutex);
+	return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(memcg_slabinfo);
+
+static int __init memcg_slabinfo_init(void)
+{
+	debugfs_create_file("memcg_slabinfo", S_IFREG | S_IRUGO,
+			    NULL, NULL, &memcg_slabinfo_fops);
+	return 0;
+}
+
+late_initcall(memcg_slabinfo_init);
+#endif /* CONFIG_DEBUG_FS && CONFIG_MEMCG */
 #endif /* CONFIG_SLAB || CONFIG_SLUB_DEBUG */
 
 static __always_inline void *__do_krealloc(const void *p, size_t new_size,
-- 
2.18.1

Re: [PATCH] mm, memcg: Add a memcg_slabinfo debugfs file

[Shakeel Butt]

On Wed, Jun 19, 2019 at 7:46 AM Waiman Long <longman@redhat.com> wrote:
>
> There are concerns about memory leaks from extensive use of memory
> cgroups as each memory cgroup creates its own set of kmem caches. There
> is a possiblity that the memcg kmem caches may remain even after the
> memory cgroup removal. Therefore, it will be useful to show how many
> memcg caches are present for each of the kmem caches.
>
> This patch introduces a new <debugfs>/memcg_slabinfo file which is
> somewhat similar to /proc/slabinfo in format, but lists only slabs that
> are in memcg kmem caches. Information available in /proc/slabinfo are
> not repeated in memcg_slabinfo.
>

At Google, we have an interface /proc/slabinfo_full which shows each
kmem cache (root and memcg) on a separate line i.e. no accumulation.
This interface has helped us a lot for debugging zombies and memory
leaks. The name of the memcg kmem caches include the memcg name, css
id and "dead" for offlined memcgs. I think these extra information is
much more useful for debugging. What do you think?

Shakeel

Re: [PATCH] mm, memcg: Add a memcg_slabinfo debugfs file

[Waiman Long]

On 6/19/19 11:18 AM, Shakeel Butt wrote:
> On Wed, Jun 19, 2019 at 7:46 AM Waiman Long <longman@redhat.com> wrote:
>> There are concerns about memory leaks from extensive use of memory
>> cgroups as each memory cgroup creates its own set of kmem caches. There
>> is a possiblity that the memcg kmem caches may remain even after the
>> memory cgroup removal. Therefore, it will be useful to show how many
>> memcg caches are present for each of the kmem caches.
>>
>> This patch introduces a new <debugfs>/memcg_slabinfo file which is
>> somewhat similar to /proc/slabinfo in format, but lists only slabs that
>> are in memcg kmem caches. Information available in /proc/slabinfo are
>> not repeated in memcg_slabinfo.
>>
> At Google, we have an interface /proc/slabinfo_full which shows each
> kmem cache (root and memcg) on a separate line i.e. no accumulation.
> This interface has helped us a lot for debugging zombies and memory
> leaks. The name of the memcg kmem caches include the memcg name, css
> id and "dead" for offlined memcgs. I think these extra information is
> much more useful for debugging. What do you think?
>
> Shakeel

Yes, I think that can be a good idea. My only concern is that it can be
very verbose. Will work on a v2 patch.

Thanks,
Longman

Re: [PATCH] mm, memcg: Add a memcg_slabinfo debugfs file

[Shakeel Butt]

On Wed, Jun 19, 2019 at 8:30 AM Waiman Long <longman@redhat.com> wrote:
>
> On 6/19/19 11:18 AM, Shakeel Butt wrote:
> > On Wed, Jun 19, 2019 at 7:46 AM Waiman Long <longman@redhat.com> wrote:
> >> There are concerns about memory leaks from extensive use of memory
> >> cgroups as each memory cgroup creates its own set of kmem caches. There
> >> is a possiblity that the memcg kmem caches may remain even after the
> >> memory cgroup removal. Therefore, it will be useful to show how many
> >> memcg caches are present for each of the kmem caches.
> >>
> >> This patch introduces a new <debugfs>/memcg_slabinfo file which is
> >> somewhat similar to /proc/slabinfo in format, but lists only slabs that
> >> are in memcg kmem caches. Information available in /proc/slabinfo are
> >> not repeated in memcg_slabinfo.
> >>
> > At Google, we have an interface /proc/slabinfo_full which shows each
> > kmem cache (root and memcg) on a separate line i.e. no accumulation.
> > This interface has helped us a lot for debugging zombies and memory
> > leaks. The name of the memcg kmem caches include the memcg name, css
> > id and "dead" for offlined memcgs. I think these extra information is
> > much more useful for debugging. What do you think?
> >
> > Shakeel
>
> Yes, I think that can be a good idea. My only concern is that it can be
> very verbose. Will work on a v2 patch.
>

Yes, it is very verbose but it is only for debugging and normal users
should not be (continuously) reading that interface.

Shakeel

Re: [PATCH] mm, memcg: Add a memcg_slabinfo debugfs file

[Waiman Long]

On 6/19/19 11:35 AM, Shakeel Butt wrote:
> On Wed, Jun 19, 2019 at 8:30 AM Waiman Long <longman@redhat.com> wrote:
>> On 6/19/19 11:18 AM, Shakeel Butt wrote:
>>> On Wed, Jun 19, 2019 at 7:46 AM Waiman Long <longman@redhat.com> wrote:
>>>> There are concerns about memory leaks from extensive use of memory
>>>> cgroups as each memory cgroup creates its own set of kmem caches. There
>>>> is a possiblity that the memcg kmem caches may remain even after the
>>>> memory cgroup removal. Therefore, it will be useful to show how many
>>>> memcg caches are present for each of the kmem caches.
>>>>
>>>> This patch introduces a new <debugfs>/memcg_slabinfo file which is
>>>> somewhat similar to /proc/slabinfo in format, but lists only slabs that
>>>> are in memcg kmem caches. Information available in /proc/slabinfo are
>>>> not repeated in memcg_slabinfo.
>>>>
>>> At Google, we have an interface /proc/slabinfo_full which shows each
>>> kmem cache (root and memcg) on a separate line i.e. no accumulation.
>>> This interface has helped us a lot for debugging zombies and memory
>>> leaks. The name of the memcg kmem caches include the memcg name, css
>>> id and "dead" for offlined memcgs. I think these extra information is
>>> much more useful for debugging. What do you think?
>>>
>>> Shakeel
>> Yes, I think that can be a good idea. My only concern is that it can be
>> very verbose. Will work on a v2 patch.
>>
> Yes, it is very verbose but it is only for debugging and normal users
> should not be (continuously) reading that interface.

I am not against it. It is just an observation. I still think we can
skip kmem caches that don't have any child memcg caches as the
information is in slabinfo already.

Cheers,
Longman