[PATCH] memcg: Add kmem.slabinfo to v2 for debugging purpose

[PATCH] memcg: Add kmem.slabinfo to v2 for debugging purpose

[Waiman Long]

With memory cgroup v1, there is a kmem.slabinfo file that can be
used to view what slabs are allocated to the memory cgroup. There
is currently no such equivalent in memory cgroup v2. This file can
be useful for debugging purpose.

This patch adds an equivalent kmem.slabinfo to v2 with the caveat that
this file will only show up as ".__DEBUG__.memory.kmem.slabinfo" when the
"cgroup_debug" parameter is specified in the kernel boot command line.
This is to avoid cluttering the cgroup v2 interface with files that
are seldom used by end users.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 mm/memcontrol.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ba9138a4a1de..236554a23f8f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5812,6 +5812,22 @@ static struct cftype memory_files[] = {
 		.seq_show = memory_oom_group_show,
 		.write = memory_oom_group_write,
 	},
+#ifdef CONFIG_MEMCG_KMEM
+	{
+		/*
+		 * This file is for debugging purpose only and will show
+		 * up as ".__DEBUG__.memory.kmem.slabinfo" when the
+		 * "cgroup_debug" parameter is specified in the kernel
+		 * boot command line.
+		 */
+		.name = "kmem.slabinfo",
+		.flags = CFTYPE_NOT_ON_ROOT | CFTYPE_DEBUG,
+		.seq_start = memcg_slab_start,
+		.seq_next = memcg_slab_next,
+		.seq_stop = memcg_slab_stop,
+		.seq_show = memcg_slab_show,
+	},
+#endif
 	{ }	/* terminate */
 };
 
-- 
2.18.1

Re: [PATCH] memcg: Add kmem.slabinfo to v2 for debugging purpose

[Andrew Morton]

On Wed, 26 Jun 2019 12:56:14 -0400 Waiman Long <longman@redhat.com> wrote:

> With memory cgroup v1, there is a kmem.slabinfo file that can be
> used to view what slabs are allocated to the memory cgroup. There
> is currently no such equivalent in memory cgroup v2. This file can
> be useful for debugging purpose.
> 
> This patch adds an equivalent kmem.slabinfo to v2 with the caveat that
> this file will only show up as ".__DEBUG__.memory.kmem.slabinfo" when the
> "cgroup_debug" parameter is specified in the kernel boot command line.
> This is to avoid cluttering the cgroup v2 interface with files that
> are seldom used by end users.
>
> ...
>
> mm/memcontrol.c | 16 ++++++++++++++++
> 1 file changed, 16 insertions(+)

A change to the kernel's user interface triggers a change to the
kernel's user interface documentation.  This should be automatic by
now :(

Re: [PATCH] memcg: Add kmem.slabinfo to v2 for debugging purpose

[Waiman Long]

On 6/26/19 6:25 PM, Andrew Morton wrote:
> On Wed, 26 Jun 2019 12:56:14 -0400 Waiman Long <longman@redhat.com> wrote:
>
>> With memory cgroup v1, there is a kmem.slabinfo file that can be
>> used to view what slabs are allocated to the memory cgroup. There
>> is currently no such equivalent in memory cgroup v2. This file can
>> be useful for debugging purpose.
>>
>> This patch adds an equivalent kmem.slabinfo to v2 with the caveat that
>> this file will only show up as ".__DEBUG__.memory.kmem.slabinfo" when the
>> "cgroup_debug" parameter is specified in the kernel boot command line.
>> This is to avoid cluttering the cgroup v2 interface with files that
>> are seldom used by end users.
>>
>> ...
>>
>> mm/memcontrol.c | 16 ++++++++++++++++
>> 1 file changed, 16 insertions(+)
> A change to the kernel's user interface triggers a change to the
> kernel's user interface documentation.  This should be automatic by
> now :(
>
>
We don't usually document debugging only files as they are subject to
change with no stability guarantee. That is the point of marking it for
debugging instead of a regular file that we need to support forever.

Cheers,
Longman

Re: [PATCH] memcg: Add kmem.slabinfo to v2 for debugging purpose

[Tejun Heo]

Hello, Waiman.

On Wed, Jun 26, 2019 at 12:56:14PM -0400, Waiman Long wrote:
> With memory cgroup v1, there is a kmem.slabinfo file that can be
> used to view what slabs are allocated to the memory cgroup. There
> is currently no such equivalent in memory cgroup v2. This file can
> be useful for debugging purpose.
> 
> This patch adds an equivalent kmem.slabinfo to v2 with the caveat that
> this file will only show up as ".__DEBUG__.memory.kmem.slabinfo" when the
> "cgroup_debug" parameter is specified in the kernel boot command line.
> This is to avoid cluttering the cgroup v2 interface with files that
> are seldom used by end users.

Can you please take a look at drgn?

  https://github.com/osandov/drgn

Baking in debug interface files always is limited and nasty and drgn
can get you way more flexible debugging / monitoring tool w/o having
to bake in anything into the kernel.  For an example, please take a
look at

  https://lore.kernel.org/bpf/20190614015620.1587672-10-tj@kernel.org/

Thanks.

-- 
tejun

Re: [PATCH] memcg: Add kmem.slabinfo to v2 for debugging purpose

[Waiman Long]

On 6/27/19 10:20 AM, Tejun Heo wrote:
> Hello, Waiman.
>
> On Wed, Jun 26, 2019 at 12:56:14PM -0400, Waiman Long wrote:
>> With memory cgroup v1, there is a kmem.slabinfo file that can be
>> used to view what slabs are allocated to the memory cgroup. There
>> is currently no such equivalent in memory cgroup v2. This file can
>> be useful for debugging purpose.
>>
>> This patch adds an equivalent kmem.slabinfo to v2 with the caveat that
>> this file will only show up as ".__DEBUG__.memory.kmem.slabinfo" when the
>> "cgroup_debug" parameter is specified in the kernel boot command line.
>> This is to avoid cluttering the cgroup v2 interface with files that
>> are seldom used by end users.
> Can you please take a look at drgn?
>
>   https://github.com/osandov/drgn
>
> Baking in debug interface files always is limited and nasty and drgn
> can get you way more flexible debugging / monitoring tool w/o having
> to bake in anything into the kernel.  For an example, please take a
> look at
>
>   https://lore.kernel.org/bpf/20190614015620.1587672-10-tj@kernel.org/
>
> Thanks.
>
Thanks for the information. Will take a serious look at that.

Cheers,
Longman