Re: [RFC PATCH] memcg: expose children memory usage for root

[Yosry Ahmed <yosryahmed@google.com>]

On Mon, Jul 22, 2024 at 3:53 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> Linux kernel does not expose memory.current on the root memcg and there
> are applications which have to traverse all the top level memcgs to
> calculate the total memory charged in the system. This is more expensive
> (directory traversal and multiple open and reads) and is racy on a busy
> machine. As the kernel already have the needed information i.e. root's
> memory.current, why not expose that?
>
> However root's memory.current will have a different semantics than the
> non-root's memory.current as the kernel skips the charging for root, so
> maybe it is better to have a different named interface for the root.
> Something like memory.children_usage only for root memcg.
>
> Now there is still a question that why the kernel does not expose
> memory.current for the root. The historical reason was that the memcg
> charging was expensice and to provide the users to bypass the memcg
> charging by letting them run in the root. However do we still want to
> have this exception today? What is stopping us to start charging the
> root memcg as well. Of course the root will not have limits but the
> allocations will go through memcg charging and then the memory.current
> of root and non-root will have the same semantics.
>
> This is an RFC to start a discussion on memcg charging for root.

I vaguely remember when running some netperf tests (tcp_rr?) in a
cgroup that the performance decreases considerably with every level
down the hierarchy. I am assuming that charging was a part of the
reason. If that's the case, charging the root will be similar to
moving all workloads one level down the hierarchy in terms of charging
overhead.

>
> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> ---
>  Documentation/admin-guide/cgroup-v2.rst | 6 ++++++
>  mm/memcontrol.c                         | 5 +++++
>  2 files changed, 11 insertions(+)
>
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 6c6075ed4aa5..e4afc05fd8ea 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1220,6 +1220,12 @@ PAGE_SIZE multiple when read back.
>         The total amount of memory currently being used by the cgroup
>         and its descendants.
>
> +  memory.children_usage
> +       A read-only single value file which exists only on root cgroup.
> +
> +       The total amount of memory currently being used by the
> +        descendants of the root cgroup.
> +
>    memory.min
>         A read-write single value file which exists on non-root
>         cgroups.  The default is "0".
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 960371788687..eba8cf76d3d3 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4304,6 +4304,11 @@ static struct cftype memory_files[] = {
>                 .flags = CFTYPE_NOT_ON_ROOT,
>                 .read_u64 = memory_current_read,
>         },
> +       {
> +               .name = "children_usage",
> +               .flags = CFTYPE_ONLY_ON_ROOT,
> +               .read_u64 = memory_current_read,
> +       },
>         {
>                 .name = "peak",
>                 .flags = CFTYPE_NOT_ON_ROOT,
> --
> 2.43.0
>
>