Re: [PATCH] mm/memcontrol: Add the drop_cache interface for cgroup v2

Michal Hocko
2020-09-21T08:12:00Z

On Mon 21-09-20 16:02:55, zangchunxin@bytedance.com wrote:
> From: Chunxin Zang <zangchunxin@bytedance.com>
>
> In the cgroup v1, we have 'force_mepty' interface. This is very
> useful for userspace to actively release memory. But the cgroup
> v2 does not.
>
> This patch reuse cgroup v1's function, but have a new name for
> the interface. Because I think 'drop_cache' may be is easier to
> understand :)

This should really explain a usecase. Global drop_caches is a terrible
interface and it has caused many problems in the past. People have
learned to use it as a remedy to any problem they might see and cause
other problems without realizing that. This is the reason why we even
log each attempt to drop caches.

I would rather not repeat the same mistake on the memcg level unless
there is a very strong reason for it.

> Signed-off-by: Chunxin Zang <zangchunxin@bytedance.com>
> ---
> Documentation/admin-guide/cgroup-v2.rst | 11 +++++++++++
> mm/memcontrol.c | 5 +++++
> 2 files changed, 16 insertions(+)
>
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index ce3e05e41724..fbff959c8116 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1181,6 +1181,17 @@ PAGE_SIZE multiple when read back.
> high limit is used and monitored properly, this limit's
> utility is limited to providing the final safety net.
>
> + memory.drop_cache
> + A write-only single value file which exists on non-root
> + cgroups.
> +
> + Provide a mechanism for users to actively trigger memory
> + reclaim. The cgroup will be reclaimed and as many pages
> + reclaimed as possible.
> +
> + It will broke low boundary. Because it tries to reclaim the
> + memory many times, until the memory drops to a certain level.
> +
> memory.oom.group
> A read-write single value file which exists on non-root
> cgroups. The default value is "0".
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 0b38b6ad547d..98646484efff 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6226,6 +6226,11 @@ static struct cftype memory_files[] = {
> .write = memory_max_write,
> },
> {
> + .name = "drop_cache",
> + .flags = CFTYPE_NOT_ON_ROOT,
> + .write = mem_cgroup_force_empty_write,
> + },
> + {
> .name = "events",
> .flags = CFTYPE_NOT_ON_ROOT,
> .file_offset = offsetof(struct mem_cgroup, events_file),
> --
> 2.11.0

--
Michal Hocko
SUSE Labs