On Mon, Sep 21, 2020 at 4:12 PM Michal Hocko [off-list ref] wrote:

On Mon 21-09-20 16:02:55, zangchunxin@bytedance.com wrote:

quoted

From: Chunxin Zang <redacted>

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.

I think we'd better add these comments above the function
mem_cgroup_force_empty() to explain why we don't want to expose this
interface in cgroup2, otherwise people will continue to send this
proposal without any strong reason.

quoted

Signed-off-by: Chunxin Zang <redacted>
---
 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

-- 
Thanks
Yafang