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Subject[PATCH resend] memcg: introduce per-memcg reclaim interface
From: Shakeel Butt <>

Introduce an memcg interface to trigger memory reclaim on a memory cgroup.

Use case: Proactive Reclaim

A userspace proactive reclaimer can continuously probe the memcg to
reclaim a small amount of memory. This gives more accurate and
up-to-date workingset estimation as the LRUs are continuously
sorted and can potentially provide more deterministic memory
overcommit behavior. The memory overcommit controller can provide
more proactive response to the changing behavior of the running
applications instead of being reactive.

A userspace reclaimer's purpose in this case is not a complete replacement
for kswapd or direct reclaim, it is to proactively identify memory savings
opportunities and reclaim some amount of cold pages set by the policy
to free up the memory for more demanding jobs or scheduling new jobs.

A user space proactive reclaimer is used in Google data centers.
Additionally, Meta's TMO paper recently referenced a very similar
interface used for user space proactive reclaim:

Benefits of a user space reclaimer:

1) More flexible on who should be charged for the cpu of the memory
reclaim. For proactive reclaim, it makes more sense to be centralized.

2) More flexible on dedicating the resources (like cpu). The memory
overcommit controller can balance the cost between the cpu usage and
the memory reclaimed.

3) Provides a way to the applications to keep their LRUs sorted, so,
under memory pressure better reclaim candidates are selected. This also
gives more accurate and uptodate notion of working set for an

Why memory.high is not enough?

- memory.high can be used to trigger reclaim in a memcg and can
potentially be used for proactive reclaim.
However there is a big downside in using memory.high. It can potentially
introduce high reclaim stalls in the target application as the
allocations from the processes or the threads of the application can hit
the temporary memory.high limit.

- Userspace proactive reclaimers usually use feedback loops to decide
how much memory to proactively reclaim from a workload. The metrics
used for this are usually either refaults or PSI, and these metrics
will become messy if the application gets throttled by hitting the
high limit.

- memory.high is a stateful interface, if the userspace proactive
reclaimer crashes for any reason while triggering reclaim it can leave
the application in a bad state.

- If a workload is rapidly expanding, setting memory.high to proactively
reclaim memory can result in actually reclaiming more memory than

The benefits of such interface and shortcomings of existing interface
were further discussed in this RFC thread:


Introducing a very simple memcg interface 'echo 10M > memory.reclaim' to
trigger reclaim in the target memory cgroup.

Possible Extensions:

- This interface can be extended with an additional parameter or flags
to allow specifying one or more types of memory to reclaim from (e.g.
file, anon, ..).

- The interface can also be extended with a node mask to reclaim from
specific nodes. This has use cases for reclaim-based demotion in memory
tiering systens.

- A similar per-node interface can also be added to support proactive
reclaim and reclaim-based demotion in systems without memcg.

For now, let's keep things simple by adding the basic functionality.

[ refreshed to current master, updated commit
message based on recent discussions and use cases]
Signed-off-by: Shakeel Butt <>
Signed-off-by: Yosry Ahmed <>
Documentation/admin-guide/cgroup-v2.rst | 9 ++++++
mm/memcontrol.c | 37 +++++++++++++++++++++++++
2 files changed, 46 insertions(+)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 69d7a6983f78..925aaabb2247 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1208,6 +1208,15 @@ 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.reclaim
+ A write-only file which exists on non-root cgroups.
+ This is a simple interface to trigger memory reclaim in the
+ target cgroup. Write the number of bytes to reclaim to this
+ file and the kernel will try to reclaim that much memory.
+ Please note that the kernel can over or under reclaim from
+ the target cgroup.
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 725f76723220..994849fab7df 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6355,6 +6355,38 @@ static ssize_t memory_oom_group_write(struct kernfs_open_file *of,
return nbytes;

+static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
+ size_t nbytes, loff_t off)
+ struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
+ unsigned int nr_retries = MAX_RECLAIM_RETRIES;
+ unsigned long nr_to_reclaim, nr_reclaimed = 0;
+ int err;
+ buf = strstrip(buf);
+ err = page_counter_memparse(buf, "", &nr_to_reclaim);
+ if (err)
+ return err;
+ while (nr_reclaimed < nr_to_reclaim) {
+ unsigned long reclaimed;
+ if (signal_pending(current))
+ break;
+ reclaimed = try_to_free_mem_cgroup_pages(memcg,
+ nr_to_reclaim - nr_reclaimed,
+ GFP_KERNEL, true);
+ if (!reclaimed && !nr_retries--)
+ break;
+ nr_reclaimed += reclaimed;
+ }
+ return nbytes;
static struct cftype memory_files[] = {
.name = "current",
@@ -6413,6 +6445,11 @@ static struct cftype memory_files[] = {
.seq_show = memory_oom_group_show,
.write = memory_oom_group_write,
+ {
+ .name = "reclaim",
+ .write = memory_reclaim,
+ },
{ } /* terminate */

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