The patch titled
Subject: memcg: introduce per-memcg reclaim interface
has been added to the -mm tree. Its filename is
memcg-introduce-per-memcg-reclaim-interface.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/memcg-introduce-per-memcg-reclaim-interface.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/memcg-introduce-per-memcg-reclaim-interface.patch
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------------------------------------------------------
From: Shakeel Butt <shakeelb@xxxxxxxxxx>
Subject: memcg: introduce per-memcg reclaim interface
Patch series "memcg: introduce per-memcg proactive reclaim", v4.
Add a memory.reclaim proactive reclaim interface. The rationale behind
the interface and how it works are in the first patch.
This patch (of 4):
Introduce a 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:
https://dl.acm.org/doi/pdf/10.1145/3503222.3507731
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
application.
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
intended.
The benefits of such interface and shortcomings of existing interface
were further discussed in this RFC thread:
https://lore.kernel.org/linux-mm/5df21376-7dd1-bf81-8414-32a73cea45dd@xxxxxxxxxx/
Interface:
----------
Introduce a very simple memcg interface 'echo 10M > memory.reclaim' to
trigger reclaim in the target memory cgroup.
The interface is introduced as a nested-keyed file to allow for future
optional arguments to be easily added to configure the behavior of
reclaim.
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.
- Add a timeout parameter to make it easier for user space to call the
interface without worrying about being blocked for an undefined amount
of time.
For now, let's keep things simple by adding the basic functionality.
[yosryahmed@xxxxxxxxxx: refreshed to current master, updated commit message based on recent discussions and use cases]
Link: https://lkml.kernel.org/r/20220421234426.3494842-2-yosryahmed@xxxxxxxxxx
Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Acked-by: Wei Xu <weixugc@xxxxxxxxxx>
Acked-by: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Zefan Li <lizefan.x@xxxxxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Wei Xu <weixugc@xxxxxxxxxx>
Cc: Greg Thelen <gthelen@xxxxxxxxxx>
Cc: Chen Wandun <chenwandun@xxxxxxxxxx>
Cc: Vaibhav Jain <vaibhav@xxxxxxxxxxxxx>
Cc: "Michal Koutný" <mkoutny@xxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: Dan Schatzberg <schatzberg.dan@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/admin-guide/cgroup-v2.rst | 21 ++++++++++
mm/memcontrol.c | 44 ++++++++++++++++++++++
2 files changed, 65 insertions(+)
--- a/Documentation/admin-guide/cgroup-v2.rst~memcg-introduce-per-memcg-reclaim-interface
+++ a/Documentation/admin-guide/cgroup-v2.rst
@@ -1208,6 +1208,27 @@ 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 nested-keyed file which exists for all cgroups.
+
+ This is a simple interface to trigger memory reclaim in the
+ target cgroup.
+
+ This file accepts a single key, the number of bytes to reclaim.
+ No nested keys are currently supported.
+
+ Example::
+
+ echo "1G" > memory.reclaim
+
+ The interface can be later extended with nested keys to
+ configure the reclaim behavior. For example, specify the
+ type of memory to reclaim from (anon, file, ..).
+
+ Please note that the kernel can over or under reclaim from
+ the target cgroup. If less bytes are reclaimed than the
+ specified amount, -EAGAIN is returned.
+
memory.oom.group
A read-write single value file which exists on non-root
cgroups. The default value is "0".
--- a/mm/memcontrol.c~memcg-introduce-per-memcg-reclaim-interface
+++ a/mm/memcontrol.c
@@ -6356,6 +6356,45 @@ static ssize_t memory_oom_group_write(st
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))
+ return -EINTR;
+
+ /* This is the final attempt, drain percpu lru caches in the
+ * hope of introducing more evictable pages for
+ * try_to_free_mem_cgroup_pages().
+ */
+ if (!nr_retries)
+ lru_add_drain_all();
+
+ reclaimed = try_to_free_mem_cgroup_pages(memcg,
+ nr_to_reclaim - nr_reclaimed,
+ GFP_KERNEL, true);
+
+ if (!reclaimed && !nr_retries--)
+ return -EAGAIN;
+
+ nr_reclaimed += reclaimed;
+ }
+
+ return nbytes;
+}
+
static struct cftype memory_files[] = {
{
.name = "current",
@@ -6414,6 +6453,11 @@ static struct cftype memory_files[] = {
.seq_show = memory_oom_group_show,
.write = memory_oom_group_write,
},
+ {
+ .name = "reclaim",
+ .flags = CFTYPE_NS_DELEGATABLE,
+ .write = memory_reclaim,
+ },
{ } /* terminate */
};
_
Patches currently in -mm which might be from shakeelb@xxxxxxxxxx are
memcg-introduce-per-memcg-reclaim-interface.patch
