From 4fb009dbe6c2fc42fbf07a57644749e005c43c0c Mon Sep 17 00:00:00 2001
From: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Date: Wed, 25 Aug 2021 12:54:15 +0200
Subject: [PATCH] cgroup/cpuset: Avoid memory migration when nodemasks match

References: bsc#1190050 (Cgroup functional and performance backports)
Patch-mainline: v5.15-rc1
Git-commit: 9f72daf7edfa8f7e86ce8940d52266b5e931dcb0

With the introduction of ee9707e8593d ("cgroup/cpuset: Enable memory
migration for cpuset v2") attaching a process to a different cgroup will
trigger a memory migration regardless of whether it's really needed.
Memory migration is an expensive operation, so bypass it if the
nodemasks passed to cpuset_migrate_mm() are equal.

Note that we're not only avoiding the migration work itself, but also a
call to lru_cache_disable(), which triggers and flushes an LRU drain
work on every online CPU.

Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 kernel/cgroup/cpuset.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 44d234b0df5e..d497a65c4f04 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1634,6 +1634,11 @@ static void cpuset_migrate_mm(struct mm_struct *mm, const nodemask_t *from,
 {
 	struct cpuset_migrate_mm_work *mwork;
 
+	if (nodes_equal(*from, *to)) {
+		mmput(mm);
+		return;
+	}
+
 	mwork = kzalloc(sizeof(*mwork), GFP_KERNEL);
 	if (mwork) {
 		mwork->mm = mm;