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From:   shisiyuan <shisiyuan19870131@gmail.com>
Cc:     shisiyuan <shisiyuan@xiaomi.com>, Tejun Heo <tj@kernel.org>,
        Li Zefan <lizefan@huawei.com>,
        Johannes Weiner <hannes@cmpxchg.org>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: [PATCH] cgroup: Return if dst_cgrp equals to src_cgrp
Date:   Sat, 21 May 2022 01:02:00 +0800
Message-Id: <1653066120-21262-1-git-send-email-shisiyuan@xiaomi.com>
To:     unlisted-recipients:; (no To-header on input)
Precedence: bulk

In function cgroup_migrate_add_src(), if dst_cgrp
equals to src_cgroup which the tasks link, dont
go on migrating tasks to another css_set.
This can save the cost of unnecessary migration.

Signed-off-by: shisiyuan <shisiyuan@xiaomi.com>
---
 kernel/cgroup/cgroup.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 6139460..97d7f68 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -2634,6 +2634,9 @@ void cgroup_migrate_add_src(struct css_set *src_cset,
 
 	src_cgrp = cset_cgroup_from_root(src_cset, dst_cgrp->root);
 
+	if (src_cgrp == dst_cgrp)
+		return;
+
 	if (!list_empty(&src_cset->mg_preload_node))
 		return;
 
@@ -2780,6 +2783,9 @@ int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
 	rcu_read_unlock();
 	spin_unlock_irq(&css_set_lock);
 
+	if (list_empty(&mgctx.preloaded_src_csets))
+		return ret;
+
 	/* prepare dst csets and commit */
 	ret = cgroup_migrate_prepare_dst(&mgctx);
 	if (!ret)
@@ -2927,7 +2933,7 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
 	struct cgroup_subsys_state *d_css;
 	struct cgroup *dsct;
 	struct css_set *src_cset;
-	int ret;
+	int ret = 0;
 
 	lockdep_assert_held(&cgroup_mutex);
 
@@ -2943,6 +2949,9 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
 	}
 	spin_unlock_irq(&css_set_lock);
 
+	if (list_empty(&mgctx.preloaded_src_csets))
+		goto out_finish;
+
 	/* NULL dst indicates self on default hierarchy */
 	ret = cgroup_migrate_prepare_dst(&mgctx);
 	if (ret)
-- 
2.7.4