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Subject[PATCH] cgroup: Make cgroup_rstat_updated() ready for root cgroup usage
From b7be022bf811d02605098fa61b7545bc146b78e7 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Mon, 2 Apr 2018 14:45:20 -0700

cgroup_rstat_updated() ensures that the cgroup's rstat is linked to
the parent. If there's no parent, it never gets linked and the
function ends up grabbing and releasing the cgroup_rstat_lock each
time for no reason which can be expensive.

This hasn't been a problem till now because nobody was calling the
function for the root cgroup but rstat is gonna be exposed to
controllers and use cases, so let's get ready. Make
cgroup_rstat_updated() an no-op for the root cgroup.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
Hello,

This should address the scalability issue reported by kenrel test bot.
All git branches updated accordingly.

Thanks.

kernel/cgroup/rstat.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index 0d1d5fc..9a30b51 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -27,6 +27,10 @@ void cgroup_rstat_updated(struct cgroup *cgrp, int cpu)
struct cgroup *parent;
unsigned long flags;

+ /* nothing to do for root */
+ if (!cgroup_parent(cgrp))
+ return;
+
/*
* Paired with the one in cgroup_rstat_cpu_pop_upated(). Either we
* see NULL updated_next or they see our updated stat.
--
2.9.5
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