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Subject[PATCH AUTOSEL 4.19 096/192] cgroup, rstat: Don't flush subtree root unless necessary
Date
From: Tejun Heo <tj@kernel.org>

[ Upstream commit b4ff1b44bcd384d22fcbac6ebaf9cc0d33debe50 ]

cgroup_rstat_cpu_pop_updated() is used to traverse the updated cgroups
on flush. While it was only visiting updated ones in the subtree, it
was visiting @root unconditionally. We can easily check whether @root
is updated or not by looking at its ->updated_next just as with the
cgroups in the subtree.

* Remove the unnecessary cgroup_parent() test. The system root cgroup
is never updated and thus its ->updated_next is always NULL. No
need to test whether cgroup_parent() exists in addition to
->updated_next.

* Terminate traverse if ->updated_next is NULL. This can only happen
for subtree @root and there's no reason to visit it if it's not
marked updated.

This reduces cpu consumption when reading a lot of rstat backed files.
In a micro benchmark reading stat from ~1600 cgroups, the sys time was
lowered by >40%.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/cgroup/rstat.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index d503d1a9007c..bb95a35e8c2d 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -87,7 +87,6 @@ static struct cgroup *cgroup_rstat_cpu_pop_updated(struct cgroup *pos,
struct cgroup *root, int cpu)
{
struct cgroup_rstat_cpu *rstatc;
- struct cgroup *parent;

if (pos == root)
return NULL;
@@ -115,8 +114,8 @@ static struct cgroup *cgroup_rstat_cpu_pop_updated(struct cgroup *pos,
* However, due to the way we traverse, @pos will be the first
* child in most cases. The only exception is @root.
*/
- parent = cgroup_parent(pos);
- if (parent && rstatc->updated_next) {
+ if (rstatc->updated_next) {
+ struct cgroup *parent = cgroup_parent(pos);
struct cgroup_rstat_cpu *prstatc = cgroup_rstat_cpu(parent, cpu);
struct cgroup_rstat_cpu *nrstatc;
struct cgroup **nextp;
@@ -140,9 +139,12 @@ static struct cgroup *cgroup_rstat_cpu_pop_updated(struct cgroup *pos,
* updated stat.
*/
smp_mb();
+
+ return pos;
}

- return pos;
+ /* only happens for @root */
+ return NULL;
}

/* see cgroup_rstat_flush() */
--
2.19.1
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