Commit a74ebf04 authored Feb 15, 2019 by Tejun Heo Committed by Greg Kroah-Hartman Apr 05, 2019

cgroup, rstat: Don't flush subtree root unless necessary



[ 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>

parent b13eb524

kernel/cgroup/rstat.c

+6 −4

Original line number	Diff line number	Diff line
		@@ -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;
		@@ -140,11 +139,14 @@ static struct cgroup cgroup_rstat_cpu_pop_updated(struct cgroup pos,
		* updated stat.
		*/
		smp_mb();
		}

		return pos;
		}

		/* only happens for @root */
		return NULL;
		}

		/* see cgroup_rstat_flush() */
		static void cgroup_rstat_flush_locked(struct cgroup *cgrp, bool may_sleep)
		__releases(&cgroup_rstat_lock) __acquires(&cgroup_rstat_lock)