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

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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>
This commit is contained in:
Tejun Heo 2019-02-15 11:01:31 -08:00
parent 05b71f6ffd
commit b4ff1b44bc
1 changed files with 6 additions and 4 deletions
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10
kernel/cgroup/rstat.c
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@ -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() */