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    Subject[PATCH 5.0 125/246] cgroup, rstat: Dont flush subtree root unless necessary
    Date
    5.0-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    [ 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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