cgroup: Make cgroup_rstat_updated() ready for root cgroup usage

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>

diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index a4901e2..d503d1a 100644 (file)
--- 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.