cgroups: add cpu_relax() calls in css_tryget() and cgroup_clear_css_refs()

css_tryget() and cgroup_clear_css_refs() contain polling loops; these
loops should have cpu_relax calls in them to reduce cross-cache traffic.

Signed-off-by: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 6 insertions, 2 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index e267e62827bb..e4e8e117d27d 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -99,6 +99,7 @@ static inline bool css_tryget(struct cgroup_subsys_state *css)
         while (!atomic_inc_not_zero(&css->refcnt)) {
                 if (test_bit(CSS_REMOVED, &css->flags))
                         return false;
+                cpu_relax();
         }
         return true;
 }
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 0066092de19a..492215d67fa5 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2509,7 +2509,7 @@ static int cgroup_clear_css_refs(struct cgroup *cgrp)
         for_each_subsys(cgrp->root, ss) {
                 struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
                 int refcnt;
-                do {
+                while (1) {
                         /* We can only remove a CSS with a refcnt==1 */
                         refcnt = atomic_read(&css->refcnt);
                         if (refcnt > 1) {
@@ -2523,7 +2523,10 @@ static int cgroup_clear_css_refs(struct cgroup *cgrp)
                          * css_tryget() to spin until we set the
                          * CSS_REMOVED bits or abort
                          */
-                } while (atomic_cmpxchg(&css->refcnt, refcnt, 0) != refcnt);
+                        if (atomic_cmpxchg(&css->refcnt, refcnt, 0) == refcnt)
+                                break;
+                        cpu_relax();
+                }
         }
  done:
         for_each_subsys(cgrp->root, ss) {