[PATCH v2 6/7] cpusets: Optimize the implementation of guarantee_online_cpus()

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

A cpuset's cpus_allowed mask is kept updated upon CPU hotplug
events in such a way that it always contains online cpus. Using
this observation, rework the body of guarantee_online_cpus().

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/cpuset.c |   15 +++++++++------
 1 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index c501a90..6446095 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -301,20 +301,23 @@ static struct file_system_type cpuset_fs_type = {
  * One way or another, we guarantee to return some non-empty subset
  * of cpu_online_mask.
  *
+ * Optimization: The cpus_allowed mask of a cpuset is maintained in such
+ * a way as to always have online cpus, by doing the cpuset hiearchy walk
+ * if/when necessary during CPU hotplug. And hence, it fits the above
+ * requirement perfectly. The only point to watch out is that cpus_allowed
+ * can be empty when the cpuset has no tasks and user_cpus_allowed is empty.
+ *
  * Call with callback_mutex held.
  */
 
 static void guarantee_online_cpus(const struct cpuset *cs,
 				  struct cpumask *pmask)
 {
-	while (cs && !cpumask_intersects(cs->user_cpus_allowed,
-							cpu_online_mask))
-		cs = cs->parent;
-
-	if (cs)
-		cpumask_and(pmask, cs->user_cpus_allowed, cpu_online_mask);
+	if (cs && !cpumask_empty(cs->cpus_allowed))
+		cpumask_copy(pmask, cs->cpus_allowed);
 	else
 		cpumask_copy(pmask, cpu_online_mask);
+
 	BUG_ON(!cpumask_intersects(pmask, cpu_online_mask));
 }