[PATCH v5 2/5] cpusets, hotplug: Implement cpuset tree traversal in a helper function

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

At present, the functions that deal with cpusets during CPU/Mem hotplug
are quite messy, since a lot of the functionality is mixed up without clear
separation. And this takes a toll on optimization as well. For example,
the function cpuset_update_active_cpus() is called on both CPU offline and CPU
online events; and it invokes scan_for_empty_cpusets(), which makes sense
only for CPU offline events. And hence, the current code ends up unnecessarily
traversing the cpuset tree during CPU online also.

As a first step towards cleaning up those functions, encapsulate the cpuset
tree traversal in a helper function, so as to facilitate upcoming changes.

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

 kernel/cpuset.c |   36 +++++++++++++++++++++++++++---------
 1 files changed, 27 insertions(+), 9 deletions(-)

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 5fc1570..15926f8 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -2001,6 +2001,32 @@ static void remove_tasks_in_empty_cpuset(struct cpuset *cs)
 }
 
 /*
+ * Helper function to traverse cpusets.
+ * It can be used to walk the cpuset tree from top to bottom, completing
+ * one layer before dropping down to the next (thus always processing a
+ * node before any of its children).
+ */
+static struct cpuset *cpuset_next(struct list_head *queue)
+{
+	struct cpuset *cp;
+	struct cpuset *child;	/* scans child cpusets of cp */
+	struct cgroup *cont;
+
+	if (list_empty(queue))
+		return NULL;
+
+	cp = list_first_entry(queue, struct cpuset, stack_list);
+	list_del(queue->next);
+	list_for_each_entry(cont, &cp->css.cgroup->children, sibling) {
+		child = cgroup_cs(cont);
+		list_add_tail(&child->stack_list, queue);
+	}
+
+	return cp;
+}
+
+
+/*
  * Walk the specified cpuset subtree and look for empty cpusets.
  * The tasks of such cpuset must be moved to a parent cpuset.
  *
@@ -2019,19 +2045,11 @@ static void scan_for_empty_cpusets(struct cpuset *root)
 {
 	LIST_HEAD(queue);
 	struct cpuset *cp;	/* scans cpusets being updated */
-	struct cpuset *child;	/* scans child cpusets of cp */
-	struct cgroup *cont;
 	static nodemask_t oldmems;	/* protected by cgroup_mutex */
 
 	list_add_tail((struct list_head *)&root->stack_list, &queue);
 
-	while (!list_empty(&queue)) {
-		cp = list_first_entry(&queue, struct cpuset, stack_list);
-		list_del(queue.next);
-		list_for_each_entry(cont, &cp->css.cgroup->children, sibling) {
-			child = cgroup_cs(cont);
-			list_add_tail(&child->stack_list, &queue);
-		}
+	while ((cp = cpuset_next(&queue)) != NULL) {
 
 		/* Continue past cpusets with all cpus, mems online */
 		if (cpumask_subset(cp->cpus_allowed, cpu_active_mask) &&