cpusets, hotplug: Implement cpuset tree traversal in a helper function

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>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20120524141635.3692.893.stgit@srivatsabhat.in.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 27 insertions, 9 deletions

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 746d1eeb5db..ba96349aa52 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1990,6 +1990,32 @@ 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.
  *
@@ -2008,19 +2034,11 @@ 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) &&