cpusets, hotplug: Restructure functions that are invoked during hotplug

Separate out the cpuset related handling for CPU/Memory online/offline.
This also helps us exploit the most obvious and basic level of optimization
that any notification mechanism (CPU/Mem online/offline) has to offer us:
"We *know* why we have been invoked. So stop pretending that we are lost,
and do only the necessary amount of processing!".

And while at it, rename scan_for_empty_cpusets() to
scan_cpusets_upon_hotplug(), which is more appropriate considering how
it is restructured.

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/20120524141650.3692.48637.stgit@srivatsabhat.in.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 63 insertions, 29 deletions

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index ba96349aa522..ba0a4d74d262 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -147,6 +147,12 @@ typedef enum {
         CS_SPREAD_SLAB,
 } cpuset_flagbits_t;
 
+/* the type of hotplug event */
+enum hotplug_event {
+        CPUSET_CPU_OFFLINE,
+        CPUSET_MEM_OFFLINE,
+};
+
 /* convenient tests for these bits */
 static inline int is_cpu_exclusive(const struct cpuset *cs)
 {
@@ -2016,8 +2022,10 @@ static struct cpuset *cpuset_next(struct list_head *queue)
 
 
 /*
- * Walk the specified cpuset subtree and look for empty cpusets.
- * The tasks of such cpuset must be moved to a parent cpuset.
+ * Walk the specified cpuset subtree upon a hotplug operation (CPU/Memory
+ * online/offline) and update the cpusets accordingly.
+ * For regular CPU/Mem hotplug, look for empty cpusets; the tasks of such
+ * cpuset must be moved to a parent cpuset.
  *
  * Called with cgroup_mutex held.  We take callback_mutex to modify
  * cpus_allowed and mems_allowed.
@@ -2030,38 +2038,58 @@ static struct cpuset *cpuset_next(struct list_head *queue)
  * that has tasks along with an empty 'mems'.  But if we did see such
  * a cpuset, we'd handle it just like we do if its 'cpus' was empty.
  */
-static void scan_for_empty_cpusets(struct cpuset *root)
+static void
+scan_cpusets_upon_hotplug(struct cpuset *root, enum hotplug_event event)
 {
         LIST_HEAD(queue);
-        struct cpuset *cp;      /* scans cpusets being updated */
+        struct cpuset *cp;              /* scans cpusets being updated */
         static nodemask_t oldmems;      /* protected by cgroup_mutex */
 
         list_add_tail((struct list_head *)&root->stack_list, &queue);
 
-        while ((cp = cpuset_next(&queue)) != NULL) {
+        switch (event) {
+        case CPUSET_CPU_OFFLINE:
+                while ((cp = cpuset_next(&queue)) != NULL) {
+
+                        /* Continue past cpusets with all cpus online */
+                        if (cpumask_subset(cp->cpus_allowed, cpu_active_mask))
+                                continue;
+
+                        /* Remove offline cpus from this cpuset. */
+                        mutex_lock(&callback_mutex);
+                        cpumask_and(cp->cpus_allowed, cp->cpus_allowed,
+                                                        cpu_active_mask);
+                        mutex_unlock(&callback_mutex);
+
+                        /* Move tasks from the empty cpuset to a parent */
+                        if (cpumask_empty(cp->cpus_allowed))
+                                remove_tasks_in_empty_cpuset(cp);
+                        else
+                                update_tasks_cpumask(cp, NULL);
+                }
+                break;
 
-                /* Continue past cpusets with all cpus, mems online */
-                if (cpumask_subset(cp->cpus_allowed, cpu_active_mask) &&
-                    nodes_subset(cp->mems_allowed, node_states[N_HIGH_MEMORY]))
-                        continue;
+        case CPUSET_MEM_OFFLINE:
+                while ((cp = cpuset_next(&queue)) != NULL) {
 
-                oldmems = cp->mems_allowed;
+                        /* Continue past cpusets with all mems online */
+                        if (nodes_subset(cp->mems_allowed,
+                                        node_states[N_HIGH_MEMORY]))
+                                continue;
 
-                /* Remove offline cpus and mems from this cpuset. */
-                mutex_lock(&callback_mutex);
-                cpumask_and(cp->cpus_allowed, cp->cpus_allowed,
-                            cpu_active_mask);
-                nodes_and(cp->mems_allowed, cp->mems_allowed,
+                        oldmems = cp->mems_allowed;
+
+                        /* Remove offline mems from this cpuset. */
+                        mutex_lock(&callback_mutex);
+                        nodes_and(cp->mems_allowed, cp->mems_allowed,
                                                 node_states[N_HIGH_MEMORY]);
-                mutex_unlock(&callback_mutex);
+                        mutex_unlock(&callback_mutex);
 
-                /* Move tasks from the empty cpuset to a parent */
-                if (cpumask_empty(cp->cpus_allowed) ||
-                     nodes_empty(cp->mems_allowed))
-                        remove_tasks_in_empty_cpuset(cp);
-                else {
-                        update_tasks_cpumask(cp, NULL);
-                        update_tasks_nodemask(cp, &oldmems, NULL);
+                        /* Move tasks from the empty cpuset to a parent */
+                        if (nodes_empty(cp->mems_allowed))
+                                remove_tasks_in_empty_cpuset(cp);
+                        else
+                                update_tasks_nodemask(cp, &oldmems, NULL);
                 }
         }
 }
@@ -2080,8 +2108,11 @@ static void scan_for_empty_cpusets(struct cpuset *root)
  *
  * Called within get_online_cpus().  Needs to call cgroup_lock()
  * before calling generate_sched_domains().
+ *
+ * @cpu_online: Indicates whether this is a CPU online event (true) or
+ * a CPU offline event (false).
  */
-void cpuset_update_active_cpus(void)
+void cpuset_update_active_cpus(bool cpu_online)
 {
         struct sched_domain_attr *attr;
         cpumask_var_t *doms;
@@ -2091,7 +2122,10 @@ void cpuset_update_active_cpus(void)
         mutex_lock(&callback_mutex);
         cpumask_copy(top_cpuset.cpus_allowed, cpu_active_mask);
         mutex_unlock(&callback_mutex);
-        scan_for_empty_cpusets(&top_cpuset);
+
+        if (!cpu_online)
+                scan_cpusets_upon_hotplug(&top_cpuset, CPUSET_CPU_OFFLINE);
+
         ndoms = generate_sched_domains(&doms, &attr);
         cgroup_unlock();
 
@@ -2122,9 +2156,9 @@ static int cpuset_track_online_nodes(struct notifier_block *self,
         case MEM_OFFLINE:
                 /*
                  * needn't update top_cpuset.mems_allowed explicitly because
-                 * scan_for_empty_cpusets() will update it.
+                 * scan_cpusets_upon_hotplug() will update it.
                  */
-                scan_for_empty_cpusets(&top_cpuset);
+                scan_cpusets_upon_hotplug(&top_cpuset, CPUSET_MEM_OFFLINE);
                 break;
         default:
                 break;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 4c1d80c6b318..4b4a63d34396 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7134,7 +7134,7 @@ static int cpuset_cpu_active(struct notifier_block *nfb, unsigned long action,
 
         case CPU_ONLINE:
         case CPU_DOWN_FAILED:
-                cpuset_update_active_cpus();
+                cpuset_update_active_cpus(true);
                 break;
         default:
                 return NOTIFY_DONE;
@@ -7147,7 +7147,7 @@ static int cpuset_cpu_inactive(struct notifier_block *nfb, unsigned long action,
 {
         switch (action) {
         case CPU_DOWN_PREPARE:
-                cpuset_update_active_cpus();
+                cpuset_update_active_cpus(false);
                 break;
         case CPU_DOWN_PREPARE_FROZEN:
                 num_cpus_frozen++;