4 files changed, 33 insertions, 3 deletions

diff --git a/Documentation/cpu-hotplug.txt b/Documentation/cpu-hotplug.txt
index b6d24c22274b..a741f658a3c9 100644
--- a/Documentation/cpu-hotplug.txt
+++ b/Documentation/cpu-hotplug.txt
@@ -220,7 +220,9 @@ A: The following happen, listed in no particular order :-)
   CPU_DOWN_PREPARE or CPU_DOWN_PREPARE_FROZEN, depending on whether or not the
   CPU is being offlined while tasks are frozen due to a suspend operation in
   progress
-- All process is migrated away from this outgoing CPU to a new CPU
+- All processes are migrated away from this outgoing CPU to new CPUs.
+  The new CPU is chosen from each process' current cpuset, which may be
+  a subset of all online CPUs.
 - All interrupts targeted to this CPU is migrated to a new CPU
 - timers/bottom half/task lets are also migrated to a new CPU
 - Once all services are migrated, kernel calls an arch specific routine
diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index 31adfde1c95f..ecae585ec3da 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -21,6 +21,7 @@ extern int cpuset_init_early(void);
 extern int cpuset_init(void);
 extern void cpuset_init_smp(void);
 extern cpumask_t cpuset_cpus_allowed(struct task_struct *p);
+extern cpumask_t cpuset_cpus_allowed_locked(struct task_struct *p);
 extern nodemask_t cpuset_mems_allowed(struct task_struct *p);
 #define cpuset_current_mems_allowed (current->mems_allowed)
 void cpuset_init_current_mems_allowed(void);
@@ -87,6 +88,10 @@ static inline cpumask_t cpuset_cpus_allowed(struct task_struct *p)
 {
         return cpu_possible_map;
 }
+static inline cpumask_t cpuset_cpus_allowed_locked(struct task_struct *p)
+{
+        return cpu_possible_map;
+}
 
 static inline nodemask_t cpuset_mems_allowed(struct task_struct *p)
 {
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index fa31cb9f9898..50f5dc463688 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1818,10 +1818,23 @@ cpumask_t cpuset_cpus_allowed(struct task_struct *tsk)
         cpumask_t mask;
 
         mutex_lock(&callback_mutex);
+        mask = cpuset_cpus_allowed_locked(tsk);
+        mutex_unlock(&callback_mutex);
+
+        return mask;
+}
+
+/**
+ * cpuset_cpus_allowed_locked - return cpus_allowed mask from a tasks cpuset.
+ * Must be  called with callback_mutex held.
+ **/
+cpumask_t cpuset_cpus_allowed_locked(struct task_struct *tsk)
+{
+        cpumask_t mask;
+
         task_lock(tsk);
         guarantee_online_cpus(task_cs(tsk), &mask);
         task_unlock(tsk);
-        mutex_unlock(&callback_mutex);
 
         return mask;
 }
diff --git a/kernel/sched.c b/kernel/sched.c
index a7e30462600f..4071306e1088 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -5160,8 +5160,16 @@ static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p)
 
                 /* No more Mr. Nice Guy. */
                 if (dest_cpu == NR_CPUS) {
+                        cpumask_t cpus_allowed = cpuset_cpus_allowed_locked(p);
+                        /*
+                         * Try to stay on the same cpuset, where the
+                         * current cpuset may be a subset of all cpus.
+                         * The cpuset_cpus_allowed_locked() variant of
+                         * cpuset_cpus_allowed() will not block.  It must be
+                         * called within calls to cpuset_lock/cpuset_unlock.
+                         */
                         rq = task_rq_lock(p, &flags);
-                        cpus_setall(p->cpus_allowed);
+                        p->cpus_allowed = cpus_allowed;
                         dest_cpu = any_online_cpu(p->cpus_allowed);
                         task_rq_unlock(rq, &flags);
 
@@ -5527,6 +5535,7 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
 
         case CPU_DEAD:
         case CPU_DEAD_FROZEN:
+                cpuset_lock(); /* around calls to cpuset_cpus_allowed_lock() */
                 migrate_live_tasks(cpu);
                 rq = cpu_rq(cpu);
                 kthread_stop(rq->migration_thread);
@@ -5540,6 +5549,7 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
                 rq->idle->sched_class = &idle_sched_class;
                 migrate_dead_tasks(cpu);
                 spin_unlock_irq(&rq->lock);
+                cpuset_unlock();
                 migrate_nr_uninterruptible(rq);
                 BUG_ON(rq->nr_running != 0);