2 files changed, 39 insertions, 0 deletions

diff --git a/Documentation/cpusets.txt b/Documentation/cpusets.txt
index 159e2a0c3e80..76b44290c154 100644
--- a/Documentation/cpusets.txt
+++ b/Documentation/cpusets.txt
@@ -217,6 +217,12 @@ exclusive cpuset.  Also, the use of a Linux virtual file system (vfs)
 to represent the cpuset hierarchy provides for a familiar permission
 and name space for cpusets, with a minimum of additional kernel code.
 
+The cpus file in the root (top_cpuset) cpuset is read-only.
+It automatically tracks the value of cpu_online_map, using a CPU
+hotplug notifier.  If and when memory nodes can be hotplugged,
+we expect to make the mems file in the root cpuset read-only
+as well, and have it track the value of node_online_map.
+
 
 1.4 What are exclusive cpusets ?
 --------------------------------
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 1a649f2bb9bb..f1dda98bdffe 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -816,6 +816,10 @@ static int update_cpumask(struct cpuset *cs, char *buf)
         struct cpuset trialcs;
         int retval, cpus_unchanged;
 
+        /* top_cpuset.cpus_allowed tracks cpu_online_map; it's read-only */
+        if (cs == &top_cpuset)
+                return -EACCES;
+
         trialcs = *cs;
         retval = cpulist_parse(buf, trialcs.cpus_allowed);
         if (retval < 0)
@@ -2033,6 +2037,33 @@ out:
         return err;
 }
 
+/*
+ * The top_cpuset tracks what CPUs and Memory Nodes are online,
+ * period.  This is necessary in order to make cpusets transparent
+ * (of no affect) on systems that are actively using CPU hotplug
+ * but making no active use of cpusets.
+ *
+ * This handles CPU hotplug (cpuhp) events.  If someday Memory
+ * Nodes can be hotplugged (dynamically changing node_online_map)
+ * then we should handle that too, perhaps in a similar way.
+ */
+
+#ifdef CONFIG_HOTPLUG_CPU
+static int cpuset_handle_cpuhp(struct notifier_block *nb,
+                                unsigned long phase, void *cpu)
+{
+        mutex_lock(&manage_mutex);
+        mutex_lock(&callback_mutex);
+
+        top_cpuset.cpus_allowed = cpu_online_map;
+
+        mutex_unlock(&callback_mutex);
+        mutex_unlock(&manage_mutex);
+
+        return 0;
+}
+#endif
+
 /**
  * cpuset_init_smp - initialize cpus_allowed
  *
@@ -2043,6 +2074,8 @@ void __init cpuset_init_smp(void)
 {
         top_cpuset.cpus_allowed = cpu_online_map;
         top_cpuset.mems_allowed = node_online_map;
+
+        hotcpu_notifier(cpuset_handle_cpuhp, 0);
 }
 
 /**