From 8a48152f36dd6a288a24e53a0fc0592d5b496773 Mon Sep 17 00:00:00 2001
From: Paul Jackson <pj@sgi.com>
Date: Sun, 27 Aug 2006 01:23:51 -0700
Subject: [PATCH] --- yaml --- r: 33646 b: refs/heads/master c:
 4c4d50f7b39cc58f1064b93a61ad617451ae41df h: refs/heads/master v: v3

---
 [refs]                          |  2 +-
 trunk/Documentation/cpusets.txt |  6 ++++++
 trunk/kernel/cpuset.c           | 33 +++++++++++++++++++++++++++++++++
 3 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/[refs] b/[refs]
index eb366d87aa13..01e1f696d4e4 100644
--- a/[refs]
+++ b/[refs]
@@ -1,2 +1,2 @@
 ---
-refs/heads/master: 6394cca54894f6a9bcf927ab78d28985944298ff
+refs/heads/master: 4c4d50f7b39cc58f1064b93a61ad617451ae41df
diff --git a/trunk/Documentation/cpusets.txt b/trunk/Documentation/cpusets.txt
index 159e2a0c3e80..76b44290c154 100644
--- a/trunk/Documentation/cpusets.txt
+++ b/trunk/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/trunk/kernel/cpuset.c b/trunk/kernel/cpuset.c
index 1a649f2bb9bb..f1dda98bdffe 100644
--- a/trunk/kernel/cpuset.c
+++ b/trunk/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 @@ int __init cpuset_init(void)
 	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);
 }
 
 /**