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-rw-r--r--Documentation/cpusets.txt6
-rw-r--r--kernel/cpuset.c33
2 files changed, 39 insertions, 0 deletions
diff --git a/Documentation/cpusets.txt b/Documentation/cpusets.txt
index 159e2a0c3e8..76b44290c15 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 1a649f2bb9b..f1dda98bdff 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);
}
/**