[PATCH] cpuset: top_cpuset tracks hotplug changes to cpu_online_map

[Paul Jackson <pj@sgi.com>]

From: Paul Jackson <pj@sgi.com>

Change the list of cpus allowed to tasks in the top (root)
cpuset to dynamically track what cpus are online, using a CPU
hotplug notifier.  Make this top cpus file read-only.

On systems that have cpusets configured in their kernel, but that
aren't actively using cpusets (for some distros, this covers
the majority of systems) all tasks end up in the top cpuset.

If that system does support CPU hotplug, then these tasks cannot
make use of CPUs that are added after system boot, because the
CPUs are not allowed in the top cpuset.  This is a surprising
regression over earlier kernels that didn't have cpusets enabled.

In order to keep the behaviour of cpusets consistent between
systems actively making use of them and systems not using them,
this patch changes the behaviour of the 'cpus' file in the top
(root) cpuset, making it read only, and making it automatically
track the value of cpu_online_map.  Thus tasks in the top
cpuset will have automatic use of hot plugged CPUs allowed by
their cpuset.

Thanks to Anton Blanchard and Nathan Lynch for reporting this
problem, driving the fix, and earlier versions of this patch.

Signed-off-by: Paul Jackson <pj@sgi.com>

---

Anton or Nathan - can you test that this actually fixes
the problem?  It should, but I don't have a hotplug
enabled test rig available at the moment.  I've tested
that it builds i386 and ia64, both with and without cpusets
enabled, and boots and behaves ok on ia64 otherwise.  - pj

 Documentation/cpusets.txt |    6 ++++++
 kernel/cpuset.c           |   33 +++++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

--- 2.6.18-rc4-mm2.orig/kernel/cpuset.c	2006-08-23 23:03:23.082328581 -0700
+++ 2.6.18-rc4-mm2/kernel/cpuset.c	2006-08-23 23:25:29.745783140 -0700
@@ -815,6 +815,10 @@ static int update_cpumask(struct cpuset 
 	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)
@@ -2032,6 +2036,33 @@ 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
  *
@@ -2042,6 +2073,8 @@ 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);
 }
 
 /**
--- 2.6.18-rc4-mm2.orig/Documentation/cpusets.txt	2006-08-23 23:45:28.543688884 -0700
+++ 2.6.18-rc4-mm2/Documentation/cpusets.txt	2006-08-23 23:45:46.515909967 -0700
@@ -217,6 +217,12 @@ exclusive cpuset.  Also, the use of a Li
 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 ?
 --------------------------------

-- 
                          I won't rest till it's the best ...
                          Programmer, Linux Scalability
                          Paul Jackson <pj@sgi.com> 1.650.933.1373
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