From: Jonathan Lim <jolim@redhat.com>
Date: Thu, 11 Oct 2007 15:13:42 -0400
Subject: [misc] sched: force /sbin/init off isolated cpus
Message-id: 20071011191342.GD5008@sgi-desktop.boston.redhat.com
O-Subject: [RHEL5.2 PATCH] BZ 328091: sched: force /sbin/init off isolated cpus
Bugzilla: 328091

Force /sbin/init off isolated cpus (unless every CPU is specified as an
isolcpu).

/sbin/init is spawned from the boot CPU's idle thread, and /sbin/init starts
the rest of userspace.  So if the boot CPU is specified to be an isolcpu, then
prior to this patch, all of userspace will be run there.

The patch has been tested on x86_64 and is upstream:

  http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=5c1e176781f43bc902a51e5832f789756bff911b

Following is the diff against the 2.6.18-52.el5 kernel source:

Acked-by: Prarit Bhargava <prarit@redhat.com>

diff --git a/kernel/sched.c b/kernel/sched.c
index b266c27..66e4fe5 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4817,7 +4817,7 @@ void show_state(void)
  * NOTE: this function does not set the idle thread's NEED_RESCHED
  * flag, to make booting more robust.
  */
-void __devinit init_idle(struct task_struct *idle, int cpu)
+void __cpuinit init_idle(struct task_struct *idle, int cpu)
 {
 	struct rq *rq = cpu_rq(cpu);
 	unsigned long flags;
@@ -5459,7 +5459,7 @@ static void cpu_attach_domain(struct sched_domain *sd, int cpu)
 }
 
 /* cpus with isolated domains */
-static cpumask_t cpu_isolated_map = CPU_MASK_NONE;
+static cpumask_t __cpuinitdata cpu_isolated_map = CPU_MASK_NONE;
 
 /* Setup the mask of cpus configured for isolated domains */
 static int __init isolated_cpu_setup(char *str)
@@ -6747,11 +6747,20 @@ static int update_sched_domains(struct notifier_block *nfb,
 
 void __init sched_init_smp(void)
 {
+	cpumask_t non_isolated_cpus;
+
 	lock_cpu_hotplug();
 	arch_init_sched_domains(&cpu_online_map);
+	cpus_andnot(non_isolated_cpus, cpu_online_map, cpu_isolated_map);
+	if (cpus_empty(non_isolated_cpus))
+		cpu_set(smp_processor_id(), non_isolated_cpus);
 	unlock_cpu_hotplug();
 	/* XXX: Theoretical race here - CPU may be hotplugged now */
 	hotcpu_notifier(update_sched_domains, 0);
+
+	/* Move init over to a non-isolated CPU */
+	if (set_cpus_allowed(current, non_isolated_cpus) < 0)
+		BUG();
 }
 #else
 void __init sched_init_smp(void)