From: Chris Lalancette <clalance@redhat.com>
Date: Mon, 28 Jan 2008 11:29:00 -0500
Subject: [xen] fix /sbin/init to use cpu_possible
Message-id: 479E02CC.40804@redhat.com
O-Subject: [RHEL5.2 PATCH]: Fix /sbin/init to use cpu_possible
Bugzilla: 430310

All,
     Attached is a patch to fix BZ 430310.  Back in October, a patch to force
/sbin/init off of isolated cpus was posted/merged:

http://post-office.corp.redhat.com/archives/rhkernel-list/2007-October/msg00248.html

However, this patch causes problems in paravirtualized SMP Xen guests.  The
problem boils down to the use of cpu_online_map to determine the
non_isolated_cpus; Xen PV guests technically start up UP, and then hotplug the
processors in.  However, this happens *after* the sched_init_smp() has forced
/sbin/init onto processor 0 only, which means that all further processes will be
on 0 unless you explicitly taskset them.  Upstream Linux has already fixed this
here by using cpu_possible_map instead of cpu_online_map:

http://git2.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=e5e5673f828623e58a401862b33173591faaeaff

The attached patch is just a simple re-diff of this patch, and fixes the issue
in my testing and Barry Marson's performance testing.

Please review and ACK.

Chris Lalancette

Acked-by: Don Dutile <ddutile@redhat.com>
Acked-by: "Stephen C. Tweedie" <sct@redhat.com>
Acked-by: Bill Burns <bburns@redhat.com>

diff --git a/kernel/sched.c b/kernel/sched.c
index 293db1f..0d71173 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -6760,7 +6760,7 @@ void __init sched_init_smp(void)
 
 	lock_cpu_hotplug();
 	arch_init_sched_domains(&cpu_online_map);
-	cpus_andnot(non_isolated_cpus, cpu_online_map, cpu_isolated_map);
+	cpus_andnot(non_isolated_cpus, cpu_possible_map, cpu_isolated_map);
 	if (cpus_empty(non_isolated_cpus))
 		cpu_set(smp_processor_id(), non_isolated_cpus);
 	unlock_cpu_hotplug();