quoted

At the end of kernel/cpu.c::cpu_up() both cpus are set as present, possible
and online. The problem is that in kernel/sched.c::find_idlest_group()
p->cpus_allowed is always 1 (it should be 3). I tried to put a dump_stack()
or a printk(mask) in set_cpu_mask() but it crashes badly when mask is not 3,
ie. anything different from khelper or kthread, so I couldn't know what sets
the wrong cpumask.

Does this fix it?

diff --git a/kernel/sched.c b/kernel/sched.c
index b515e3c..3c8b1c5 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c

@@ -6875,7 +6875,7 @@ 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();