sched: Allow per-cpu kernel threads to run on online && !active

In order to enable symmetric hotplug, we must mirror the online &&
!active state of cpu-down on the cpu-up side.

However, to retain sanity, limit this state to per-cpu kthreads.

Aside from the change to set_cpus_allowed_ptr(), which allow moving
the per-cpu kthreads on, the other critical piece is the cpu selection
for pinned tasks in select_task_rq(). This avoids dropping into
select_fallback_rq().

select_fallback_rq() cannot be allowed to select !active cpus because
its used to migrate user tasks away. And we do not want to move user
tasks onto cpus that are in transition.

Requested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Jan H. Schönherr <jschoenh@amazon.de>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: rt@linutronix.de
Link: http://lkml.kernel.org/r/20160301152303.GV6356@twins.programming.kicks-ass.net
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 2 insertions, 4 deletions

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 40cee6b77a93..e828cf65d7df 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -743,12 +743,10 @@ set_cpu_present(unsigned int cpu, bool present)
 static inline void
 set_cpu_online(unsigned int cpu, bool online)
 {
-        if (online) {
+        if (online)
                 cpumask_set_cpu(cpu, &__cpu_online_mask);
-                cpumask_set_cpu(cpu, &__cpu_active_mask);
-        } else {
+        else
                 cpumask_clear_cpu(cpu, &__cpu_online_mask);
-        }
 }
 
 static inline void