sched: Fix balance vs hotplug race

Since (e761b77: cpu hotplug, sched: Introduce cpu_active_map and redo
sched domain managment) we have cpu_active_mask which is suppose to rule
scheduler migration and load-balancing, except it never (fully) did.

The particular problem being solved here is a crash in try_to_wake_up()
where select_task_rq() ends up selecting an offline cpu because
select_task_rq_fair() trusts the sched_domain tree to reflect the
current state of affairs, similarly select_task_rq_rt() trusts the
root_domain.

However, the sched_domains are updated from CPU_DEAD, which is after the
cpu is taken offline and after stop_machine is done. Therefore it can
race perfectly well with code assuming the domains are right.

Cure this by building the domains from cpu_active_mask on
CPU_DOWN_PREPARE.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 789cf5f920ce..d77b54733c5b 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -84,6 +84,7 @@ extern const struct cpumask *const cpu_active_mask;
 #define num_online_cpus()       cpumask_weight(cpu_online_mask)
 #define num_possible_cpus()     cpumask_weight(cpu_possible_mask)
 #define num_present_cpus()      cpumask_weight(cpu_present_mask)
+#define num_active_cpus()       cpumask_weight(cpu_active_mask)
 #define cpu_online(cpu)         cpumask_test_cpu((cpu), cpu_online_mask)
 #define cpu_possible(cpu)       cpumask_test_cpu((cpu), cpu_possible_mask)
 #define cpu_present(cpu)        cpumask_test_cpu((cpu), cpu_present_mask)
@@ -92,6 +93,7 @@ extern const struct cpumask *const cpu_active_mask;
 #define num_online_cpus()       1
 #define num_possible_cpus()     1
 #define num_present_cpus()      1
+#define num_active_cpus()       1
 #define cpu_online(cpu)         ((cpu) == 0)
 #define cpu_possible(cpu)       ((cpu) == 0)
 #define cpu_present(cpu)        ((cpu) == 0)