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Subject[PATCH 2/4] sched/fair: Plug hole between hotplug and active load_balance
The load balancer applies cpu_active_mask to whatever sched_domains it
finds, however in the case of active_balance there is a hole between
setting rq->{active_balance,push_cpu} and running the stop_machine
work doing the actual migration.

The @push_cpu can go offline in this window, which would result in us
moving a task onto a dead cpu, which is a fairly bad thing.

Double check the active mask before the stop work does the migration.


CPU0 CPU1

<SoftIRQ>
stop_machine(takedown_cpu)
load_balance() cpu_stopper_thread()
... work = multi_cpu_stop
stop_one_cpu_nowait( /* wait for CPU0 */
.func = active_load_balance_cpu_stop
);
</SoftIRQ>

cpu_stopper_thread()
work = multi_cpu_stop
/* sync with CPU1 */
take_cpu_down()
<idle>
play_dead();

work = active_load_balance_cpu_stop
set_task_cpu(p, CPU1); /* oops!! */

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/sched/fair.c | 7 +++++++
1 file changed, 7 insertions(+)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8560,6 +8560,13 @@ static int active_load_balance_cpu_stop(
struct rq_flags rf;

rq_lock_irq(busiest_rq, &rf);
+ /*
+ * Between queueing the stop-work and running it is a hole in which
+ * CPUs can become inactive. We should not move tasks from or to
+ * inactive CPUs.
+ */
+ if (!cpu_active(busiest_cpu) || !cpu_active(target_cpu))
+ goto out_unlock;

/* make sure the requested cpu hasn't gone down in the meantime */
if (unlikely(busiest_cpu != smp_processor_id() ||

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