From edd8e41d2e3cbd6ebe13ead30eb1adc6f48cbb33 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Thu, 7 Sep 2017 17:03:51 +0200 Subject: [PATCH] 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 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 ); cpu_stopper_thread() work = multi_cpu_stop /* sync with CPU1 */ take_cpu_down() play_dead(); work = active_load_balance_cpu_stop set_task_cpu(p, CPU1); /* oops!! */ Reported-by: Thomas Gleixner Signed-off-by: Peter Zijlstra (Intel) Cc: Linus Torvalds Cc: Peter Zijlstra Link: http://lkml.kernel.org/r/20170907150614.044460912@infradead.org Signed-off-by: Ingo Molnar --- kernel/sched/fair.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 3bcea40d3029..efeebed935ae 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -8560,6 +8560,13 @@ static int active_load_balance_cpu_stop(void *data) 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() || -- 2.20.1