Messages in this thread Patch in this message | | | Date | Thu, 07 Sep 2017 17:03:51 +0200 | From | Peter Zijlstra <> | Subject | [PATCH 2/4] sched/fair: Plug hole between hotplug and active load_balance |
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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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