Commit edd8e41d authored Sep 07, 2017 by Peter Zijlstra Committed by Ingo Molnar Sep 12, 2017

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>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170907150614.044460912@infradead.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 2800486e

kernel/sched/fair.c

+7 −0

Original line number	Diff line number	Diff line
		@@ -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() \|\|