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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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