linux-kernel - Re: [PATCH] sched/core: Fix kick offline cpu to do nohz idle load balance

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Date:   Mon, 10 Oct 2016 16:34:48 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     Wanpeng Li <wanpeng.li@...mail.com>,
        Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched/core: Fix kick offline cpu to do nohz idle load balance

2016-10-10 12:10 GMT+08:00 Wanpeng Li <kernellwp@...il.com>:
> From: Wanpeng Li <wanpeng.li@...mail.com>
>
>  WARNING: CPU: 0 PID: 3404 at arch/x86/kernel/smp.c:125 native_smp_send_reschedule+0x3f/0x50
>  CPU: 0 PID: 3404 Comm: qemu-system-x86 Not tainted 4.8.0+ #21
>  Call Trace:
>    __warn+0xd1/0xf0
>    warn_slowpath_null+0x1d/0x20
>    native_smp_send_reschedule+0x3f/0x50
>    trigger_load_balance+0x29c/0x4a0
>    ? trigger_load_balance+0x72/0x4a0
>    scheduler_tick+0x9f/0xd0
>    ? tick_sched_do_timer+0x50/0x50
>    update_process_times+0x47/0x60
>    tick_sched_handle.isra.24+0x25/0x60
>    tick_sched_timer+0x3d/0x70
>    __hrtimer_run_queues+0xf4/0x510
>    hrtimer_interrupt+0xb7/0x1d0
>    local_apic_timer_interrupt+0x35/0x60
>    smp_apic_timer_interrupt+0x3d/0x50
>    apic_timer_interrupt+0x96/0xa0
>
> If there is a need to kick the idle load balancer, an ILB will be selected
> to perform nohz idle load balance, however, if the selected ILB is in the
> process of offline, smp_sched_reschedule() which generates a sched IPI will
> splat as above.
>
>            CPU0                      CPU1
>
>                                  find_new_ilb()
>     set_rq_offline()
>                                  smp_sched_reschedule()  Oops
>     nohz_balance_exit_idle()
>
> This patch fix it by exiting nohz idle balance before set cpu offline.

             CPU 0                       CPU1

                                          find_new_ilb()
    nohz_balance_exit_idle()
    set_rq_offline()
                                          smp_sched_reschedule()

It seems that the patch still can't avoid this race, so any proposal
is a great appreciated. :)

Regards,
Wanpeng Li