Re: Can RCU stall lead to hard lockups?

Paul E. McKenney
2018-01-09T04:23:54Z

On Mon, Jan 08, 2018 at 07:52:07PM -0800, Tejun Heo wrote:
> Hello, Paul.
>
> So, I was looking at a machine which triggered crashdump from NMI hard
> lockup. The dmesg was filled up with backtraces - many were stuck in
> reclaim path, which seems to be the culprit, the others were stuck in
> RCU path. It looks like by the time crashdump was created, all CPUs
> got stuck in RCU path.
>
> One of the CPUs looked like the following.
>
> PID: 4149739 TASK: ffff881a46baa880 CPU: 13 COMMAND: "CPUThreadPool8"
> #0 [ffff881fff945e48] crash_nmi_callback at ffffffff8103f7d0
> #1 [ffff881fff945e58] nmi_handle at ffffffff81020653
> #2 [ffff881fff945eb0] default_do_nmi at ffffffff81020c36
> #3 [ffff881fff945ed0] do_nmi at ffffffff81020d32
> #4 [ffff881fff945ef0] end_repeat_nmi at ffffffff81956a7e
> [exception RIP: io_serial_in+21]
> RIP: ffffffff81630e55 RSP: ffff881fff943b88 RFLAGS: 00000002
> RAX: 000000000000ca00 RBX: ffffffff8230e188 RCX: 0000000000000000
> RDX: 00000000000002fd RSI: 0000000000000005 RDI: ffffffff8230e188
> RBP: ffff881fff943bb0 R8: 0000000000000000 R9: ffffffff820cb3c4
> R10: 0000000000000019 R11: 0000000000002000 R12: 00000000000026e1
> R13: 0000000000000020 R14: ffffffff820cd398 R15: 0000000000000035
> ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0000
> --- <NMI exception stack> ---
> #5 [ffff881fff943b88] io_serial_in at ffffffff81630e55
> #6 [ffff881fff943b90] wait_for_xmitr at ffffffff8163175c
> #7 [ffff881fff943bb8] serial8250_console_putchar at ffffffff816317dc
> #8 [ffff881fff943bd8] uart_console_write at ffffffff8162ac00
> #9 [ffff881fff943c08] serial8250_console_write at ffffffff81634691
> #10 [ffff881fff943c80] univ8250_console_write at ffffffff8162f7c2
> #11 [ffff881fff943c90] console_unlock at ffffffff810dfc55
> #12 [ffff881fff943cf0] vprintk_emit at ffffffff810dffb5
> #13 [ffff881fff943d50] vprintk_default at ffffffff810e01bf
> #14 [ffff881fff943d60] vprintk_func at ffffffff810e1127
> #15 [ffff881fff943d70] printk at ffffffff8119a8a4
> #16 [ffff881fff943dd0] print_cpu_stall_info at ffffffff810eb78c
> #17 [ffff881fff943e88] rcu_check_callbacks at ffffffff810ef133
> #18 [ffff881fff943ee8] update_process_times at ffffffff810f3497
> #19 [ffff881fff943f10] tick_sched_timer at ffffffff81103037
> #20 [ffff881fff943f38] __hrtimer_run_queues at ffffffff810f3f38
> #21 [ffff881fff943f88] hrtimer_interrupt at ffffffff810f442b

You don't have a low-bandwidth console, do you? (For example, the
output being directed to slow mass storage, across a slow network link,
or the like.)

> The CPU which triggered hardlockup:
>
> PID: 4149709 TASK: ffff88010f88c380 CPU: 26 COMMAND: "CPUThreadPool35"
> #0 [ffff883fff1059d0] machine_kexec at ffffffff8104a874
> #1 [ffff883fff105a30] __crash_kexec at ffffffff811116cc
> #2 [ffff883fff105af0] __crash_kexec at ffffffff81111795
> #3 [ffff883fff105b08] panic at ffffffff8119a6ae
> #4 [ffff883fff105b98] watchdog_overflow_callback at ffffffff81135dbd
> #5 [ffff883fff105bb0] __perf_event_overflow at ffffffff81186866
> #6 [ffff883fff105be8] perf_event_overflow at ffffffff81192bc4
> #7 [ffff883fff105bf8] intel_pmu_handle_irq at ffffffff8100b265
> #8 [ffff883fff105df8] perf_event_nmi_handler at ffffffff8100489f
> #9 [ffff883fff105e58] nmi_handle at ffffffff81020653
> #10 [ffff883fff105eb0] default_do_nmi at ffffffff81020b94
> #11 [ffff883fff105ed0] do_nmi at ffffffff81020d32
> #12 [ffff883fff105ef0] end_repeat_nmi at ffffffff81956a7e
> [exception RIP: queued_spin_lock_slowpath+248]
> RIP: ffffffff810da958 RSP: ffff883fff103e68 RFLAGS: 00000046
> RAX: 0000000000000000 RBX: 0000000000000046 RCX: 00000000006d0000
> RDX: ffff883fff49a950 RSI: 0000000000d10101 RDI: ffffffff81e54300
> RBP: ffff883fff103e80 R8: ffff883fff11a950 R9: 0000000000000000
> R10: 000000000e5873ba R11: 000000000000010f R12: ffffffff81e54300
> R13: 0000000000000000 R14: ffff88010f88c380 R15: ffffffff81e54300
> ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
> --- <NMI exception stack> ---
> #13 [ffff883fff103e68] queued_spin_lock_slowpath at ffffffff810da958
> #14 [ffff883fff103e70] _raw_spin_lock_irqsave at ffffffff8195550b
> #15 [ffff883fff103e88] rcu_check_callbacks at ffffffff810eed18
> #16 [ffff883fff103ee8] update_process_times at ffffffff810f3497
> #17 [ffff883fff103f10] tick_sched_timer at ffffffff81103037
> #18 [ffff883fff103f38] __hrtimer_run_queues at ffffffff810f3f38
> #19 [ffff883fff103f88] hrtimer_interrupt at ffffffff810f442b
> --- <IRQ stack> ---
> bt: read error: kernel virtual address: ffffffffffffffd8 type: "pt_regs"

This might be unable to acquire the spinlock that CPU 13 was holding
while printing to the console.

> All other CPUs have something similar to the following:
>
> PID: 4150460 TASK: ffff88162c891b00 CPU: 8 COMMAND: "GlobalIOThreadP"
> #0 [ffff881fff805e48] crash_nmi_callback at ffffffff8103f7d0
> #1 [ffff881fff805e58] nmi_handle at ffffffff81020653
> #2 [ffff881fff805eb0] default_do_nmi at ffffffff81020c36
> #3 [ffff881fff805ed0] do_nmi at ffffffff81020d32
> #4 [ffff881fff805ef0] end_repeat_nmi at ffffffff81956a7e
> [exception RIP: queued_spin_lock_slowpath+248]
> RIP: ffffffff810da958 RSP: ffff881fff803e68 RFLAGS: 00000046
> RAX: 0000000000000000 RBX: 0000000000000046 RCX: 0000000000250000
> RDX: ffff881fffb1a950 RSI: 00000000002d0101 RDI: ffffffff81e54300
> RBP: ffff881fff803e80 R8: ffff881fff81a950 R9: 0000000000000000
> R10: 000000000e5873ba R11: ffff88207fffd000 R12: ffffffff81e54300
> R13: 0000000000000000 R14: ffff88162c891b00 R15: ffffffff81e54300
> ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0000
> --- <NMI exception stack> ---
> #5 [ffff881fff803e68] queued_spin_lock_slowpath at ffffffff810da958
> #6 [ffff881fff803e70] _raw_spin_lock_irqsave at ffffffff8195550b
> #7 [ffff881fff803e88] rcu_check_callbacks at ffffffff810eed18
> #8 [ffff881fff803ee8] update_process_times at ffffffff810f3497
> #9 [ffff881fff803f10] tick_sched_timer at ffffffff81103037
> #10 [ffff881fff803f38] __hrtimer_run_queues at ffffffff810f3f38
> #11 [ffff881fff803f88] hrtimer_interrupt at ffffffff810f442b
> --- <IRQ stack> ---
>
> I don't know the RCU code at all but it *looks* like the first CPU is
> taking a sweet while flushing printk buffer while holding a lock (the
> console is IPMI serial console, which faithfully emulates 115200 baud
> rate), and everyone else seems stuck waiting for that spinlock in
> rcu_check_callbacks().
>
> Does this sound possible?

115200 baud? Ouch!!! That -will- result in trouble from console
printing, and often also in RCU CPU stall warnings.

Thanx, Paul