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Kernel hard LOCKUP and rcu_sched stalls caused by a slow responding serial console interface

This document (000020516) is provided subject to the disclaimer at the end of this document.


SUSE Linux Enterprise Server 15
SUSE Linux Enterprise Server 12


A modified kernel module with printk() enabled calls has been loaded for debugging purposes. Kernel crashed due to a hard LOCKUP on CPU#10:
[ 5653.173525] NMI watchdog: Watchdog detected hard LOCKUP on cpu 10
[ 5653.173539] RIP: 0010:native_queued_spin_lock_slowpath+0x65/0x1e0
[ 5653.173542] Call Trace:
[ 5653.173542]  _raw_spin_lock_irqsave+0x30/0x40
[ 5653.173542]  force_qs_rnp+0xa8/0x160
[ 5653.173542]  rcu_gp_kthread+0x7b2/0xfa0
[ 5653.173542]  ? __schedule+0x305/0x750
[ 5653.173542]  ? rcu_barrier+0x590/0x590
[ 5653.173543]  kthread+0x10d/0x130
[ 5653.173543]  ? kthread_park+0xa0/0xa0
[ 5653.173543]  ret_from_fork+0x35/0x40

The current stack of CPU#10, running rcu_sched with PID 12:
 PID: 12     TASK: ffff967981712680  CPU: 10  COMMAND: "rcu_sched"
 #0 [fffffe00002079c8] machine_kexec at ffffffffa3e6d861
 #1 [fffffe0000207a20] __crash_kexec at ffffffffa3f53ca5
 #2 [fffffe0000207ae8] panic at ffffffffa3e9b1e7
 #3 [fffffe0000207b70] nmi_panic at ffffffffa3e9ae45
 #4 [fffffe0000207b78] watchdog_overflow_callback at ffffffffa3f89fb4
 #5 [fffffe0000207b88] __perf_event_overflow at ffffffffa40041c2
 #6 [fffffe0000207bb8] handle_pmi_common at ffffffffa3e0efa6
 #7 [fffffe0000207e08] intel_pmu_handle_irq at ffffffffa3e0f22f
 #8 [fffffe0000207e48] perf_event_nmi_handler at ffffffffa3e0746d
 #9 [fffffe0000207e60] nmi_handle at ffffffffa3e36043
#10 [fffffe0000207eb8] default_do_nmi at ffffffffa3e364be
#11 [fffffe0000207ed8] do_nmi at ffffffffa3e366d3
#12 [fffffe0000207ef0] end_repeat_nmi at ffffffffa480164b
    [exception RIP: native_queued_spin_lock_slowpath+101]
    RIP: ffffffffa3ef8345  RSP: ffffa4e94c51be40  RFLAGS: 00000002
    RAX: 0000000000000101  RBX: 0000000000000246  RCX: ffffffffa52c21a0
    RDX: 0000000000000000  RSI: 0000000000000000  RDI: ffffffffa5279600
    RBP: ffffffffa5279600   R8: 0000000000000001   R9: 0000000000000002
    R10: ffffa4e94c57bdc8  R11: 0000000000000006  R12: 0000000000000000
    R13: ffffffffa3f1c560  R14: 000000000002d7c0  R15: 0000000000000001
    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
--- <NMI exception stack> ---
#13 [ffffa4e94c51be40] native_queued_spin_lock_slowpath at ffffffffa3ef8345
#14 [ffffa4e94c51be40] _raw_spin_lock_irqsave at ffffffffa46a5ea0
#15 [ffffa4e94c51be50] force_qs_rnp at ffffffffa3f1ccc8
#16 [ffffa4e94c51be90] rcu_gp_kthread at ffffffffa3f21322
#17 [ffffa4e94c51bf10] kthread at ffffffffa3ec06ed
#18 [ffffa4e94c51bf50] ret_from_fork at ffffffffa4800215

All CPU are running idle, except for CPU 56,52,10, which also have expired timers:
crash> timer -r -C 10,56,52
CPU: 10  HRTIMER_CPU_BASE: ffff96987f49ff80
  CLOCK: 0  HRTIMER_CLOCK_BASE: ffff96987f49ffc0  [ktime_get]
   SOFTEXPIRES      EXPIRES          TTE          HRTIMER           FUNCTION
  5626674278491  5626674278491  -23733721509  ffff96987f4a04e0  ffffffffa3f42bf0  <tick_sched_timer>
  5628834278491  5628834278491  -21573721509  ffff96987f4a0740  ffffffffa3f89540  <watchdog_timer_fn>

CPU: 52  HRTIMER_CPU_BASE: ffff96987fa9ff80
  CLOCK: 0  HRTIMER_CLOCK_BASE: ffff96987fa9ffc0  [ktime_get]
   SOFTEXPIRES      EXPIRES         TTE          HRTIMER           FUNCTION
  5649262278491  5649262278491  -1145721509  ffff96987faa04e0  ffffffffa3f42bf0  <tick_sched_timer>

CPU: 56  HRTIMER_CPU_BASE: ffff96b8ff91ff80
  CLOCK: 0  HRTIMER_CLOCK_BASE: ffff96b8ff91ffc0  [ktime_get]
   SOFTEXPIRES      EXPIRES          TTE          HRTIMER           FUNCTION
  5629282278491  5629282278491  -21125721509  ffff96b8ff920740  ffffffffa3f89540  <watchdog_timer_fn>

The referenced spinlock seems to be only used by PID#12:
crash> qspinlock ffffffffa5279600
struct qspinlock {
    val = {
      counter = 0x101
      locked = 0x1,
      pending = 0x1
      locked_pending = 0x101,
      tail = 0x0

While CPU#52 is writing the printk() debugging messages to the serial console:
PID: 28052  TASK: ffff96a1e5fa0000  CPU: 52  COMMAND: "sshd"
 #0 [fffffe0000a65e58] crash_nmi_callback at ffffffffa3e61113
 #1 [fffffe0000a65e60] nmi_handle at ffffffffa3e36043
 #2 [fffffe0000a65eb8] default_do_nmi at ffffffffa3e364be
 #3 [fffffe0000a65ed8] do_nmi at ffffffffa3e366d3
 #4 [fffffe0000a65ef0] end_repeat_nmi at ffffffffa480164b
    [exception RIP: io_serial_in+20]
    RIP: ffffffffa43d8424  RSP: ffffa4e94d204c20  RFLAGS: 00000002
    RAX: ffffffffa43d8400  RBX: ffffffffa6458540  RCX: 0000000000000000
    RDX: 00000000000002fd  RSI: 0000000000000005  RDI: ffffffffa6458540
    RBP: 00000000000026fc   R8: 000b4bf3fa09edba   R9: 0000000000000034
    R10: 0000000000000001  R11: ffffa4e94d204cb0  R12: 0000000000000020
    R13: ffffffffa5a89605  R14: ffffffffa5a895c0  R15: 0000000000000045
    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
--- <NMI exception stack> ---
 #5 [ffffa4e94d204c20] io_serial_in at ffffffffa43d8424
 #6 [ffffa4e94d204c20] wait_for_xmitr at ffffffffa43d8e20
 #7 [ffffa4e94d204c40] serial8250_console_putchar at ffffffffa43d8ea6
 #8 [ffffa4e94d204c58] uart_console_write at ffffffffa43d16f5
 #9 [ffffa4e94d204c80] serial8250_console_write at ffffffffa43dcd18
#10 [ffffa4e94d204d00] console_unlock at ffffffffa3f06ae2
#11 [ffffa4e94d204d48] vprintk_emit at ffffffffa3f086f9
#12 [ffffa4e94d204d98] printk at ffffffffa3f08fb0
#13 [ffffa4e94d204e00] show_trace_log_lvl at ffffffffa3e3591d
#14 [ffffa4e94d204ef0] watchdog_timer_fn at ffffffffa3f89758
#15 [ffffa4e94d204f20] __hrtimer_run_queues at ffffffffa3f31c08
#16 [ffffa4e94d204f80] hrtimer_interrupt at ffffffffa3f323a5
#17 [ffffa4e94d204fd8] smp_apic_timer_interrupt at ffffffffa48026ba
#18 [ffffa4e94d204ff0] apic_timer_interrupt at ffffffffa4801bef
--- <IRQ stack> ---

The system serial interface is active:
crash> p saved_command_line
saved_command_line = $1 = 0xffff96b8fffb7a80 "BOOT_IMAGE=/boot/vmlinuz-5.3.18-24.37-default root=/dev/mapper/vg_system-lv_root ipv6.disable=1 showopts ignore_loglevel textmode=1 splash=verbose transparent_hugepage=never crashkernel=768M,high crashkernel=72M,low console=tty0 console=ttyS1,115200 vga=0x"...


The situation can be omitted by removing/disabling the serial console configuration on grub, as it is not to fast enough to bear with the huge amount of logs during a printk storm, for as long as the modified kernel module is running. 


The hard LOCUP and rcu_sched stalls are caused by a slowly responding serial console interface, during a massive amount of printk messages (printk storm). 


Reported to Engineering


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  • Document ID:000020516
  • Creation Date: 25-Nov-2021
  • Modified Date:03-Dec-2021
    • SUSE Linux Enterprise Server
    • SUSE Linux Enterprise Server for SAP Applications

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