Re: tick/sched: iowait_sleeptime resides in tick_cpu_sched structure is a negative value

[Frederic Weisbecker <frederic@kernel.org>]

On Thu, Jan 06, 2022 at 05:14:02PM +0800, luanshi wrote:
> Hi All,
> 
> We encounted a hardlockup problem on ARM server, after some debug found
> that:
> 
> PID: 0      TASK: ffff0400064de300  CPU: 126  COMMAND: "swapper/126"
>  #0 [ffff8000250f3c90] __crash_kexec at ffff80001013a064
>  #1 [ffff8000250f3e30] panic at ffff800010afd028
>  #2 [ffff8000250f3f10] nmi_panic at ffff80001004a9e0
>  #3 [ffff8000250f3f20] watchdog_hardlockup_check at ffff80001017b7f0
>  #4 [ffff8000250f3f40] sdei_watchdog_callback at ffff80001003c9a4
>  #5 [ffff8000250f3f50] sdei_event_handler at ffff8000108816b8
>  #6 [ffff8000250f3f80] _sdei_handler at ffff800010b1c2e4
>  #7 [ffff8000250f3fd0] __sdei_handler at ffff800010b139b0
>  #8 [ffff8000250f3ff0] __sdei_asm_handler at ffff800010014c18
> --- <IRQ stack> ---
>  #9 [ffff800013973f10] __cpu_do_idle at ffff800010b13764
> #10 [ffff800013973f20] arch_cpu_idle at ffff800010b137ec
> #11 [ffff800013973f30] default_idle_call at ffff800010b1b5cc
> #12 [ffff800013973f50] cpuidle_idle_call at ffff80001009648c
> #13 [ffff800013973f90] do_idle at ffff8000100965b0
> #14 [ffff800013973fc0] cpu_startup_entry at ffff8000100967d4
> #15 [ffff800013973fe0] secondary_start_kernel at ffff800010026bb0
> 
> 
> per_cpu(tick_cpu_sched, 126) = $1 = {
>   sched_timer = {
>     node = {
>       node = {
>         __rb_parent_color = 18446603337117384112,
>         rb_right = 0x0,
>         rb_left = 0x0
>       },
>       expires = 6108564000000
>     },
>     _softexpires = 6108564000000,
>     function = 0xffff800010122ec0 <tick_sched_timer>,
>     base = 0xffff04473bbcc780,
>     state = 1 '\001',
>     is_rel = 0 '\000',
>     is_soft = 0 '\000',
>     is_hard = 1 '\001'
>   },
>   check_clocks = 0,
>   nohz_mode = NOHZ_MODE_INACTIVE,
>   inidle = 1,
>   tick_stopped = 0,
>   idle_active = 1,
>   do_timer_last = 0,
>   got_idle_tick = 1,
>   last_tick = 0,
>   next_tick = 0,
>   idle_jiffies = 0,
>   idle_calls = 0,
>   idle_sleeps = 0,
>   idle_entrytime = 5012087709249,
>   idle_waketime = 0,
>   idle_exittime = 0,
>   idle_sleeptime = 4936136669951,
>   iowait_sleeptime = -1942739704,
>   last_jiffies = 0,
>   timer_expires = 0,
>   timer_expires_base = 0,
>   next_timer = 0,
>   idle_expires = 0,
>   tick_dep_mask = {
>     counter = 0
>   }
> }
> 
> iowait_sleeptime = -1942739704,
> ----------
> iowait_sleeptime is monotonically increasing, under what circumstances iowait_sleeptime
> can be a negative value?
> 
> For detailed information:
> https://bugzilla.kernel.org/show_bug.cgi?id=215458
> 
> 
> Can you give me some suggestions for debugging.

Some racy updates can happen if cpufreq or "cat /proc/stat" do concurrent
updates. But for that nohz needs to be running and I see your CPU clock has
NOHZ_MODE_INACTIVE. Perhaps it's only for that CPU though.

What is the value of tick_nohz_active in your dump?


> 
> 
> 
> Thanks,
> Liguang
>