Re: shell hung in fork system call

> On Jul 9, 2023, at 19:59, Konstantin Belousov <kostikbel@gmail.com> wrote:
> 
> On Sun, Jul 09, 2023 at 11:36:03PM +0000, John F Carr wrote:
>> 
>> 
>>> On Jul 9, 2023, at 19:25, Konstantin Belousov <kostikbel@gmail.com> wrote:
>>> 
>>> On Sun, Jul 09, 2023 at 10:41:27PM +0000, John F Carr wrote:
>>>> Kernel and system at a146207d66f320ed239c1059de9df854b66b55b7 plus some irrelevant local changes, four 64 bit ARM processors, make.conf sets CPUTYPE?=cortex-a57.
>>>> 
>>>> I typed ^C while /bin/sh was starting a pipeline and my shell got hung in the middle of fork().
>>>> 
>>>>> From the terminal:
>>>> 
>>>> # git log --oneline --|more
>>>> ^C^C^C
>>>> load: 3.26  cmd: sh 95505 [fork] 5308.67r 0.00u 0.03s 0% 2860k
>>>> mi_switch+0x198 sleepq_switch+0xfc sleepq_timedwait+0x40 _sleep+0x264 fork1+0x67c sys_fork+0x34 do_el0_sync+0x4c8 handle_el0_sync+0x44 
>>>> load: 3.16  cmd: sh 95505 [fork] 5311.75r 0.00u 0.03s 0% 2860k
>>>> mi_switch+0x198 sleepq_switch+0xfc sleepq_timedwait+0x40 _sleep+0x264 fork1+0x67c sys_fork+0x34 do_el0_sync+0x4c8 handle_el0_sync+0x44 
>>>> 
>>>> According to ps -d on another terminal the shell has no children:
>>>> 
>>>> PID TT  STAT       TIME COMMAND
>>>> [...]
>>>> 873 u0  IWs     0:00.00 `-- login [pam] (login)
>>>> 874 u0  I       0:00.17   `-- -sh (sh)
>>>> 95504 u0  I       0:00.01     `-- su -
>>>> 95505 u0  D+      0:00.05       `-- -su (sh)
>>>> [...]
>>>> 
>>>> Nothing on the (115200 bps serial) console.  No change in system performance.
>>>> 
>>>> The system is busy copying a large amount of data from the network to a ZFS pool on spinning disks.  The git|more pipeline could have taken some time to get going while I/O requests worked their way through the queue.  It would not have touched the busy pool, only the zroot pool on an SSD.
>>>> 
>>>> Has anything changed recently that might cause this?
>>> 
>>> There was some change around fork, but your sleep seems to be not from
>>> that change.  Can you show the wait channel for the process?  Do something
>>> like
>>> $ ps alxww
>>> 
>> 
>> UID   PID  PPID  C PRI NI   VSZ   RSS MWCHAN   STAT TT        TIME COMMAND
>>   0 95505 95504  2  20  0 13508  2876 fork     D+   u0     0:00.13 -su (sh)
>> 
>> This is probably the same information displayed as [fork] in the output from ^T.
>> 
>> Does it correspond to the source line
>> 
>> pause("fork", hz / 2);
>> 
>> ?
> 
> Yes, it is rate-limiting code.  Still it is interesting to see the whole
> ps output.
> 
> Do you have 7a70f17ac4bd64dc1a5020f in your source?

No, I do not have that commit.

The comment mentions livelock.  CPU use as reported by iostat did not change after the process hung.