Re: Question about kill a process group

[Eric W. Biederman]

Zhang Qiao <zhangqiao22@xxxxxxxxxx> writes:

> Gentle ping. Any comments on this problem?

Is fork12 a new test?

Is there a real world use case that connects to this?

How many children are being created in this test?  Several million?

I would like to blame this on the old issue that tasklist_lock being
a global lock.  Given the number of child processes (as many as can be
created) I don't think we are hurt much by using a global lock.  The
problem for solubility is that we have a lock.

Fundamentally there must be a lock taken to maintain the parent's
list of children.

I only see SIGQUIT being called once in the parent process so that
should not be an issue.

There is a minor issue in fork12 that it calls exit(0) instead of
_exit(0) in the children.  Not the problem you are dealing with
but it does look like it can be a distraction.

I suspect the issue really is the thundering hurd of a million+
processes synchronizing on a single lock.

I don't think this is a hard lockup, just a global slow down.
I expect everything will eventually exit.


To do something about this is going to take a deep and fundamental
redesign of how we maintain process lists to handle a parent
with millions of children well.

Is there any real world reason to care about this case?  Without
real world motivation I am inclined to just note that this is
something that is handled poorly, and leave it as is.

Eric

>
> 在 2022/4/2 10:22, Zhang Qiao 写道:
>> ping...
>> 
>> Any suggestions for this problem？
>> 
>> thank!
>> Qiao
>> 
>> 
>> 在 2022/3/29 16:07, Zhang Qiao 写道:
>>> hello everyone,
>>>
>>> I got a hradlockup panic when run the ltp syscall testcases.
>>>
>>> 348439.713178] NMI watchdog: Watchdog detected hard LOCKUP on cpu 32
>>> [348439.713236] irq event stamp: 0
>>> [348439.713237] hardirqs last  enabled at (0): [<0000000000000000>] 0x0
>>> [348439.713238] hardirqs last disabled at (0): [<ffffffff87cd1ea5>] copy_process+0x7f5/0x2160
>>> [348439.713239] softirqs last  enabled at (0): [<ffffffff87cd1ea5>] copy_process+0x7f5/0x2160
>>> [348439.713240] softirqs last disabled at (0): [<0000000000000000>] 0x0
>>> [348439.713241] CPU: 32 PID: 1151212 Comm: fork12 Kdump: loaded Tainted: G S                5.10.0+ #1
>>> [348439.713242] Hardware name: Huawei RH2288H V3/BC11HGSA0, BIOS 3.35 10/20/2016
>>> [348439.713243] RIP: 0010:queued_write_lock_slowpath+0x4d/0x80
>>> [348439.713245] RSP: 0018:ffffa3a6bed4fe60 EFLAGS: 00000006
>>> [348439.713246] RAX: 0000000000000500 RBX: ffffffff892060c0 RCX: 00000000000000ff
>>> [348439.713247] RDX: 0000000000000500 RSI: 0000000000000100 RDI: ffffffff892060c0
>>> [348439.713248] RBP: ffffffff892060c4 R08: 0000000000000001 R09: 0000000000000000
>>> [348439.713249] R10: ffffa3a6bed4fde8 R11: 0000000000000000 R12: ffff96dfd3b68001
>>> [348439.713250] R13: ffff96dfd3b68000 R14: ffff96dfd3b68c38 R15: ffff96e2cf1f51c0
>>> [348439.713251] FS:  0000000000000000(0000) GS:ffff96edbc200000(0000) knlGS:0000000000000000
>>> [348439.713252] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> [348439.713253] CR2: 0000000000416ea0 CR3: 0000002d91812004 CR4: 00000000001706e0
>>> [348439.713254] Call Trace:
>>> [348439.713255]  do_raw_write_lock+0xa9/0xb0
>>> [348439.713256]  _raw_write_lock_irq+0x5a/0x70
>>> [348439.713256]  do_exit+0x429/0xd00
>>> [348439.713257]  do_group_exit+0x39/0xb0
>>> [348439.713258]  __x64_sys_exit_group+0x14/0x20
>>> [348439.713259]  do_syscall_64+0x33/0x40
>>> [348439.713260]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>> [348439.713260] RIP: 0033:0x7f59295a7066
>>> [348439.713261] Code: Unable to access opcode bytes at RIP 0x7f59295a703c.
>>> [348439.713262] RSP: 002b:00007fff0afeac38 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
>>> [348439.713264] RAX: ffffffffffffffda RBX: 00007f5929694530 RCX: 00007f59295a7066
>>> [348439.713265] RDX: 0000000000000002 RSI: 000000000000003c RDI: 0000000000000002
>>> [348439.713266] RBP: 0000000000000002 R08: 00000000000000e7 R09: ffffffffffffff80
>>> [348439.713267] R10: 0000000000000002 R11: 0000000000000246 R12: 00007f5929694530
>>> [348439.713268] R13: 0000000000000001 R14: 00007f5929697f68 R15: 0000000000000000
>>> [348439.713269] Kernel panic - not syncing: Hard LOCKUP
>>> [348439.713270] CPU: 32 PID: 1151212 Comm: fork12 Kdump: loaded Tainted: G S                5.10.0+ #1
>>> [348439.713272] Hardware name: Huawei RH2288H V3/BC11HGSA0, BIOS 3.35 10/20/2016
>>> [348439.713272] Call Trace:
>>> [348439.713273]  <NMI>
>>> [348439.713274]  dump_stack+0x77/0x97
>>> [348439.713275]  panic+0x10c/0x2fb
>>> [348439.713275]  nmi_panic+0x35/0x40
>>> [348439.713276]  watchdog_hardlockup_check+0xeb/0x110
>>> [348439.713277]  __perf_event_overflow+0x52/0xf0
>>> [348439.713278]  handle_pmi_common+0x21a/0x320
>>> [348439.713286]  intel_pmu_handle_irq+0xc9/0x1b0
>>> [348439.713287]  perf_event_nmi_handler+0x24/0x40
>>> [348439.713288]  nmi_handle+0xc3/0x2a0
>>> [348439.713289]  default_do_nmi+0x49/0xf0
>>> [348439.713289]  exc_nmi+0x146/0x160
>>> [348439.713290]  end_repeat_nmi+0x16/0x55
>>> [348439.713291] RIP: 0010:queued_write_lock_slowpath+0x4d/0x80
>>> [348439.713293] RSP: 0018:ffffa3a6bed4fe60 EFLAGS: 00000006
>>> [348439.713295] RAX: 0000000000000500 RBX: ffffffff892060c0 RCX: 00000000000000ff
>>> [348439.713296] RDX: 0000000000000500 RSI: 0000000000000100 RDI: ffffffff892060c0
>>> [348439.713296] RBP: ffffffff892060c4 R08: 0000000000000001 R09: 0000000000000000
>>> [348439.713297] R10: ffffa3a6bed4fde8 R11: 0000000000000000 R12: ffff96dfd3b68001
>>> [348439.713298] R13: ffff96dfd3b68000 R14: ffff96dfd3b68c38 R15: ffff96e2cf1f51c0
>>> [348439.713300]  </NMI>
>>> [348439.713301]  do_raw_write_lock+0xa9/0xb0
>>> [348439.713302]  _raw_write_lock_irq+0x5a/0x70
>>> [348439.713303]  do_exit+0x429/0xd00
>>> [348439.713303]  do_group_exit+0x39/0xb0
>>> [348439.713304]  __x64_sys_exit_group+0x14/0x20
>>> [348439.713305]  do_syscall_64+0x33/0x40
>>> [348439.713306]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>> [348439.713307] RIP: 0033:0x7f59295a7066
>>> [348439.713308] Code: Unable to access opcode bytes at RIP 0x7f59295a703c.
>>>
>>>
>>> when analyzing vmcore, i notice lots of fork12 processes are waiting for tasklist read lock or write
>>> lock (see the attachment file all_cpu_stacks.log)，and every fork12 process(belongs to the same
>>> process group) call kill(0, SIGQUIT) in their signal handler()[1], it will traverse all the processes in the
>>> same process group and send signal to them one by one, which is a very time-costly work and hold tasklist
>>> read lock long time. At the same time, other processes will exit after receive signal, they try to get
>>> the tasklist write lock at exit_notify().
>>>
>>> [1] fork12 testcase: https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/syscalls/fork/fork12.c
>>>
>>> some processes call kill(0, SIGQUIT), wait for tasklist read lock:
>>>
>>>  #5 [ffff972a9b16fd78] native_queued_spin_lock_slowpath at ffffffff9931ed47
>>>  #6 [ffff972a9b16fd78] queued_read_lock_slowpath at ffffffff99320a58
>>>  #7 [ffff972a9b16fd90] do_wait at ffffffff992bc17d
>>>  #8 [ffff972a9b16fdd0] kernel_wait4 at ffffffff992bd88d
>>>  #9 [ffff972a9b16fe58] __do_sys_wait4 at ffffffff992bd9e5
>>> #10 [ffff972a9b16ff30] do_syscall_64 at ffffffff9920432d
>>> #11 [ffff972a9b16ff50] entry_SYSCALL_64_after_hwframe at ffffffff99c000ad
>>>
>>> As the same time, some processes are exiting, wait for tasklist write lock:
>>>
>>>  #5 [ffff972aa49a7e60] native_queued_spin_lock_slowpath at ffffffff9931ecb0
>>>  #6 [ffff972aa49a7e60] queued_write_lock_slowpath at ffffffff993209e4
>>>  #7 [ffff972aa49a7e78] do_raw_write_lock at ffffffff99320834
>>>  #8 [ffff972aa49a7e88] do_exit at ffffffff992bcd78
>>>  #9 [ffff972aa49a7f00] do_group_exit at ffffffff992bd719
>>> #10 [ffff972aa49a7f28] __x64_sys_exit_group at ffffffff992bd7a4
>>> #11 [ffff972aa49a7f30] do_syscall_64 at ffffffff9920432d
>>> #12 [ffff972aa49a7f50] entry_SYSCALL_64_after_hwframe at ffffffff99c000ad
>>>
>>> In this scenario,there are lots of process are waiting for tasklist read lock or the tasklist
>>> write lock, so they will queue. if the wait queue is long enough, it might cause a hardlockup issue when a
>>> process wait for taking the write lock at exit_notify().
>>>
>>> I tried to solve this problem by avoiding traversing the process group multiple times when kill(0, xxxx)
>>> is called multiple times form the same process group, but it doesn't look like a good solution.
>>>
>>> Is there any good idea for fixing this problem ?
>>>
>>> Thanks!
>>>
>>> Qiao
>>> .
>>>
>> .
>>