Date:   Mon, 11 Oct 2021 15:42:38 +0800
From:   "guanghui.fgh" <guanghuifeng@...ux.alibaba.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     jirislaby@...nel.org, baolin.wang@...ux.alibaba.com,
        linux-kernel@...r.kernel.org, zhuo.song@...ux.alibaba.com,
        zhangliguang@...ux.alibaba.com
Subject: Re: [PATCH] tty: tty_buffer: Fix the softlockup issue in
 flush_to_ldisc


在 2021/10/10 21:18, Greg KH 写道:
> On Fri, Oct 08, 2021 at 03:50:15PM +0800, guanghui.fgh wrote:
>> 在 2021/9/30 13:38, Greg KH 写道:
>>> On Thu, Sep 30, 2021 at 11:11:38AM +0800, Guanghui Feng wrote:
>>>> When I run ltp testcase(ltp/testcases/kernel/pty/pty04.c) with arm64, there is a soft lockup,
>>>> which look like this one:
>>>>
>>>>     watchdog: BUG: soft lockup - CPU#41 stuck for 67s! [kworker/u192:2:106867]
>>>>     CPU: 41 PID: 106867 Comm: kworker/u192:2 Kdump: loaded Tainted: G           OE     5.10.23 #1
>>>>     Hardware name: H3C R4960 G3/BC82AMDDA, BIOS 1.70 01/07/2021
>>>>     Workqueue: events_unbound flush_to_ldisc
>>>>     pstate: 00c00009 (nzcv daif +PAN +UAO -TCO BTYPE=--)
>>>>     pc : slip_unesc+0x80/0x214 [slip]
>>>>     lr : slip_receive_buf+0x84/0x100 [slip]
>>>>     sp : ffff80005274bce0
>>>>     x29: ffff80005274bce0 x28: 0000000000000000
>>>>     x27: ffff00525626fcc8 x26: ffff800011921078
>>>>     x25: 0000000000000000 x24: 0000000000000004
>>>>     x23: ffff00402b4059c0 x22: ffff00402b405940
>>>>     x21: ffff205d87b81e21 x20: ffff205d87b81b9b
>>>>     x19: 0000000000000000 x18: 0000000000000000
>>>>     x17: 0000000000000000 x16: 0000000000000000
>>>>     x15: 0000000000000000 x14: 5f5f5f5f5f5f5f5f
>>>>     x13: 5f5f5f5f5f5f5f5f x12: 5f5f5f5f5f5f5f5f
>>>>     x11: 5f5f5f5f5f5f5f5f x10: 5f5f5f5f5f5f5f5f
>>>>     x9 : ffff8000097d7628 x8 : ffff205d87b85e20
>>>>     x7 : 0000000000000000 x6 : 0000000000000001
>>>>     x5 : ffff8000097dc008 x4 : ffff8000097d75a4
>>>>     x3 : ffff205d87b81e1f x2 : 0000000000000005
>>>>     x1 : 000000000000005f x0 : ffff00402b405940
>>>>     Call trace:
>>>>      slip_unesc+0x80/0x214 [slip]
>>>>      tty_ldisc_receive_buf+0x64/0x80
>>>>      tty_port_default_receive_buf+0x50/0x90
>>>>      flush_to_ldisc+0xbc/0x110
>>>>      process_one_work+0x1d4/0x4b0
>>>>      worker_thread+0x180/0x430
>>>>      kthread+0x11c/0x120
>>>>     Kernel panic - not syncing: softlockup: hung tasks
>>>>     CPU: 41 PID: 106867 Comm: kworker/u192:2 Kdump: loaded Tainted: G           OEL    5.10.23 #1
>>>>     Hardware name: H3C R4960 G3/BC82AMDDA, BIOS 1.70 01/07/2021
>>>>     Workqueue: events_unbound flush_to_ldisc
>>>>     Call trace:
>>>>      dump_backtrace+0x0/0x1ec
>>>>      show_stack+0x24/0x30
>>>>      dump_stack+0xd0/0x128
>>>>      panic+0x15c/0x374
>>>>      watchdog_timer_fn+0x2b8/0x304
>>>>      __run_hrtimer+0x88/0x2c0
>>>>      __hrtimer_run_queues+0xa4/0x120
>>>>      hrtimer_interrupt+0xfc/0x270
>>>>      arch_timer_handler_phys+0x40/0x50
>>>>      handle_percpu_devid_irq+0x94/0x220
>>>>      __handle_domain_irq+0x88/0xf0
>>>>      gic_handle_irq+0x84/0xfc
>>>>      el1_irq+0xc8/0x180
>>>>      slip_unesc+0x80/0x214 [slip]
>>>>      tty_ldisc_receive_buf+0x64/0x80
>>>>      tty_port_default_receive_buf+0x50/0x90
>>>>      flush_to_ldisc+0xbc/0x110
>>>>      process_one_work+0x1d4/0x4b0
>>>>      worker_thread+0x180/0x430
>>>>      kthread+0x11c/0x120
>>>>     SMP: stopping secondary CPUs
>>>>
>>>> In the testcase pty04, there are multple processes and we only pay close attention to the
>>>> first three actually. The first process call the write syscall to send data to the pty master
>>>> with all one's strength(tty_write->file_tty_write->do_tty_write->n_tty_write call chain).
>>>> The second process call the read syscall to receive data by the pty slave(with PF_PACKET socket).
>>>> The third process will wait a moment in which the first two processes will do there work and then
>>>> it call ioctl to hangup the pty pair which will cease the first two process read/write to the pty.
>>>> Before hangup the pty, the first process send data to the pty buffhead with high speed. At the same
>>>> time if the workqueue is waken up, the workqueue will do the flush_to_ldisc to pop data from pty
>>>> master's buffhead to line discipline in a loop until there is no more data left without any on one's
>>>> own schedule which will result in doing work in flush_to_ldisc for a long time. As kernel configured
>>>> without CONFIG_PREEMPT, there maybe occurs softlockup in the flush_to_ldisc. So I add cond_resched
>>>> in the flush_to_ldisc while loop to avoid it.
>>> Please properly wrap your changelog text at 72 columns.
>> When I run ltp testcase(ltp/testcases/kernel/pty/pty04.c) with arm64, there is a soft lockup,
>> which look like this one:
>> Call trace:
>>     dump_backtrace+0x0/0x1ec
>>     show_stack+0x24/0x30
>>     dump_stack+0xd0/0x128
>>     panic+0x15c/0x374
>>     watchdog_timer_fn+0x2b8/0x304
>>     __run_hrtimer+0x88/0x2c0
>>     __hrtimer_run_queues+0xa4/0x120
>>     hrtimer_interrupt+0xfc/0x270
>>     arch_timer_handler_phys+0x40/0x50
>>     handle_percpu_devid_irq+0x94/0x220
>>     __handle_domain_irq+0x88/0xf0
>>     gic_handle_irq+0x84/0xfc
>>     el1_irq+0xc8/0x180
>>     slip_unesc+0x80/0x214 [slip]
>>     tty_ldisc_receive_buf+0x64/0x80
>>     tty_port_default_receive_buf+0x50/0x90
>>     flush_to_ldisc+0xbc/0x110
>>     process_one_work+0x1d4/0x4b0
>>     worker_thread+0x180/0x430
>>     kthread+0x11c/0x120
>>
>> In the testcase pty04, The first process call the write syscall to send data to the pty master.
>> At the same time if the workqueue is waken up, the workqueue will do the flush_to_ldisc to pop data
>> in a loop until there is no more data left which will result in doing work in flush_to_ldisc for a
>> long time. As kernel configured without CONFIG_PREEMPT, there maybe occurs softlockup in the flush_to_ldisc.
> Is this a "real" test for something that you have seen in a normal
> workload?  ltp is known for having buggy/confusing tests in it in the
> past, you might wish to consult with the authors of that test.

Firstly, thanks for your response.

I have check the ltp pty testcase. At the same time, I find the pty 
softlockup in arm64, and it is similar to others.

https://github.com/victronenergy/venus/issues/350

https://groups.google.com/g/syzkaller-lts-bugs/c/SpkH8yH26js/m/3aifBl_GAwAJ

>>>
>>>> Signed-off-by: Guanghui Feng <guanghuifeng@...ux.alibaba.com>
>>>> ---
>>>>    drivers/tty/tty_buffer.c | 1 +
>>>>    1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
>>>> index bd2d915..77b92f9 100644
>>>> --- a/drivers/tty/tty_buffer.c
>>>> +++ b/drivers/tty/tty_buffer.c
>>>> @@ -534,6 +534,7 @@ static void flush_to_ldisc(struct work_struct *work)
>>>>    		if (!count)
>>>>    			break;
>>>>    		head->read += count;
>>>> +		cond_resched();
>>> This is almost never the correct solution for fixing a problem in the
>>> kernel anymore.
>>>
>>> And if it is, it needs to be documented really really well.  I think you
>>> just slowed down the overall throughput of a tty device by adding this
>>> call, so are you sure you didn't break something?
>> OK, it should be:
>>
>> diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
>> index bd2d915..77b92f9 100644
>> --- a/drivers/tty/tty_buffer.c
>> +++ b/drivers/tty/tty_buffer.c
>> @@ -534,6 +534,7 @@ static void flush_to_ldisc(struct work_struct *work)
>>   		if (!count)
>>   			break;
>>   		head->read += count;
>> +		if (need_resched())
>> +			cond_resched();
> Still feels really wrong, we do not sprinkle this all around the kernel
> if we do not have to.
>
>>> And why are you not running with a preempt kernel here?  What prevents
>>> that from being enabled to solve issues like this?
>> In server mode, we usually running without preempt kernel for
>> performance(with less scheduling)
> You are trading off throughput for this very reason, you are sending
> data faster than you could normally have, so why are you wanting to stop
> that?
>
>>> Also, having only one CPU burning through a network workload like this
>>> seems correct to me, why would you want the CPU to stop handling the
>>> data being sent to it like this?  You have at least 40 other ones to do
>>> other things here :)
>>>
>>> thanks,
>>>
>>> greg k-h
>> When only using one core, the pty data sending and workqueue can't do work
>> simultaneously. When the sender and workqueue
>>
>> running in different core, the workqueue will do the flush_to_ldisc in a
>> loop until there is no more data left which will result in
>>
>> occuring softlockup when the sender sends data fastly in full time. So I add
>> need_resched check and cond_resched in the
>>
>> flush_to_ldisc while loop to avoid it(without preempt kernel).
> Why not just switch to preempt kernel then if this specific workload
> really is important to you?
>
> Again, is this a real workload, or just a contrived test that is trying
> to get as much throughput as possible for a single pty device?
>
> thanks,
>
> greg k-h

Because there are many enviroment working right for a long time, and 
normally only doing bug fix,

and the consumers don't be willing to change the kernel configues.

thanks

Guanghui Feng

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