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Message-ID: <a4229d29-a512-2dee-1989-9bc2ac10ed55@linux.alibaba.com>
Date:   Fri, 8 Oct 2021 15:50:15 +0800
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Subject: Re: [PATCH] tty: tty_buffer: Fix the softlockup issue in
 flush_to_ldisc
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     jirislaby@kernel.org, baolin.wang@linux.alibaba.com,
        linux-kernel@vger.kernel.org
References: <1632971498-57869-1-git-send-email-guanghuifeng@linux.alibaba.com>
 <YVVNSS/+RInLPpx/@kroah.com>
From:   "guanghui.fgh" <guanghuifeng@linux.alibaba.com>
In-Reply-To: <YVVNSS/+RInLPpx/@kroah.com>
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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.

>
>
>> Signed-off-by: Guanghui Feng <guanghuifeng@linux.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();

> 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)
> 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).

thanks

Guanghui Feng