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Date:   Sun, 10 Oct 2021 15:18:56 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     "guanghui.fgh" <guanghuifeng@...ux.alibaba.com>
Cc:     jirislaby@...nel.org, baolin.wang@...ux.alibaba.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: tty_buffer: Fix the softlockup issue in
 flush_to_ldisc

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.

> 
> > 
> > 
> > > 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

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