ast: cursor flashing softlockups

[David Daney]

On 05/17/2016 10:39 AM, David Daney wrote:
> I can confirm this.

I have a patch that I think is the proper fix.  You should see it soon 
(after I test it a bit more)

Thanks,
David Daney


[...]

>>>
>>> On Tue, May 17, 2016 at 4:07 AM, Dann Frazier
>>> <dann.frazier at canonical.com> wrote:
>>>> Hi,
>>>>   I'm observing a soft lockup issue w/ the ASPEED controller on an
>>>> arm64 server platform. This was originally seen on Ubuntu's 4.4
>>>> kernel, but it is reproducible w/ vanilla 4.6-rc7 as well.
>>>>
>>>> [   32.792656] NMI watchdog: BUG: soft lockup - CPU#38 stuck for 22s!
>>>> [swapper/38:0]
>>>>
>>>> I observe this just once each time I boot into debian-installer (I'm
>>>> using a serial console, but the ast module gets loaded during
>>>> startup).
>>>
>>> I have figured out that it is caused by 'mod_timer(timer, jiffies)' and
>>> 'ops->cur_blink_jiffies' is observed as zero in cursor_timer_handler()
>>> when the issue happened.
>>
>> Thanks for tracking this down.
>>
>> This softlockup looks to be caused by:
>>
>>     commit 27a4c827c34ac4256a190cc9d24607f953c1c459
>>     Author: Scot Doyle <lkml14 at scotdoyle.com>
>>     Date:   Thu Mar 26 13:56:38 2015 +0000
>>
>>         fbcon: use the cursor blink interval provided by vt
>>
>>         vt now provides a cursor blink interval via vc_data. Use this
>>         interval instead of the currently hardcoded 200 msecs. Store
>> it in
>>         fbcon_ops to avoid locking the console in cursor_timer_handler().
>>
>>         Signed-off-by: Scot Doyle <lkml14 at scotdoyle.com>
>>         Acked-by: Pavel Machek <pavel at ucw.cz>
>>         Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
>>
>> and
>>
>>     commit bd63364caa8df38bad2b25b11b2a1b849475cce5
>>     Author: Scot Doyle <lkml14 at scotdoyle.com>
>>     Date:   Thu Mar 26 13:54:39 2015 +0000
>>
>>         vt: add cursor blink interval escape sequence
>>
>>         Add an escape sequence to specify the current console's cursor
>> blink
>>         interval. The interval is specified as a number of
>> milliseconds until
>>         the next cursor display state toggle, from 50 to 65535.
>> /proc/loadavg
>>         did not show a difference with a one msec interval, but the lower
>>         bound is set to 50 msecs since slower hardware wasn't tested.
>>
>>         Store the interval in the vc_data structure for later access
>> by fbcon,
>>         initializing the value to fbcon's current hardcoded value of
>> 200 msecs.
>>
>>         Signed-off-by: Scot Doyle <lkml14 at scotdoyle.com>
>>         Acked-by: Pavel Machek <pavel at ucw.cz>
>>         Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
>>
>>
>>
>>> Looks it is a real fbcon/vt issue, see following:
>>>
>>> fbcon_init()
>>>      <-.con_init
>>>            <-visual_init()
>>>
>>> reset_terminal()
>>>      <-vc_init()
>>>
>>> vc->vc_cur_blink_ms is just set in reset_terminal() from vc_init() path,
>>> and ops->cur_blink_jiffies is figured out from vc->vc_cur_blink_ms
>>> in fbcon_init().
>>>
>>> And visual_init() is always run before vc_init(), so
>>> ops->cur_blink_jiffies
>>> is initialized as zero and cause the soft lockup issue finally.
>>>
>>> Thanks,
>>> Ming
>>>
>>>>
>>>> perf shows that the CPU caught by the NMI is typically in code
>>>> updating the cursor timer:
>>>>
>>>> -   16.92%  swapper  [kernel.kallsyms]      [k]
>>>> _raw_spin_unlock_irqrestore
>>>>     - _raw_spin_unlock_irqrestore
>>>>        + 16.87% mod_timer
>>>>        + 0.05% cursor_timer_handler
>>>> -   12.15%  swapper  [kernel.kallsyms]      [k] queue_work_on
>>>>     - queue_work_on
>>>>        + 12.00% cursor_timer_handler
>>>>        + 0.15% call_timer_fn
>>>> +   10.98%  swapper  [kernel.kallsyms]      [k] run_timer_softirq
>>>> -    2.23%  swapper  [kernel.kallsyms]      [k] mod_timer
>>>>     - mod_timer
>>>>        + 1.97% cursor_timer_handler
>>>>        + 0.26% call_timer_fn
>>>>
>>>> During the same period, I can see that another CPU is actively
>>>> executing the timer function:
>>>>
>>>> -   42.18%  kworker/u96:2  [kernel.kallsyms]  [k] ww_mutex_unlock
>>>>     - ww_mutex_unlock
>>>>        - 40.70% ast_dirty_update
>>>>             ast_imageblit
>>>>             soft_cursor
>>>>             bit_cursor
>>>>             fb_flashcursor
>>>>             process_one_work
>>>>             worker_thread
>>>>             kthread
>>>>             ret_from_fork
>>>>        + 1.48% ast_imageblit
>>>> -   40.15%  kworker/u96:2  [kernel.kallsyms]  [k] __memcpy_toio
>>>>     - __memcpy_toio
>>>>        + 31.54% ast_dirty_update
>>>>        + 8.61% ast_imageblit
>>>>
>>>> Using the graph function tracer on fb_flashcursor(), I see that
>>>> ast_dirty_update usually takes around 60 us, in which it makes 16
>>>> calls to __memcpy_toio(). However, there is always one instance on
>>>> every boot of the installer where ast_dirty_update() takes ~98 *ms* to
>>>> complete, during which it makes 743 calls to __memcpy_toio(). While
>>>> that  doesn't directly account for the full 22s, I do wonder if that
>>>> maybe a smoking gun.
>>>>
>>>> fyi, this is being tracked at: https://bugs.launchpad.net/bugs/1574814
>>>>
>>>>    -dann
>>
>