ast: cursor flashing softlockups

[Ming Lei]

Hi,

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.

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