Re: + panic-avoid-the-extra-noise-dmesg.patch added to -mm tree

Petr Mladek
2018-12-04T10:20:37Z

On Mon 2018-12-03 23:15:31, Andrew Morton wrote:
>
> The patch titled
> Subject: panic: Avoid the extra noise dmesg
> has been added to the -mm tree. Its filename is
> panic-avoid-the-extra-noise-dmesg.patch
>
> This patch should soon appear at
> http://ozlabs.org/~akpm/mmots/broken-out/panic-avoid-the-extra-noise-dmesg.patch
> and later at
> http://ozlabs.org/~akpm/mmotm/broken-out/panic-avoid-the-extra-noise-dmesg.patch
>
> Before you just go and hit "reply", please:
> a) Consider who else should be cc'ed
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> c) Ideally: find the original patch on the mailing list and do a
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>
> *** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
>
> The -mm tree is included into linux-next and is updated
> there every 3-4 working days
>
> ------------------------------------------------------
> From: Feng Tang <feng.tang@intel.com>
> Subject: panic: Avoid the extra noise dmesg
>
> When kernel panic happens, it will first print the panic call stack,
> then the ending msg like:
>
> [ 35.743249] ---[ end Kernel panic - not syncing: Fatal exception
> [ 35.749975] ------------[ cut here ]------------
>
> The above message are very useful for debugging.
>
> But if system is configured to not reboot on panic, say the "panic_timeout"
> parameter equals 0, it will likely print out many noisy message like
> WARN() call stack for each and every CPU except the panic one, messages
> like below:
>
> WARNING: CPU: 1 PID: 280 at kernel/sched/core.c:1198 set_task_cpu+0x183/0x190
> Call Trace:
> <IRQ>
> try_to_wake_up
> default_wake_function
> autoremove_wake_function
> __wake_up_common
> __wake_up_common_lock
> __wake_up
> wake_up_klogd_work_func
> irq_work_run_list
> irq_work_tick
> update_process_times
> tick_sched_timer
> __hrtimer_run_queues
> hrtimer_interrupt
> smp_apic_timer_interrupt
> apic_timer_interrupt

I guess that it is a warning about migrating tasks to an offline CPU.

> For people working in console mode, the screen will first show the panic
> call stack, but immediately overridded by these noisy extra messages, which
> makes debugging much more difficult, as the original context gets lost on
> screen.
>
> Also these noisy messages will confuse some users, as I have seen many bug
> reporters posted the noisy message into bugzilla, instead of the real panic
> call stack and context.
>
> Removing the "local_irq_enable" will avoid the noisy message.
>
> The justification for the removing is: when code runs to this point, it
> means user has chosed to not reboot, or do any special handling by using
> the panic notifier method, no much point in re-enabling the interrupt.
>
> Link: http://lkml.kernel.org/r/1543902228-23834-1-git-send-email-feng.tang@intel.com
> Signed-off-by: Feng Tang <feng.tang@intel.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
>
>
> --- a/kernel/panic.c~panic-avoid-the-extra-noise-dmesg
> +++ a/kernel/panic.c
> @@ -322,7 +322,6 @@ void panic(const char *fmt, ...)
> }
> #endif
> pr_emerg("---[ end Kernel panic - not syncing: %s ]---\n", buf);
> - local_irq_enable();
> for (i = 0; ; i += PANIC_TIMER_STEP) {
> touch_softlockup_watchdog();
> if (i >= i_next) {

Hmm, this calls panic_blink(). It seems that it depends on workqueues
and the scheduler:

+ led_panic_blink()
+ led_trigger_event()
+ led_set_brightness()
+ schedule_work(set_brightness_work)

I guess that blinking might be important in some situations and
on some devices. On the other hand, we are interested only into
blinking from this point on.

The easiest solution seems to be to make a noop from printk().
For example, we could add a global flag:

int panic_blinking;

and add the following into vprintk_func()

/*
* Do not push away real panic() message by warnings from led
* blinking code.
*/
if (panic_blinking)
return 0;

How does that sound?

Best Regards,
Petr