linux-kernel - Re: [PATCH v5] panic: Avoid the extra noise dmesg

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Message-Id: <20190228120014.19c0ab9227809341c1d5acc6@linux-foundation.org>
Date:   Thu, 28 Feb 2019 12:00:14 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Feng Tang <feng.tang@...el.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Petr Mladek <pmladek@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kees Cook <keescook@...omium.org>,
        Borislav Petkov <bp@...e.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH v5] panic: Avoid the extra noise dmesg

On Fri, 22 Feb 2019 14:09:59 +0800 Feng Tang <feng.tang@...el.com> wrote:

> 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

It's a fairly ugly-looking patch but I am inclined to agree.

The panicing CPU is spinning and blinking a LED and all CPUs have
interrupts enabled and the system is known to be in a messed up state. 
All sorts of kernel code could emit all sorts of output in such
circumstances.  So a global printk-killing knob seems appropriate.

Thoughts:

- why do the suppression in vprintk_emit()?  Doing it right at entry
  to printk() seems cleaner, more explicit?

- Other code sites may wish to suppress all printks.  Perhaps
  `panic_suppress_printk' should just be called `suppress_printk'?