linux-kernel - Re: [PATCH v6] panic: Avoid the extra noise dmesg

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Date:   Fri, 1 Mar 2019 11:06:14 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, 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 v6] panic: Avoid the extra noise dmesg

On Fri 2019-03-01 16:49:46, Feng Tang 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
> 
> For people working in console mode, the screen will first show the panic
> call stack, but immediately overridden 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.
> 
> Adding a flag "suppress_printk" which gets set in panic() to avoid those
> noisy messages, without changing current kernel behavior that both panic
> blinking and sysrq magic key can work as is, suggested by Petr Mladek.
> 
> To verify this, make sure kernel is not configured to reboot on panic and
> in console
>  # echo c > /proc/sysrq-trigger
> to see if console only prints out the panic call stack.
> 
> Suggested-by: Petr Mladek <pmladek@...e.com>
> Signed-off-by: Feng Tang <feng.tang@...el.com>

The patch looks fine to me:

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr