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Date:   Fri, 1 Mar 2019 09:54:01 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <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 v5] panic: Avoid the extra noise dmesg

On Fri 2019-03-01 12:11:31, Feng Tang wrote:
> Hi Andrew,
> 
> Thanks for the review!
> 
> On Thu, Feb 28, 2019 at 12:00:14PM -0800, Andrew Morton wrote:
> > 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.
> 
> Yes, it's ugly :) we've changed 3 methods to tackle this.
> 
> > 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?
> 
> Yes, I put it in printk() in one earlier post, and Petr suggested to
> put it into vprintk_emit so that it works for all printk() interfaces,
> like the devkmsg_write -> printk_emit -> vprintk_emit path.

Yes, there are more printk interfaces. The check in vprintk_emit()
allows to calm down also prink_deferred() and dev_printk().

> > - Other code sites may wish to suppress all printks.  Perhaps
> >   `panic_suppress_printk' should just be called `suppress_printk'?
> Ok, then I'll move the definition from panic.c to printk code.

This change looks fine to me.

Best Regards,
Petr

PS: I am sorry for the late review. I have spent many days with
reviewing a proposal of rewrite of printk() internals.

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