linux-kernel - Re: [PATCH 2/2] panic: Enable to print out all printk msg in buffer

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Date:   Wed, 10 Apr 2019 10:02:24 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Feng Tang <feng.tang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH 2/2] panic: Enable to print out all printk msg in buffer

On Wed 2019-04-10 10:59:26, Sergey Senozhatsky wrote:
> On (04/09/19 16:14), Petr Mladek wrote:
> > We should:
> > 
> >    + Flush the latest messages before we replay the log.
> 
> Do you mean the pending messages? When we replay the log we also should
> print "header line" and panic-cpu backtrace. So we will print panic-cpu
> oops twice

console_flush_on_panic() is just the last resort. I believe
that the panic header and backtrace reach the console even
without it in most cases. Explicit flush before reply
would just make it consistent.

> 	// from panic-cpu flush_on_panic
> 	[header]
> 	backtrace
> 	[end of header]
> 
>         // from console_replay
> 	then all logbuf messages
> 	and then the same header/backtrace one more time
> 
> 	[header]
> 	backtrace]
> 	[end of backtrace]
> 
> Is there any particular reason to flush pending messages before
> we play the buffer? Replaying the logbuf can take some time, so
> my guess would be that you want to print panic-cpu backtrace as
> soon as possible. Is there something else?

The panic() message is usually the most important one for debugging.
I feel a bit uneasy that we would delay it until full replay that
might get killed from several reasons:

   + external monitoring system would force reboot

   + user might realize, e.g. after 20 minutes, that the full
     log reply was probably not worth it.

I understand that people enabling this option would most likely
wait but still. I do not see it as a big deal to repeat
the messages.

Best Regards,
Petr