linux-kernel - Re: [PATCH 8/8] printk: Add config option for disabling printk offloading

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Message-Id: <20140423111750.88ad799bb271a1fe0bed119f@linux-foundation.org>
Date:	Wed, 23 Apr 2014 11:17:50 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jan Kara <jack@...e.cz>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	LKML <linux-kernel@...r.kernel.org>, pmladek@...e.cz,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 8/8] printk: Add config option for disabling printk
 offloading

On Wed, 23 Apr 2014 13:08:47 +0200 Jan Kara <jack@...e.cz> wrote:

> > If you want reliable crash logging then we need to be able to set a
> > printk level mask per console and just set the serial console for
> > "crit/err" and the queue console for the rest, with a 'cat
> > >/dev/ttywhatever' running if this feature was in use ?
>   Ok, now I understand. Thanks for an interesting idea. IMO people
> definitely need messages logged directly into serial console when e.g. oops
> is happening because very likely they won't get logged to disk and even
> userspace won't have a chance to run and copy messages to the serial
> console. Plus for useful softlockup reports or oops messages you need also
> the KERN_NOTICE and KERN_INFO messages - stack traces, cpu numbers, process
> information - all this is printed with these levels.
> 
> These obvious places could be changed to print with lower log level I
> assume but still I'm somewhat worried that some KERN_INFO messages that
> would be useful for debugging a crash won't make it to console before the
> crash happens.
> 
> But if both you and Andrew think that the above problems are smaller than
> the complexity connected with printk offloading, I can give it a try.
> Andrew?

I'm curious about the idea of writing a new(?) console driver which the
problematic machines can use.

The problem of course will be in sizing the driver's queue.  Perhaps we
can have a driver which uses a huge queue, temporarily use that driver
during boot then switch over to a conventional console driver?

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