linux-kernel - Re: Printk mulitple line message support

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Message-ID: <1321318714.13860.54.camel@yhuang-dev>
Date:	Tue, 15 Nov 2011 08:58:34 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	Bruno Prémont <bonbons@...ux-vserver.org>
Cc:	Joe Perches <joe@...ches.com>,
	William Douglas <william.r.douglas@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Printk mulitple line message support

On Tue, 2011-11-15 at 02:20 +0800, Bruno Prémont wrote:
> On Mon, 14 November 2011 Joe Perches <joe@...ches.com> wrote:
> 
> > On Mon, 2011-11-14 at 14:58 +0800, Huang Ying wrote:
> > > Hi,
> > > 
> > > In most cases, printk only guarantees messages from different printk
> > > calling will not be interleaved between each other.  But many printk
> > > users uses multiple line to form a complete message and call printk
> > > for each line.  So the following situation is possible for two printk
> > > users running on two CPUs.
> > > 
> > > line 1 of message from printk user1
> > > line 1 of message from printk user2
> > > line 2 of message from printk user1
> > > line 2 of message from printk user2
> > > 
> > > This makes kernel log hard to read.  One possible solution to this
> > > issue is to give a sequence number (or ID) to each complete message.
> > > So the above lines will be:
> > > 
> > > {1}line 1 of message from printk user1
> > > {2}line 1 of message from printk user2
> > > {1}line 2 of message from printk user1
> > > {2}line 2 of message from printk user2
> > > 
> > > Then some simple script can be used to group lines together according
> > > to sequence number in lines.
> > > 
> > > What do you think about that?
> > 
> > This makes the typical multi-part but non-interleaved
> > output difficult to read.
> > 
> > How about determining if there is interleaving and
> > emitting sequence # only in those cases?
> > 
> > Perhaps test the atomic for the last sequence #.
> 
> Wouldn't another option be to let printk() handle '\n' in messages
> so the multi-line messages could be done with a single call to printk.
> 
> Those messages could contain optional severity information after the
> linefeed (if none given, the one of previous line would be repeated
> internally by printk).
> 
> This way consumers of printk (and all variations of it like dev_err)
> would all benefit without need to redefine them for multi-line use.

Yes.  That is another solution.  But sometimes, it is just not so
convenient to do that.  The printk users may have to allocate a big
buffer to hold all these lines.  The solution here is for those users.

Best Regards,
Huang Ying


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