linux-kernel - Re: Printk mulitple line message support

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Date:	Mon, 14 Nov 2011 17:18:03 -0800
From:	Joe Perches <joe@...ches.com>
To:	Huang Ying <ying.huang@...el.com>
Cc:	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 08:50 +0800, Huang Ying wrote:
> On Mon, 2011-11-14 at 22:40 +0800, Joe Perches 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.
> 
> With a simple script, we can strip out the sequence # easily.
> 
> > How about determining if there is interleaving and
> > emitting sequence # only in those cases?
> > 
> > Perhaps test the atomic for the last sequence #.
> 
> So we will have no sequence # prefix for printk user1's lines if printk
> user 2 comes in the middle?  Something as follow?
> 
> line 1 of message from printk user1
> {2}line 1 of message from printk user2
> line 2 of message from printk user1
> {2}line 2 of message from printk user2
> 
> This will make it hard for a script to sort the lines.  Where should it
> insert lines from printk user2 in the sort result?

90+% it's the previous line.

I believe you are not solving any real problem
with pr_<level>_ml.

Most all interleaved complete line uses have some
pr_fmt prefix that distinguishes between the sources.

A perhaps larger problem is interleaved partial
lines with pr_cont.

I believe that an initiator/terminator is necessary
for reassembly.  Something that could be used
with pr_<level>, dev_<level>, netdev_<level>, et al.

mp_start(&cookie)
pr_<level>(fmt, ...);
pr_mp_cont(&cookie, fmt, ...);
pr_mp_cont(&cookie, fmt "\n", ...);
mp_end(&cookie);


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