Date:	Tue, 15 Nov 2011 09:52:37 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	Joe Perches <joe@...ches.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 09:18 +0800, Joe Perches wrote:
> 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.

Except dev_<level> and netdev_<level>, it appears that many general
pr_<level> do not have the prefix.

> A perhaps larger problem is interleaved partial
> lines with pr_cont.

In an interleaved situation, how to determine <level> for pr_cont?
Maybe we should avoid use it if we care about interleaving?

> 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, ...);

Do not need cookie here?

> pr_mp_cont(&cookie, fmt, ...);
> pr_mp_cont(&cookie, fmt "\n", ...);
> mp_end(&cookie);

With initiator/terminator the kernel log will be more structural, but
that will make kernel log a little harder to read too.

Best Regards,
Huang Ying


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