linux-kernel - Re: [PATCH 00/11] [GIT PULL] more updates for the tag format

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Message-ID: <20090610160303.GA10240@mit.edu>
Date:	Wed, 10 Jun 2009 12:03:03 -0400
From:	Theodore Tso <tytso@....edu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frédéric Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Mel Gorman <mel@....ul.ie>,
	Christoph Hellwig <hch@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Peter Zijlstra <peterz@...radead.org>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Zhaolei <zhaolei@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Jason Baron <jbaron@...hat.com>,
	Jiaying Zhang <jiayingz@...gle.com>,
	Tom Zanussi <tzanussi@...il.com>,
	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Subject: Re: [PATCH 00/11] [GIT PULL] more updates for the tag format

On Wed, Jun 10, 2009 at 09:49:29AM -0400, Steven Rostedt wrote:
> > 
> > Maybe I'm missing something, but looks like the this new format, while
> > simpler and easier to read, doesn't have support for using a more
> > complicated C expression as a printk argument.  For example:
> > 
> > 	TP_printk("dev %s ino %lu mode %d uid %u gid %u blocks %llu",
> > 		  jbd2_dev_to_name(__entry->dev), __entry->ino, __entry->mode,
> > 		  __entry->uid, __entry->gid, __entry->blocks)
> > 
> > How should I handle the "jbd2_dev_to_name(__entry->dev)" argument to
> > TP_printk?  The whole point of calling jbd2_dev_to_name() at TP_printk
> > time is to not bloat the ring buffer with a 32 byte devname.
> 
> Understood, and the example you just gave also has the flaw that a 
> userspace tool could not parse it, because it would not know what to do 
> with "jbd2_dev_to_name()".
> 
> This is why I suggested keeping the TP_printk, for cases like this. Since 
> it is also currently useless in userspace.
> 
> But we really should convert all cases, and I was toying with an idea to 
> dynamically make your own data type, and be able to make a way to print 
> it.

Yes, another approach for handling this case would be to take my
"jbd2_dev_to_name" function and support it as a first-class tagged
type; after all, I'm sure ext4 won't be the only place that would like
to take a dev_t and print the device name.  So this could certainly be
fixed by adding some kind of "<dev:xxx>" sort of tagged name.

But I think it would be good to keep TP_printk because otherwise I'll
have to scramble and change my marker->tracepoint patches during the
merge window, which would invalidate all of the testing to date.

I agree that the new tagged format is superior, but I'm wondering
whether it really makes sense to try to scramble and try to switch my
ext4/jbd2 users in the 36 hours or so before Linus opens the merge
window....

						- Ted
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