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

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Message-ID: <alpine.DEB.2.00.0906101214400.30552@gandalf.stny.rr.com>
Date:	Wed, 10 Jun 2009 12:17:35 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Theodore Tso <tytso@....edu>
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, 10 Jun 2009, Theodore Tso wrote:

> 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.

Yep that could be done as long as we know the mapping will never change.
Userspace needs know what those numbers mean.

> 
> 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.

Understood, I made it that both TP_printk and TP_FORMAT can exist
together, but Christoph Hellwig doesn't like that idea. I'm thinking
for quick debug sessions, TP_printk() be used. In fact, if we go to
TP_FORMAT, I'll just make TP_printk no longer show up in the user format.

Then TP_printk() can be used for quick hacks, but if you want something 
merged, it would need to be added to the tag format.

> 
> 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....

Relax, we already decided this is .32 material ;-)

-- Steve

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