linux-kernel - Re: [ANNOUNCE] New utility: 'trace'

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Date:	Wed, 17 Nov 2010 08:49:03 -0800
From:	Darren Hart <dvhart@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Tom Zanussi <tzanussi@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Jason Baron <jbaron@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Christoph Hellwig <hch@....de>,
	Pekka Enberg <penberg@...nel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [ANNOUNCE] New utility: 'trace'

On 11/17/2010 05:02 AM, Peter Zijlstra wrote:
> On Wed, 2010-11-17 at 13:53 +0100, Frederic Weisbecker wrote:
>> On Wed, Nov 17, 2010 at 12:35:50PM +0100, Peter Zijlstra wrote:
>>> On Wed, 2010-11-17 at 09:30 +0100, Ingo Molnar wrote:
>>>>> For example I'm currently working with dozens of trace_printk() and I would be
>>>>> very happy to turn some of them off half of the time.
>>>>
>>>> I guess we could try such a patch. If you send a prototype i'd be interested in
>>>> testing it out.
>>>
>>> I don't see the point, the kernel shouldn't contain any trace_printk()s
>>> to begin with..
>>
>>
>> It's oriented toward developers. Those who use dozens of tracepoints in
>> their tree because they are debugging something or developing a new feature,
>> they might to deactivate/reactivate some of these independant points.
>>
>> This can also apply to dynamic_printk of course.
>>
>> Well, the very first and main point is to standardize trace_printk into
>> a trace event so that it gets usable by perf tools. I have been asked many
>> times "how to use trace_printk() with perf?".
>
> Thing is, since its these dev who add the trace_printk()s to begin with,
> I don't see the point in splitting them out, if you didn't want them why
> did you add them to begin with?!

What I understood from Frederic's email was that during a debug session 
it is sometimes helpful to be able to enable and disable the 
trace_printk's. This makes sense as it reduces the number of kernel 
build/reboot cycles. However, I would think most of that could be 
accomplished with some judicious message tagging and post-processing to 
filter out the unwanted trace_printk's. The only exception might be when 
the trace_printk's add enough overhead to mask a timing related bug. In 
this case, I'd probably be tempted to remove the stubs anyway.

-- 
Darren Hart
Yocto Linux Kernel
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