linux-kernel - Re: [PATCH 2/8] tracing: create automated trace defines

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Date:	Thu, 16 Apr 2009 20:13:17 -0400
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Theodore Tso <tytso@....edu>,
	Arjan van de Ven <arjan@...radead.org>,
	Christoph Hellwig <hch@....de>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Zhaolei <zhaolei@...fujitsu.com>, Li Zefan <lizf@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	"Frank Ch. Eigler" <fche@...stic.org>,
	Tom Zanussi <tzanussi@...il.com>,
	Jiaying Zhang <jiayingz@...gle.com>,
	Michael Rubin <mrubin@...gle.com>,
	Martin Bligh <mbligh@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Neil Horman <nhorman@...driver.com>,
	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH 2/8] tracing: create automated trace defines

* Jeremy Fitzhardinge (jeremy@...p.org) wrote:
> Mathieu Desnoyers wrote:
>> The other point I dislike about the out-of-line approach is that the
>> tracer will suffer from a pointless supplementary function call at each
>> event. Given how slow function calls are, at least on x86, I'd prefer
>> leaving the handler call chain inline, unless there is a very strong
>> reason not to do so.
>>   
>
> Are they?  They're generally considered to be "free", because the call  
> and return are predicted 100% accurately.
>

Adding a simple function call within the tracer fast path, in LTTng, has
a very measurable performance impact on the tbench workload. This is why
I don't use any function call-based trace clocks in LTTng, but rather my
own inline trace clock.

Mathieu

>> I'll come up with a patch that leaves the tracepoint inline, but fixes
>> the header dependency.
>>   
> OK.
>
>    J
>




-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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