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

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Message-ID: <20090421212347.GC402@elastic.org>
Date:	Tue, 21 Apr 2009 17:23:47 -0400
From:	"Frank Ch. Eigler" <fche@...stic.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Mathieu Desnoyers <compudj@...stal.dyndns.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>,
	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@...stfloor.org
Subject: Re: [PATCH 2/8] tracing: create automated trace defines

Hi -

On Tue, Apr 21, 2009 at 05:17:17PM -0400, Steven Rostedt wrote:

> [...]  Perhaps we should fork off gcc and ship Linux with its own
> compiler. This way we can optimize it for the kernel and not worry
> about any userland optimizations.

In this regard, kernel land does not seem that unlike user land.

> 	if (unlikely(err)) {
> 		__section__(".error_sect") {
> 			/* put error code here */
> 		}
> 	}
> 
> And have gcc in the error section (if it is big enough perhaps) do:
> 	jmp .L123
> .L124   [...]
> [...]
> 	jmp .L124

> We could do the same for trace points. That is, any part of code that 
> really would happen once in a while (error handling for one) we can move 
> off to its own section and keep hot paths hot.

This is called -freorder-blocks or -freorder-blocks-and-partition
(depending on how far you would like gcc to move unlikely blocks).

- FChE
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