linux-kernel - Re: [PATCH 10/11] ring-buffer: move big if statement down

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Date:	Wed, 6 May 2009 09:47:34 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 10/11] ring-buffer: move big if statement down


On Wed, 6 May 2009, Ingo Molnar wrote:

> 
> * Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > This patch changes it to a goto:
> > 
> > 	code;
> > 
> > 	if (cross to next page)
> > 		goto next_page;
> > 
> > 	more code;
> > 
> > 	return;
> > 
> > next_page:
> > 
> > 	[ lots of code]
> 
> I have pulled it, but could you please change it to a helper 
> function instead? There's almost never a good reason to combine 
> 'more code' with 'lots of code' in a single function. It also 
> documents the unlikeliness, etc.

As I stated in the change log, I did not want to convert it to a helper 
function because it uses the variables created before. It would end up 
going from:

	if (write > BUF_PAGE_SIZE)
		goto next_page;


to:

	if (write > BUF_PAGE_SIZE)
		return rb_move_tail(cpu_buffer, length, tail,
				    commit_page, tail_page);


Although it is the "unlikely" case, it is still a fast path. It happens 
every time a write into the page buffer crosses a page boundary. Since it 
is only used once, gcc would hopefully inline it. If it does not, then we 
are copying a bunch of parameters for nothing.

I could still do this and see what gcc does with it.

-- Steve

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