linux-kernel - Re: [PATCH 3/7] ring-buffer: make moving the tail page a separate function

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Message-ID: <alpine.DEB.2.00.0905070924250.32734@gandalf.stny.rr.com>
Date:	Thu, 7 May 2009 09:26:27 -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>,
	Li Zefan <lizf@...fujitsu.com>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 3/7] ring-buffer: make moving the tail page a separate
 function


On Thu, 7 May 2009, Ingo Molnar wrote:

> 
> * Steven Rostedt <rostedt@...dmis.org> wrote:
> >  
> > +static struct ring_buffer_event *
> > +__rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
> > +		  unsigned type, unsigned long length, u64 *ts)
> > +{
> > +	struct buffer_page *tail_page, *commit_page;
> > +	struct ring_buffer_event *event;
> > +	unsigned long tail, write;
> > +
> > +	commit_page = cpu_buffer->commit_page;
> > +	/* we just need to protect against interrupts */
> > +	barrier();
> > +	tail_page = cpu_buffer->tail_page;
> > +	write = local_add_return(length, &tail_page->write);
> > +	tail = write - length;
> > +
> > +	/* See if we shot pass the end of this buffer page */
> > +	if (write > BUF_PAGE_SIZE)
> > +		return rb_move_tail(cpu_buffer, length, tail,
> > +				    commit_page, tail_page, ts);
> 
> Nice! The __rb_reserve_next() fast-path logic became a lot clearer.

Thanks!

> 
> The above branch might be unlikely(), right? With usual record sizes 
> of around 40 bytes, we'll have a 100 records for every page 
> overflow. That's i think within the reach of unlikely().
> 
> Depends on how much of a mess GCC makes of it though.

I looked at the assembly that gcc generates, and it is fine. gcc inlines 
the function and puts it at the end, thus it already treats it as an 
unlikely. I would like to avoid adding unlikely annotations when possible.

-- Steve

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