linux-kernel - Re: [PATCH 5/8] readahead: add /debug/readahead/stats

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Date:	Mon, 28 Nov 2011 20:49:50 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Linux Memory Management List <linux-mm@...ck.org>,
	linux-fsdevel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Jens Axboe <jens.axboe@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH 5/8] readahead: add /debug/readahead/stats

On Tue, 29 Nov 2011 11:23:23 +0800 Wu Fengguang <fengguang.wu@...el.com> wrote:

> > > +{
> > > +#ifdef CONFIG_READAHEAD_STATS
> > > +	if (readahead_stats_enable) {
> > > +		readahead_stats(mapping, offset, req_size, ra_flags,
> > > +				start, size, async_size, actual);
> > > +		readahead_stats(mapping, offset, req_size,
> > > +				RA_PATTERN_ALL << READAHEAD_PATTERN_SHIFT,
> > > +				start, size, async_size, actual);
> > > +	}
> > > +#endif
> > > +}
> > 
> > The stub should be inlined, methinks.  The overhead of evaluating and
> > preparing eight arguments is significant.  I don't think the compiler
> > is yet smart enough to save us.
> 
> The parameter list actually becomes even out of control when doing the
> bit fields:
> 
> +       readahead_event(mapping, offset, req_size,
> +                       ra->pattern, ra->for_mmap, ra->for_metadata,
> +                       ra->start + ra->size >= eof,
> +                       ra->start, ra->size, ra->async_size, actual);
> 
> So I end up passing file_ra_state around. The added cost is, I'll have
> to dynamically create a file_ra_state for the fadvise case, which
> should be acceptable since it's a cold path.

That will reduce the cost of something which would have zero cost by
making this function a static inline when CONFIG_READAHEAD_STATS=n.

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