linux-kernel - Re: [PATCH 3/5] writeback: dirty rate control

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Date:	Tue, 09 Aug 2011 19:02:02 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Andrea Righi <arighi@...eler.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] writeback: dirty rate control

On Sat, 2011-08-06 at 16:44 +0800, Wu Fengguang wrote:

> +       pos_bw = bw * pos_ratio >> BANDWIDTH_CALC_SHIFT;
> +       pos_bw++;  /* this avoids bdi->dirty_ratelimit get stuck in 0 */
> +

> +       pos_ratio *= bdi->avg_write_bandwidth;
> +       do_div(pos_ratio, dirty_bw | 1);
> +       ref_bw = bw * pos_ratio >> BANDWIDTH_CALC_SHIFT; 

when written out that results in:

           bw * pos_ratio * bdi->avg_write_bandwidth
  ref_bw = -----------------------------------------
                         dirty_bw

which would suggest you write it like:

  ref_bw = div_u64((u64)pos_bw * bdi->avg_write_bandwidth, dirty_bw | 1);

since pos_bw is already bw * pos_ratio per the above.

Or am I missing something?
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