linux-kernel - Re: [PATCH 2/5] writeback: dirty position control

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Date:	Fri, 12 Aug 2011 11:45:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	"linux-fsdevel@...r.kernel.org" <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 2/5] writeback: dirty position control

On Fri, 2011-08-12 at 13:45 +0800, Wu Fengguang wrote:
> Code is
> 
>         unsigned long freerun = (thresh + bg_thresh) / 2;
> 
>         setpoint = (limit + freerun) / 2;
>         pos_ratio = abs(dirty - setpoint);
>         pos_ratio <<= BANDWIDTH_CALC_SHIFT;
>         do_div(pos_ratio, limit - setpoint + 1);

Why do you use do_div()? from the code those things are unsigned long,
and you can divide that just fine.

Also, there's div64_s64 that can do signed divides for s64 types.
That'll loose the extra conditionals you used for abs and putting the
sign back.

>         x = pos_ratio;
>         pos_ratio = pos_ratio * x >> BANDWIDTH_CALC_SHIFT;
>         pos_ratio = pos_ratio * x >> BANDWIDTH_CALC_SHIFT;

So on 32bit with unsigned long that gets 32=2*(10+b) bits for x, that
solves to 6, which isn't going to be enough I figure since
(dirty-setpoint) !< 64.

So you really need to use u64/s64 types here, unsigned long just won't
do, with u64 you have 64=2(10+b) 22 bits for x, which should fit.


>         if (dirty > setpoint)
>                 pos_ratio = -pos_ratio;
>         pos_ratio += 1 << BANDWIDTH_CALC_SHIFT; 


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