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SubjectRe: [PATCH 2/5] writeback: dirty position control
On Mon, Aug 08, 2011 at 09:46:33PM +0800, Peter Zijlstra wrote:
> On Sat, 2011-08-06 at 16:44 +0800, Wu Fengguang wrote:
> > +static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
> > + unsigned long thresh,
> > + unsigned long dirty,
> > + unsigned long bdi_thresh,
> > + unsigned long bdi_dirty)
> > +{
> > + unsigned long limit = hard_dirty_limit(thresh);
> > + unsigned long origin;
> > + unsigned long goal;
> > + unsigned long long span;
> > + unsigned long long pos_ratio; /* for scaling up/down the rate limit */
> > +
> > + if (unlikely(dirty >= limit))
> > + return 0;
> > +
> > + /*
> > + * global setpoint
> > + */
> > + goal = thresh - thresh / DIRTY_SCOPE;
> > + origin = 4 * thresh;
> > +
> > + if (unlikely(origin < limit && dirty > (goal + origin) / 2)) {
> > + origin = limit; /* auxiliary control line */
> > + goal = (goal + origin) / 2;
> > + pos_ratio >>= 1;
>
> use before init?

Yeah it's embarrassing, I find this bug all the way back to the initial version...

It's actually dead code because (origin < limit) should never happen.
I feel so good being able to drop 5 more lines of code :)

Thanks,
Fengguang
---
--- linux-next.orig/mm/page-writeback.c 2011-08-08 21:56:11.000000000 +0800
+++ linux-next/mm/page-writeback.c 2011-08-08 22:04:48.000000000 +0800
@@ -538,11 +538,6 @@ static unsigned long bdi_position_ratio(
goal = thresh - thresh / DIRTY_SCOPE;
origin = 4 * thresh;

- if (unlikely(origin < limit && dirty > (goal + origin) / 2)) {
- origin = limit; /* auxiliary control line */
- goal = (goal + origin) / 2;
- pos_ratio >>= 1;
- }
pos_ratio = origin - dirty;
pos_ratio <<= BANDWIDTH_CALC_SHIFT;
do_div(pos_ratio, origin - goal + 1);

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