Re: [PATCH 3/6] writeback: avoid unnecessary calculation of bdi dirty thresholds

[Peter Zijlstra <peterz@infradead.org>]

On Sun, 2010-07-11 at 10:06 +0800, Wu Fengguang wrote:
> plain text document attachment (writeback-less-bdi-calc.patch)
> Split get_dirty_limits() into global_dirty_limits()+bdi_dirty_limit(),
> so that the latter can be avoided when under global dirty background
> threshold (which is the normal state for most systems).

The patch looks OK, although esp with the proposed comments in the
follow up email, bdi_dirty_limit() gets a bit confusing wrt to how and
what the limit is.

Maybe its clearer to not call task_dirty_limit() from bdi_dirty_limit(),
that way the comment can focus on the device write request completion
proportion thing.

> +unsigned long bdi_dirty_limit(struct backing_dev_info *bdi,
> +			       unsigned long dirty)
> +{
> +	u64 bdi_dirty;
> +	long numerator, denominator;
>  
> +	/*
> +	 * Calculate this BDI's share of the dirty ratio.
> +	 */
> +	bdi_writeout_fraction(bdi, &numerator, &denominator);
>  
> +	bdi_dirty = (dirty * (100 - bdi_min_ratio)) / 100;
> +	bdi_dirty *= numerator;
> +	do_div(bdi_dirty, denominator);
>  
> +	bdi_dirty += (dirty * bdi->min_ratio) / 100;
> +	if (bdi_dirty > (dirty * bdi->max_ratio) / 100)
> +		bdi_dirty = dirty * bdi->max_ratio / 100;
> +
  +       return bdi_dirty;
>  }

And then add the call to task_dirty_limit() here:

> +++ linux-next/mm/backing-dev.c	2010-07-11 08:53:44.000000000 +0800
> @@ -83,7 +83,8 @@ static int bdi_debug_stats_show(struct s
>  		nr_more_io++;
>  	spin_unlock(&inode_lock);
>  
> -	get_dirty_limits(&background_thresh, &dirty_thresh, &bdi_thresh, bdi);
> +	global_dirty_limits(&background_thresh, &dirty_thresh);
> +	bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
  +       bdi_thresh = task_dirty_limit(current, bdi_thresh);

And add a comment to task_dirty_limit() as well, explaining its reason
for existence (protecting light/slow dirtying tasks from heavier/fast
ones).