On Tue, Aug 03, 2010 at 11:03:42PM +0800, Peter Zijlstra wrote:
> 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).
Good suggestions, that would be much less confusing. Will post updated
patches tomorrow.
Thanks,
Fengguang
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