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Subject: Re: [PATCH 11/18] block: add bdi flag to indicate risk of io queue
 underrun
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Wed, 07 Sep 2011 09:31:09 +0200
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To: Wu Fengguang <fengguang.wu@intel.com>
Cc: "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>, Tejun Heo <tj@kernel.org>, Jens Axboe <axboe@kernel.dk>, "Li, Shaohua" <shaohua.li@intel.com>, Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>, Christoph Hellwig <hch@lst.de>, Dave Chinner <david@fromorbit.com>, Greg Thelen <gthelen@google.com>, Minchan Kim <minchan.kim@gmail.com>, Vivek Goyal <vgoyal@redhat.com>, Andrea Righi <arighi@develer.com>, linux-mm <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>

On Wed, 2011-09-07 at 10:37 +0800, Wu Fengguang wrote:
> On Tue, Sep 06, 2011 at 10:22:48PM +0800, Peter Zijlstra wrote:
> > On Sun, 2011-09-04 at 09:53 +0800, Wu Fengguang wrote:
> > > +++ linux-next/mm/page-writeback.c      2011-08-31 14:40:58.000000000=
 +0800
> > > @@ -1067,6 +1067,9 @@ static void balance_dirty_pages(struct a
> > >                                      nr_dirty, bdi_thresh, bdi_dirty,
> > >                                      start_time);
> > > =20
> > > +               if (unlikely(!dirty_exceeded && bdi_async_underrun(bd=
i)))
> > > +                       break;
> > > +
> > >                 dirty_ratelimit =3D bdi->dirty_ratelimit;
> > >                 pos_ratio =3D bdi_position_ratio(bdi, dirty_thresh,
> > >                                                background_thresh, nr_=
dirty,
> >=20
> > So dirty_exceeded looks like:
> >=20
> >=20
> > 1109                 dirty_exceeded =3D (bdi_dirty > bdi_thresh) ||
> > 1110                                   (nr_dirty > dirty_thresh);
> >=20
> > Would it make sense to write it as:
> >=20
> > 	if (nr_dirty > dirty_thresh ||=20
> > 	    (nr_dirty > freerun && bdi_dirty > bdi_thresh))
> > 		dirty_exceeded =3D 1;
> >=20
> > So that we don't actually throttle bdi thingies when we're still in the
> > freerun area?
>=20
> Sounds not necessary -- (nr_dirty > freerun) is implicitly true
> because there is a big break early in the loop:
>=20
>         if (nr_dirty > freerun)
>                 break;

Ah, totally didn't see that. Thanks!

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