linux-kernel - Re: [PATCH 01/13] writeback: IO-less balance_dirty

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Date:	Thu, 2 Dec 2010 09:56:46 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Theodore Ts'o <tytso@....edu>,
	Chris Mason <chris.mason@...cle.com>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
	Jens Axboe <axboe@...nel.dk>, Mel Gorman <mel@....ul.ie>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Hellwig <hch@....de>, linux-mm <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/13] writeback: IO-less balance_dirty_pages()

On Thu, Dec 02, 2010 at 07:03:33AM +0800, Andrew Morton wrote:
> On Wed, 1 Dec 2010 21:38:18 +0800
> Wu Fengguang <fengguang.wu@...el.com> wrote:
> 
> > It shows that
> > 
> > 1) io_schedule_timeout(200ms) always return immediately for iostat,
> >    forming a busy loop.  How can this happen? When iostat received
> >    some signal? Then we may have to break out of the loop on catching
> >    signals. Note that I already have
> >                 if (fatal_signal_pending(current))
> >                         break;
> >    in the balance_dirty_pages() loop. Obviously that's not enough.
> 
> Presumably the calling task has singal_pending().
> 
> Using TASK_INTERRUPTIBLE in balance_dirty_pages() seems wrong.  If it's
> going to do that then it must break out if signal_pending(), otherwise
> it's pretty much guaranteed to degenerate into a busywait loop.

Right. It seems not rewarding enough to check signal_pending().  We've
already been able to response to signals much faster than before
(which takes more time to block in get_request_wait()).

> Plus we *do* want these processes to appear in D state and to
> contribute to load average.
> 
> So it should be TASK_UNINTERRUPTIBLE.

Fair enough. I do missed the D state (without the long wait :).
Here is the patch.

Thanks,
Fengguang
---
Subject: writeback: do uninterruptible sleep in balance_dirty_pages()
Date: Thu Dec 02 09:31:19 CST 2010

Using TASK_INTERRUPTIBLE in balance_dirty_pages() seems wrong.  If it's
going to do that then it must break out if signal_pending(), otherwise
it's pretty much guaranteed to degenerate into a busywait loop.  Plus
we *do* want these processes to appear in D state and to contribute to
load average.

So it should be TASK_UNINTERRUPTIBLE.                 -- Andrew Morton

Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
---
 mm/page-writeback.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- linux-next.orig/mm/page-writeback.c	2010-12-02 09:30:29.000000000 +0800
+++ linux-next/mm/page-writeback.c	2010-12-02 09:30:34.000000000 +0800
@@ -636,7 +636,7 @@ pause:
 					  pages_dirtied,
 					  pause);
 		bdi_update_write_bandwidth(bdi, &bw_time, &bw_written);
-		__set_current_state(TASK_INTERRUPTIBLE);
+		__set_current_state(TASK_UNINTERRUPTIBLE);
 		io_schedule_timeout(pause);
 		bdi_update_write_bandwidth(bdi, &bw_time, &bw_written);
 
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