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

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Message-Id: <20101201150333.fa4b8955.akpm@linux-foundation.org>
Date:	Wed, 1 Dec 2010 15:03:33 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
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 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.  Plus
we *do* want these processes to appear in D state and to contribute to
load average.

So it should be TASK_UNINTERRUPTIBLE.
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