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Date: Wed, 1 Dec 2010 15:03:33 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Wu Fengguang <fengguang.wu@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>, "Theodore Ts'o" <tytso@mit.edu>,
        Chris Mason <chris.mason@oracle.com>,
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        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 01/13] writeback: IO-less balance_dirty_pages()
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On Wed, 1 Dec 2010 21:38:18 +0800
Wu Fengguang <fengguang.wu@intel.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.