linux-kernel - Re: Excessive stall times on ext4 in 3.9-rc2

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Date:	Tue, 23 Apr 2013 11:31:07 +0200
From:	Jan Kara <jack@...e.cz>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Mel Gorman <mgorman@...e.de>, Theodore Ts'o <tytso@....edu>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
	linux-ext4@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>, Jiri Slaby <jslaby@...e.cz>
Subject: Re: Excessive stall times on ext4 in 3.9-rc2

On Mon 22-04-13 18:42:23, Jeff Moyer wrote:
> Jan, if I were to come up with a way of promoting a particular async
> queue to the front of the line, where would I put such a call in the
> ext4/jbd2 code to be effective?
  As Ted wrote the simplies might be to put his directly in
__lock_buffer(). Something like:

diff --git a/fs/buffer.c b/fs/buffer.c
index b4dcb34..e026a3e 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -69,6 +69,12 @@ static int sleep_on_buffer(void *word)
 
 void __lock_buffer(struct buffer_head *bh)
 {
+       /*
+        * Likely under async writeback? Tell io scheduler we are
+        * now waiting for the IO...
+        */
+       if (PageWriteback(bh->b_page))
+               io_now_sync(bh->b_bdev, bh->b_blocknr);
        wait_on_bit_lock(&bh->b_state, BH_Lock, sleep_on_buffer,
                                                        TASK_UNINTERRUPTIBLE);
}

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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