[patch] Give kjournald a IOPRIO_CLASS_RT io priority

[Arjan van de Ven <arjan@infradead.org>]

Subject: Give kjournald a IOPRIO_CLASS_RT io priority
From: Arjan van de Ven <arjan@linux.intel.com>

With latencytop, I noticed that the (in memory) atime updates during a
kernel build had latencies of 600 msec or longer; this is obviously not so
nice behavior. Other EXT3 journal related operations had similar or even
longer latencies.

Digging into this a bit more, it appears to be an interaction between EXT3
and CFQ in that CFQ tries to be fair to everyone, including kjournald.
However, in reality, kjournald is "special" in that it does a lot of journal
work and effectively this leads to a twisted kind of "mass priority
inversion" type of behavior.

The good news is that CFQ already has the infrastructure to make certain
processes special... JBD just wasn't using that quite yet.

The patch below makes kjournald of the IOPRIO_CLASS_RT priority to break
this priority inversion behavior. With this patch, the latencies for atime
updates (and similar operation) go down by a factor of 3x to 4x !


Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>


diff -purN linux-2.6.23-rc9.org/fs/jbd/journal.c linux-2.6.23-rc9.lt/fs/jbd/journal.c
--- linux-2.6.23-rc9.org/fs/jbd/journal.c	2007-10-02 05:24:52.000000000 +0200
+++ linux-2.6.23-rc9.lt/fs/jbd/journal.c	2007-10-14 00:06:55.000000000 +0200
@@ -35,6 +35,7 @@
 #include <linux/kthread.h>
 #include <linux/poison.h>
 #include <linux/proc_fs.h>
+#include <linux/ioprio.h>
 
 #include <asm/uaccess.h>
 #include <asm/page.h>
@@ -131,6 +132,8 @@ static int kjournald(void *arg)
 	printk(KERN_INFO "kjournald starting.  Commit interval %ld seconds\n",
 			journal->j_commit_interval / HZ);
 
+	current->ioprio =  (IOPRIO_CLASS_RT << IOPRIO_CLASS_SHIFT) | 4;
+
 	/*
 	 * And now, wait forever for commit wakeup events.
 	 */