[PATCH] Give kjournald a higher io priority

[Arjan van de Ven]

With latencytop, I noticed that the (in memory) file updates during my
workload (reading mail) had latencies of 6 seconds 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 a slighlty higher priority than normal
applications, reducing these latencies significantly.

Signed-off-by: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
---
 fs/ioprio.c            |    3 ++-
 fs/jbd/journal.c       |   12 ++++++++++++
 include/linux/ioprio.h |    2 ++
 3 files changed, 16 insertions(+), 1 deletions(-)

diff --git a/fs/ioprio.c b/fs/ioprio.c
index da3cc46..3bd95dc 100644
--- a/fs/ioprio.c
+++ b/fs/ioprio.c
@@ -27,7 +27,7 @@
 #include <linux/security.h>
 #include <linux/pid_namespace.h>
 
-static int set_task_ioprio(struct task_struct *task, int ioprio)
+int set_task_ioprio(struct task_struct *task, int ioprio)
 {
 	int err;
 	struct io_context *ioc;
@@ -64,6 +64,7 @@ static int set_task_ioprio(struct task_struct *task, int ioprio)
 	task_unlock(task);
 	return err;
 }
+EXPORT_SYMBOL_GPL(set_task_ioprio);
 
 asmlinkage long sys_ioprio_set(int which, int who, int ioprio)
 {
diff --git a/fs/jbd/journal.c b/fs/jbd/journal.c
index aa7143a..a859a46 100644
--- a/fs/jbd/journal.c
+++ b/fs/jbd/journal.c
@@ -36,6 +36,7 @@
 #include <linux/poison.h>
 #include <linux/proc_fs.h>
 #include <linux/debugfs.h>
+#include <linux/ioprio.h>
 
 #include <asm/uaccess.h>
 #include <asm/page.h>
@@ -131,6 +132,17 @@ static int kjournald(void *arg)
 			journal->j_commit_interval / HZ);
 
 	/*
+	 * kjournald is the process on which most other processes depend on
+	 * for doing the filesystem portion of their IO. As such, there exists
+	 * the equivalent of a priority inversion situation, where kjournald
+	 * would get less priority as it should.
+	 *
+	 * For this reason we set to "medium real time priority", which is higher
+	 * than regular tasks, but not infinitely powerful.
+	 */
+	set_task_ioprio(current, IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, 0));
+
+	/*
 	 * And now, wait forever for commit wakeup events.
 	 */
 	spin_lock(&journal->j_state_lock);
diff --git a/include/linux/ioprio.h b/include/linux/ioprio.h
index f98a656..76dad48 100644
--- a/include/linux/ioprio.h
+++ b/include/linux/ioprio.h
@@ -86,4 +86,6 @@ static inline int task_nice_ioclass(struct task_struct *task)
  */
 extern int ioprio_best(unsigned short aprio, unsigned short bprio);
 
+extern int set_task_ioprio(struct task_struct *task, int ioprio);
+
 #endif
-- 
1.5.5.1



-- 
Arjan van de Ven 	Intel Open Source Technology Centre
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