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Subject[084/200] writeback: disable periodic old data writeback for !dirty_writeback_centisecs
2.6.34-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jens Axboe <jens.axboe@oracle.com>

commit 69b62d01ec44fe0d505d89917392347732135a4d upstream.

Prior to 2.6.32, setting /proc/sys/vm/dirty_writeback_centisecs disabled
periodic dirty writeback from kupdate. This got broken and now causes
excessive sys CPU usage if set to zero, as we'll keep beating on
schedule().

Reported-by: Justin Maggard <jmaggard10@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
fs/fs-writeback.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)

--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -852,6 +852,12 @@ static long wb_check_old_data_flush(stru
unsigned long expired;
long nr_pages;

+ /*
+ * When set to zero, disable periodic writeback
+ */
+ if (!dirty_writeback_interval)
+ return 0;
+
expired = wb->last_old_flush +
msecs_to_jiffies(dirty_writeback_interval * 10);
if (time_before(jiffies, expired))
@@ -947,8 +953,12 @@ int bdi_writeback_task(struct bdi_writeb
break;
}

- wait_jiffies = msecs_to_jiffies(dirty_writeback_interval * 10);
- schedule_timeout_interruptible(wait_jiffies);
+ if (dirty_writeback_interval) {
+ wait_jiffies = msecs_to_jiffies(dirty_writeback_interval * 10);
+ schedule_timeout_interruptible(wait_jiffies);
+ } else
+ schedule();
+
try_to_freeze();
}




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