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[PATCH 18/34] xfs: reduce the number of AIL push wakeups

To: xfs@xxxxxxxxxxx
Subject: [PATCH 18/34] xfs: reduce the number of AIL push wakeups
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Tue, 21 Dec 2010 18:29:14 +1100
In-reply-to: <1292916570-25015-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1292916570-25015-1-git-send-email-david@xxxxxxxxxxxxx>
From: Dave Chinner <dchinner@xxxxxxxxxx>

The xfaild often tries to rest to wait for congestion to pass of for
IO to complete, but is regularly woken in tail-pushing situations.
In severe cases, the xfsaild is getting woken tens of thousands of
times a second. Reduce the number needless wakeups by only waking
the xfsaild if the new target is larger than the old one. Further
make short sleeps uninterruptible as they occur when the xfsaild has
decided it needs to back off to allow some IO to complete and being
woken early is counter-productive.

Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
---
 fs/xfs/linux-2.6/xfs_super.c |   20 ++++++++++++++++----
 1 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/fs/xfs/linux-2.6/xfs_super.c b/fs/xfs/linux-2.6/xfs_super.c
index abcda07..6a12da3 100644
--- a/fs/xfs/linux-2.6/xfs_super.c
+++ b/fs/xfs/linux-2.6/xfs_super.c
@@ -834,8 +834,11 @@ xfsaild_wakeup(
        struct xfs_ail          *ailp,
        xfs_lsn_t               threshold_lsn)
 {
-       ailp->xa_target = threshold_lsn;
-       wake_up_process(ailp->xa_task);
+       /* only ever move the target forwards */
+       if (XFS_LSN_CMP(threshold_lsn, ailp->xa_target) > 0) {
+               ailp->xa_target = threshold_lsn;
+               wake_up_process(ailp->xa_task);
+       }
 }
 
 STATIC int
@@ -847,8 +850,17 @@ xfsaild(
        long            tout = 0; /* milliseconds */
 
        while (!kthread_should_stop()) {
-               schedule_timeout_interruptible(tout ?
-                               msecs_to_jiffies(tout) : MAX_SCHEDULE_TIMEOUT);
+               /*
+                * for short sleeps indicating congestion, don't allow us to
+                * get woken early. Otherwise all we do is bang on the AIL lock
+                * without making progress.
+                */
+               if (tout && tout <= 20)
+                       __set_current_state(TASK_KILLABLE);
+               else
+                       __set_current_state(TASK_INTERRUPTIBLE);
+               schedule_timeout(tout ?
+                                msecs_to_jiffies(tout) : MAX_SCHEDULE_TIMEOUT);
 
                /* swsusp */
                try_to_freeze();
-- 
1.7.2.3

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