xfs: bulk AIL insertion during transaction commit

When inserting items into the AIL from the transaction committed
callbacks, we take the AIL lock for every single item that is to be
inserted. For a CIL checkpoint commit, this can be tens of thousands
of individual inserts, yet almost all of the items will be inserted
at the same point in the AIL because they have the same index.

To reduce the overhead and contention on the AIL lock for such
operations, introduce a "bulk insert" operation which allows a list
of log items with the same LSN to be inserted in a single operation
via a list splice. To do this, we need to pre-sort the log items
being committed into a temporary list for insertion.

The complexity is that not every log item will end up with the same
LSN, and not every item is actually inserted into the AIL. Items
that don't match the commit LSN will be inserted and unpinned as per
the current one-at-a-time method (relatively rare), while items that
are not to be inserted will be unpinned and freed immediately. Items
that are to be inserted at the given commit lsn are placed in a
temporary array and inserted into the AIL in bulk each time the
array fills up.

As a result of this, we trade off AIL hold time for a significant
reduction in traffic. lock_stat output shows that the worst case
hold time is unchanged, but contention from AIL inserts drops by an
order of magnitude and the number of lock traversal decreases
significantly.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

1 files changed, 107 insertions, 2 deletions

diff --git a/fs/xfs/xfs_trans_ail.c b/fs/xfs/xfs_trans_ail.c
index 645928cab42d..fe991a76bf14 100644
--- a/fs/xfs/xfs_trans_ail.c
+++ b/fs/xfs/xfs_trans_ail.c
@@ -29,6 +29,7 @@
 #include "xfs_error.h"
 
 STATIC void xfs_ail_insert(struct xfs_ail *, xfs_log_item_t *);
+STATIC void xfs_ail_splice(struct xfs_ail *, struct list_head *, xfs_lsn_t);
 STATIC void xfs_ail_delete(struct xfs_ail *, xfs_log_item_t *);
 STATIC xfs_log_item_t * xfs_ail_min(struct xfs_ail *);
 STATIC xfs_log_item_t * xfs_ail_next(struct xfs_ail *, xfs_log_item_t *);
@@ -502,6 +503,79 @@ xfs_trans_ail_update(
 }       /* xfs_trans_update_ail */
 
 /*
+ * xfs_trans_ail_update - bulk AIL insertion operation.
+ *
+ * @xfs_trans_ail_update takes an array of log items that all need to be
+ * positioned at the same LSN in the AIL. If an item is not in the AIL, it will
+ * be added.  Otherwise, it will be repositioned  by removing it and re-adding
+ * it to the AIL. If we move the first item in the AIL, update the log tail to
+ * match the new minimum LSN in the AIL.
+ *
+ * This function takes the AIL lock once to execute the update operations on
+ * all the items in the array, and as such should not be called with the AIL
+ * lock held. As a result, once we have the AIL lock, we need to check each log
+ * item LSN to confirm it needs to be moved forward in the AIL.
+ *
+ * To optimise the insert operation, we delete all the items from the AIL in
+ * the first pass, moving them into a temporary list, then splice the temporary
+ * list into the correct position in the AIL. This avoids needing to do an
+ * insert operation on every item.
+ *
+ * This function must be called with the AIL lock held.  The lock is dropped
+ * before returning.
+ */
+void
+xfs_trans_ail_update_bulk(
+        struct xfs_ail          *ailp,
+        struct xfs_log_item     **log_items,
+        int                     nr_items,
+        xfs_lsn_t               lsn) __releases(ailp->xa_lock)
+{
+        xfs_log_item_t          *mlip;
+        xfs_lsn_t               tail_lsn;
+        int                     mlip_changed = 0;
+        int                     i;
+        LIST_HEAD(tmp);
+
+        mlip = xfs_ail_min(ailp);
+
+        for (i = 0; i < nr_items; i++) {
+                struct xfs_log_item *lip = log_items[i];
+                if (lip->li_flags & XFS_LI_IN_AIL) {
+                        /* check if we really need to move the item */
+                        if (XFS_LSN_CMP(lsn, lip->li_lsn) <= 0)
+                                continue;
+
+                        xfs_ail_delete(ailp, lip);
+                        if (mlip == lip)
+                                mlip_changed = 1;
+                } else {
+                        lip->li_flags |= XFS_LI_IN_AIL;
+                }
+                lip->li_lsn = lsn;
+                list_add(&lip->li_ail, &tmp);
+        }
+
+        xfs_ail_splice(ailp, &tmp, lsn);
+
+        if (!mlip_changed) {
+                spin_unlock(&ailp->xa_lock);
+                return;
+        }
+
+        /*
+         * It is not safe to access mlip after the AIL lock is dropped, so we
+         * must get a copy of li_lsn before we do so.  This is especially
+         * important on 32-bit platforms where accessing and updating 64-bit
+         * values like li_lsn is not atomic.
+         */
+        mlip = xfs_ail_min(ailp);
+        tail_lsn = mlip->li_lsn;
+        spin_unlock(&ailp->xa_lock);
+        xfs_log_move_tail(ailp->xa_mount, tail_lsn);
+}
+
+/*
  * Delete the given item from the AIL.  It must already be in
  * the AIL.
  *
@@ -642,8 +716,8 @@ xfs_ail_insert(
                         break;
         }
 
-        ASSERT((&next_lip->li_ail == &ailp->xa_ail) ||
-               (XFS_LSN_CMP(next_lip->li_lsn, lip->li_lsn) <= 0));
+        ASSERT(&next_lip->li_ail == &ailp->xa_ail ||
+               XFS_LSN_CMP(next_lip->li_lsn, lip->li_lsn) <= 0);
 
         list_add(&lip->li_ail, &next_lip->li_ail);
 
@@ -652,6 +726,37 @@ xfs_ail_insert(
 }
 
 /*
+ * splice the log item list into the AIL at the given LSN.
+ */
+STATIC void
+xfs_ail_splice(
+        struct xfs_ail  *ailp,
+        struct list_head *list,
+        xfs_lsn_t       lsn)
+{
+        xfs_log_item_t  *next_lip;
+
+        /*
+         * If the list is empty, just insert the item.
+         */
+        if (list_empty(&ailp->xa_ail)) {
+                list_splice(list, &ailp->xa_ail);
+                return;
+        }
+
+        list_for_each_entry_reverse(next_lip, &ailp->xa_ail, li_ail) {
+                if (XFS_LSN_CMP(next_lip->li_lsn, lsn) <= 0)
+                        break;
+        }
+
+        ASSERT((&next_lip->li_ail == &ailp->xa_ail) ||
+               (XFS_LSN_CMP(next_lip->li_lsn, lsn) <= 0));
+
+        list_splice_init(list, &next_lip->li_ail);
+        return;
+}
+
+/*
  * Delete the given item from the AIL.  Return a pointer to the item.
  */
 STATIC void