[XFS] Make xfs_ail_check check less by default

Checking the entire AIL on every insert and remove is prohibitively
expensive - the sustained sequntial create rate on a single disk drops
from about 1800/s to 60/s because of this checking resulting in the
xfslogd becoming cpu bound.

By default on debug builds, only check the next and previous entries in
the list to ensure they are ordered correctly. If you really want, define
XFS_TRANS_DEBUG to use the old behaviour.

SGI-PV: 972759
SGI-Modid: xfs-linux-melb:xfs-kern:30372a

Signed-off-by: David Chinner <[e-mail]>
Signed-off-by: Lachlan McIlroy <[e-mail]>

fs/xfs/xfs_trans_ail.c

···
 STATIC xfs_log_item_t * xfs_ail_next(xfs_ail_entry_t *, xfs_log_item_t *);
 
 #ifdef DEBUG
-STATIC void xfs_ail_check(xfs_ail_entry_t *);
+STATIC void xfs_ail_check(xfs_ail_entry_t *, xfs_log_item_t *);
 #else
-#define	xfs_ail_check(a)
+#define	xfs_ail_check(a,l)
 #endif /* DEBUG */
 
 
···
 	next_lip->li_ail.ail_forw = lip;
 	lip->li_ail.ail_forw->li_ail.ail_back = lip;
 
-	xfs_ail_check(base);
+	xfs_ail_check(base, lip);
 	return;
 }
 
···
 	xfs_log_item_t	*lip)
 /* ARGSUSED */
 {
+	xfs_ail_check(base, lip);
 	lip->li_ail.ail_forw->li_ail.ail_back = lip->li_ail.ail_back;
 	lip->li_ail.ail_back->li_ail.ail_forw = lip->li_ail.ail_forw;
 	lip->li_ail.ail_forw = NULL;
 	lip->li_ail.ail_back = NULL;
 
-	xfs_ail_check(base);
 	return lip;
 }
 
···
  */
 STATIC void
 xfs_ail_check(
-	xfs_ail_entry_t *base)
+	xfs_ail_entry_t *base,
+	xfs_log_item_t	*lip)
 {
-	xfs_log_item_t	*lip;
 	xfs_log_item_t	*prev_lip;
 
-	lip = base->ail_forw;
-	if (lip == (xfs_log_item_t*)base) {
+	prev_lip = base->ail_forw;
+	if (prev_lip == (xfs_log_item_t*)base) {
 		/*
 		 * Make sure the pointers are correct when the list
 		 * is empty.
···
 	}
 
 	/*
+	 * Check the next and previous entries are valid.
+	 */
+	ASSERT((lip->li_flags & XFS_LI_IN_AIL) != 0);
+	prev_lip = lip->li_ail.ail_back;
+	if (prev_lip != (xfs_log_item_t*)base) {
+		ASSERT(prev_lip->li_ail.ail_forw == lip);
+		ASSERT(XFS_LSN_CMP(prev_lip->li_lsn, lip->li_lsn) <= 0);
+	}
+	prev_lip = lip->li_ail.ail_forw;
+	if (prev_lip != (xfs_log_item_t*)base) {
+		ASSERT(prev_lip->li_ail.ail_back == lip);
+		ASSERT(XFS_LSN_CMP(prev_lip->li_lsn, lip->li_lsn) >= 0);
+	}
+
+
+#ifdef XFS_TRANS_DEBUG
+	/*
 	 * Walk the list checking forward and backward pointers,
 	 * lsn ordering, and that every entry has the XFS_LI_IN_AIL
-	 * flag set.
+	 * flag set. This is really expensive, so only do it when
+	 * specifically debugging the transaction subsystem.
 	 */
 	prev_lip = (xfs_log_item_t*)base;
 	while (lip != (xfs_log_item_t*)base) {
···
 	}
 	ASSERT(lip == (xfs_log_item_t*)base);
 	ASSERT(base->ail_back == prev_lip);
+#endif /* XFS_TRANS_DEBUG */
 }
 #endif /* DEBUG */