ocfs2: Optimize inode group allocation by recording last used group.

In ocfs2, the block group search looks for the "emptiest" group
to allocate from. So if the allocator has many equally(or almost
equally) empty groups, new block group will tend to get spread
out amongst them.

So we add osb_inode_alloc_group in ocfs2_super to record the last
used inode allocation group.
For more details, please see
http://oss.oracle.com/osswiki/OCFS2/DesignDocs/InodeAllocationStrategy.

I have done some basic test and the results are a ten times improvement on
some cold-cache stat workloads.
Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>

fs/ocfs2/ocfs2.h

@@ -343,6 +343,9 @@ struct ocfs2_super
 
 	unsigned int			osb_dx_mask;
 	u32				osb_dx_seed[4];
+
+	/* the group we used to allocate inodes. */
+	u64				osb_inode_alloc_group;
 };
 
 #define OCFS2_SB(sb)	    ((struct ocfs2_super *)(sb)->s_fs_info)

fs/ocfs2/suballoc.c

@@ -66,6 +66,7 @@ static int ocfs2_block_group_alloc(struct ocfs2_super *osb,
 				   struct inode *alloc_inode,
 				   struct buffer_head *bh,
 				   u64 max_block,
+				   u64 *last_alloc_group,
 				   int flags);
 
 static int ocfs2_cluster_group_search(struct inode *inode,
@@ -407,6 +408,7 @@ static int ocfs2_block_group_alloc(struct ocfs2_super *osb,
 				   struct inode *alloc_inode,
 				   struct buffer_head *bh,
 				   u64 max_block,
+				   u64 *last_alloc_group,
 				   int flags)
 {
 	int status, credits;
@@ -444,6 +446,11 @@ static int ocfs2_block_group_alloc(struct ocfs2_super *osb,
 		goto bail;
 	}
 
+	if (last_alloc_group && *last_alloc_group != 0) {
+		mlog(0, "use old allocation group %llu for block group alloc\n",
+		     (unsigned long long)*last_alloc_group);
+		ac->ac_last_group = *last_alloc_group;
+	}
 	status = ocfs2_claim_clusters(osb,
 				      handle,
 				      ac,
@@ -518,6 +525,11 @@ static int ocfs2_block_group_alloc(struct ocfs2_super *osb,
 	alloc_inode->i_blocks = ocfs2_inode_sector_count(alloc_inode);
 
 	status = 0;
+
+	/* save the new last alloc group so that the caller can cache it. */
+	if (last_alloc_group)
+		*last_alloc_group = ac->ac_last_group;
+
 bail:
 	if (handle)
 		ocfs2_commit_trans(osb, handle);
@@ -535,6 +547,7 @@ static int ocfs2_reserve_suballoc_bits(struct ocfs2_super *osb,
 				       struct ocfs2_alloc_context *ac,
 				       int type,
 				       u32 slot,
+				       u64 *last_alloc_group,
 				       int flags)
 {
 	int status;
@@ -600,7 +613,8 @@ static int ocfs2_reserve_suballoc_bits(struct ocfs2_super *osb,
 		}
 
 		status = ocfs2_block_group_alloc(osb, alloc_inode, bh,
-						 ac->ac_max_block, flags);
+						 ac->ac_max_block,
+						 last_alloc_group, flags);
 		if (status < 0) {
 			if (status != -ENOSPC)
 				mlog_errno(status);
@@ -644,7 +658,7 @@ int ocfs2_reserve_new_metadata_blocks(struct ocfs2_super *osb,
 
 	status = ocfs2_reserve_suballoc_bits(osb, (*ac),
 					     EXTENT_ALLOC_SYSTEM_INODE,
-					     slot, ALLOC_NEW_GROUP);
+					     slot, NULL, ALLOC_NEW_GROUP);
 	if (status < 0) {
 		if (status != -ENOSPC)
 			mlog_errno(status);
@@ -690,7 +704,8 @@ static int ocfs2_steal_inode_from_other_nodes(struct ocfs2_super *osb,
 
 		status = ocfs2_reserve_suballoc_bits(osb, ac,
 						     INODE_ALLOC_SYSTEM_INODE,
-						     slot, NOT_ALLOC_NEW_GROUP);
+						     slot, NULL,
+						     NOT_ALLOC_NEW_GROUP);
 		if (status >= 0) {
 			ocfs2_set_inode_steal_slot(osb, slot);
 			break;
@@ -707,6 +722,7 @@ int ocfs2_reserve_new_inode(struct ocfs2_super *osb,
 {
 	int status;
 	s16 slot = ocfs2_get_inode_steal_slot(osb);
+	u64 alloc_group;
 
 	*ac = kzalloc(sizeof(struct ocfs2_alloc_context), GFP_KERNEL);
 	if (!(*ac)) {
@@ -742,14 +758,22 @@ int ocfs2_reserve_new_inode(struct ocfs2_super *osb,
 		goto inode_steal;
 
 	atomic_set(&osb->s_num_inodes_stolen, 0);
+	alloc_group = osb->osb_inode_alloc_group;
 	status = ocfs2_reserve_suballoc_bits(osb, *ac,
 					     INODE_ALLOC_SYSTEM_INODE,
 					     osb->slot_num,
+					     &alloc_group,
 					     ALLOC_NEW_GROUP |
 					     ALLOC_GROUPS_FROM_GLOBAL);
 	if (status >= 0) {
 		status = 0;
 
+		spin_lock(&osb->osb_lock);
+		osb->osb_inode_alloc_group = alloc_group;
+		spin_unlock(&osb->osb_lock);
+		mlog(0, "after reservation, new allocation group is "
+		     "%llu\n", (unsigned long long)alloc_group);
+
 		/*
 		 * Some inodes must be freed by us, so try to allocate
 		 * from our own next time.
@@ -796,7 +820,7 @@ int ocfs2_reserve_cluster_bitmap_bits(struct ocfs2_super *osb,
 
 	status = ocfs2_reserve_suballoc_bits(osb, ac,
 					     GLOBAL_BITMAP_SYSTEM_INODE,
-					     OCFS2_INVALID_SLOT,
+					     OCFS2_INVALID_SLOT, NULL,
 					     ALLOC_NEW_GROUP);
 	if (status < 0 && status != -ENOSPC) {
 		mlog_errno(status);