afs: Make callback processing more efficient.

afs_vol_interest objects represent the volume IDs currently being accessed
from a fileserver.  These hold lists of afs_cb_interest objects that
repesent the superblocks using that volume ID on that server.

When a callback notification from the server telling of a modification by
another client arrives, the volume ID specified in the notification is
looked up in the server's afs_vol_interest list.  Through the
afs_cb_interest list, the relevant superblocks can be iterated over and the
specific inode looked up and marked in each one.

Make the following efficiency improvements:

 (1) Hold rcu_read_lock() over the entire processing rather than locking it
     each time.

 (2) Do all the callbacks for each vid together rather than individually.
     Each volume then only needs to be looked up once.

 (3) afs_vol_interest objects are now stored in an rb_tree rather than a
     flat list to reduce the lookup step count.

 (4) afs_vol_interest lookup is now done with RCU, but because it's in an
     rb_tree which may rotate under us, a seqlock is used so that if it
     changes during the walk, we repeat the walk with a lock held.

With this and the preceding patch which adds RCU-based lookups in the inode
cache, target volumes/vnodes can be taken without the need to take any
locks, except on the target itself.
Signed-off-by: David Howells <dhowells@redhat.com>

fs/afs/callback.c

@@ -28,7 +28,7 @@ static struct afs_cb_interest *afs_create_interest(struct afs_server *server,
 {
 	struct afs_vol_interest *new_vi, *vi;
 	struct afs_cb_interest *new;
-	struct hlist_node **pp;
+	struct rb_node *parent, **pp;
 
 	new_vi = kzalloc(sizeof(struct afs_vol_interest), GFP_KERNEL);
 	if (!new_vi)
@@ -42,7 +42,6 @@ static struct afs_cb_interest *afs_create_interest(struct afs_server *server,
 
 	new_vi->usage = 1;
 	new_vi->vid = vnode->volume->vid;
-	INIT_HLIST_NODE(&new_vi->srv_link);
 	INIT_HLIST_HEAD(&new_vi->cb_interests);
 
 	refcount_set(&new->usage, 1);
@@ -51,31 +50,31 @@ static struct afs_cb_interest *afs_create_interest(struct afs_server *server,
 	new->server = afs_get_server(server, afs_server_trace_get_new_cbi);
 	INIT_HLIST_NODE(&new->cb_vlink);
 
-	write_lock(&server->cb_break_lock);
+	write_seqlock(&server->cb_break_lock);
 
-	for (pp = &server->cb_volumes.first; *pp; pp = &(*pp)->next) {
-		vi = hlist_entry(*pp, struct afs_vol_interest, srv_link);
-		if (vi->vid < new_vi->vid)
-			continue;
-		if (vi->vid > new_vi->vid)
-			break;
-		vi->usage++;
-		goto found_vi;
+	pp = &server->cb_volumes.rb_node;
+	while ((parent = *pp)) {
+		vi = rb_entry(parent, struct afs_vol_interest, srv_node);
+		if (vi->vid < new_vi->vid) {
+			pp = &(*pp)->rb_left;
+		} else if (vi->vid > new_vi->vid) {
+			pp = &(*pp)->rb_right;
+		} else {
+			vi->usage++;
+			goto found_vi;
+		}
 	}
 
-	new_vi->srv_link.pprev = pp;
-	new_vi->srv_link.next = *pp;
-	if (*pp)
-		(*pp)->pprev = &new_vi->srv_link.next;
-	*pp = &new_vi->srv_link;
 	vi = new_vi;
 	new_vi = NULL;
-found_vi:
+	rb_link_node_rcu(&vi->srv_node, parent, pp);
+	rb_insert_color(&vi->srv_node, &server->cb_volumes);
 
+found_vi:
 	new->vol_interest = vi;
 	hlist_add_head(&new->cb_vlink, &vi->cb_interests);
 
-	write_unlock(&server->cb_break_lock);
+	write_sequnlock(&server->cb_break_lock);
 	kfree(new_vi);
 	return new;
 }
@@ -182,17 +181,17 @@ void afs_put_cb_interest(struct afs_net *net, struct afs_cb_interest *cbi)
 
 	if (cbi && refcount_dec_and_test(&cbi->usage)) {
 		if (!hlist_unhashed(&cbi->cb_vlink)) {
-			write_lock(&cbi->server->cb_break_lock);
+			write_seqlock(&cbi->server->cb_break_lock);
 
 			hlist_del_init(&cbi->cb_vlink);
 			vi = cbi->vol_interest;
 			cbi->vol_interest = NULL;
 			if (--vi->usage == 0)
-				hlist_del(&vi->srv_link);
+				rb_erase(&vi->srv_node, &cbi->server->cb_volumes);
 			else
 				vi = NULL;
 
-			write_unlock(&cbi->server->cb_break_lock);
+			write_sequnlock(&cbi->server->cb_break_lock);
 			if (vi)
 				kfree_rcu(vi, rcu);
 			afs_put_server(net, cbi->server, afs_server_trace_put_cbi);
@@ -237,6 +236,45 @@ void afs_break_callback(struct afs_vnode *vnode, enum afs_cb_break_reason reason
 	write_sequnlock(&vnode->cb_lock);
 }
 
+/*
+ * Look up a volume interest by volume ID under RCU conditions.
+ */
+static struct afs_vol_interest *afs_lookup_vol_interest_rcu(struct afs_server *server,
+							    afs_volid_t vid)
+{
+	struct afs_vol_interest *vi = NULL;
+	struct rb_node *p;
+	int seq = 0;
+
+	do {
+		/* Unfortunately, rbtree walking doesn't give reliable results
+		 * under just the RCU read lock, so we have to check for
+		 * changes.
+		 */
+		read_seqbegin_or_lock(&server->cb_break_lock, &seq);
+
+		p = rcu_dereference_raw(server->cb_volumes.rb_node);
+		while (p) {
+			vi = rb_entry(p, struct afs_vol_interest, srv_node);
+
+			if (vi->vid < vid)
+				p = rcu_dereference_raw(p->rb_left);
+			else if (vi->vid > vid)
+				p = rcu_dereference_raw(p->rb_right);
+			else
+				break;
+			/* We want to repeat the search, this time with the
+			 * lock properly locked.
+			 */
+			vi = NULL;
+		}
+
+	} while (need_seqretry(&server->cb_break_lock, seq));
+
+	done_seqretry(&server->cb_break_lock, seq);
+	return vi;
+}
+
 /*
  * allow the fileserver to explicitly break one callback
  * - happens when
@@ -244,37 +282,18 @@ void afs_break_callback(struct afs_vnode *vnode, enum afs_cb_break_reason reason
  *   - a lock is released
  */
 static void afs_break_one_callback(struct afs_server *server,
-				   struct afs_fid *fid)
+				   struct afs_fid *fid,
+				   struct afs_vol_interest *vi)
 {
-	struct afs_vol_interest *vi;
 	struct afs_cb_interest *cbi;
 	struct afs_iget_data data;
 	struct afs_vnode *vnode;
 	struct inode *inode;
 
-	rcu_read_lock();
-	read_lock(&server->cb_break_lock);
-	hlist_for_each_entry(vi, &server->cb_volumes, srv_link) {
-		if (vi->vid < fid->vid)
-			continue;
-		if (vi->vid > fid->vid) {
-			vi = NULL;
-			break;
-		}
-		//atomic_inc(&vi->usage);
-		break;
-	}
-
-	/* TODO: Find all matching volumes if we couldn't match the server and
-	 * break them anyway.
-	 */
-	if (!vi)
-		goto out;
-
 	/* Step through all interested superblocks.  There may be more than one
 	 * because of cell aliasing.
 	 */
-	hlist_for_each_entry(cbi, &vi->cb_interests, cb_vlink) {
+	hlist_for_each_entry_rcu(cbi, &vi->cb_interests, cb_vlink) {
 		if (fid->vnode == 0 && fid->unique == 0) {
 			/* The callback break applies to an entire volume. */
 			struct afs_super_info *as = AFS_FS_S(cbi->sb);
@@ -303,10 +322,36 @@ static void afs_break_one_callback(struct afs_server *server,
 			}
 		}
 	}
+}
 
-out:
-	read_unlock(&server->cb_break_lock);
-	rcu_read_unlock();
+static void afs_break_some_callbacks(struct afs_server *server,
+				     struct afs_callback_break *cbb,
+				     size_t *_count)
+{
+	struct afs_callback_break *residue = cbb;
+	struct afs_vol_interest *vi;
+	afs_volid_t vid = cbb->fid.vid;
+	size_t i;
+
+	vi = afs_lookup_vol_interest_rcu(server, vid);
+
+	/* TODO: Find all matching volumes if we couldn't match the server and
+	 * break them anyway.
+	 */
+
+	for (i = *_count; i > 0; cbb++, i--) {
+		if (cbb->fid.vid == vid) {
+			_debug("- Fid { vl=%08llx n=%llu u=%u }",
+			       cbb->fid.vid,
+			       cbb->fid.vnode,
+			       cbb->fid.unique);
+			--*_count;
+			if (vi)
+				afs_break_one_callback(server, &cbb->fid, vi);
+		} else {
+			*residue++ = *cbb;
+		}
+	}
 }
 
 /*
@@ -319,17 +364,12 @@ void afs_break_callbacks(struct afs_server *server, size_t count,
 
 	ASSERT(server != NULL);
 
-	/* TODO: Sort the callback break list by volume ID */
+	rcu_read_lock();
 
-	for (; count > 0; callbacks++, count--) {
-		_debug("- Fid { vl=%08llx n=%llu u=%u }",
-		       callbacks->fid.vid,
-		       callbacks->fid.vnode,
-		       callbacks->fid.unique);
-		afs_break_one_callback(server, &callbacks->fid);
-	}
+	while (count > 0)
+		afs_break_some_callbacks(server, callbacks, &count);
 
-	_leave("");
+	rcu_read_unlock();
 	return;
 }
 

fs/afs/internal.h

@@ -524,9 +524,9 @@ struct afs_server {
 	rwlock_t		fs_lock;	/* access lock */
 
 	/* callback promise management */
-	struct hlist_head	cb_volumes;	/* List of volume interests on this server */
+	struct rb_root		cb_volumes;	/* List of volume interests on this server */
 	unsigned		cb_s_break;	/* Break-everything counter. */
-	rwlock_t		cb_break_lock;	/* Volume finding lock */
+	seqlock_t		cb_break_lock;	/* Volume finding lock */
 
 	/* Probe state */
 	unsigned long		probed_at;	/* Time last probe was dispatched (jiffies) */
@@ -552,7 +552,7 @@ struct afs_server {
  * Volume collation in the server's callback interest list.
  */
 struct afs_vol_interest {
-	struct hlist_node	srv_link;	/* Link in server->cb_volumes */
+	struct rb_node		srv_node;	/* Link in server->cb_volumes */
 	struct hlist_head	cb_interests;	/* List of callback interests on the server */
 	union {
 		struct rcu_head	rcu;

fs/afs/server.c

@@ -212,8 +212,8 @@ static struct afs_server *afs_alloc_server(struct afs_net *net,
 	server->addr_version = alist->version;
 	server->uuid = *uuid;
 	rwlock_init(&server->fs_lock);
-	INIT_HLIST_HEAD(&server->cb_volumes);
-	rwlock_init(&server->cb_break_lock);
+	server->cb_volumes = RB_ROOT;
+	seqlock_init(&server->cb_break_lock);
 	init_waitqueue_head(&server->probe_wq);
 	INIT_LIST_HEAD(&server->probe_link);
 	spin_lock_init(&server->probe_lock);