net/mlx5: Support lockless FTE read lookups

During connection tracking offloads with high number of connections,
(40K connections per second), flow table group lock contention is
observed.
To improve the performance by reducing lock contention, lockless
FTE read lookup is performed as described below.

Each flow table entry is refcounted.
Flow table entry is removed when refcount drops to zero.
rhash table allows rcu protected lookup.
Each hash table entry insertion and removal is write lock protected.

Hence, it is possible to perform lockless lookup in rhash table using
following scheme.

(a) Guard FTE entry lookup per group using rcu read lock.
(b) Before freeing the FTE entry, wait for all readers to finish
accessing the FTE.

Below example of one reader and write in parallel racing, shows
protection in effect with rcu lock.

lookup_fte_locked()
  rcu_read_lock();
  search_hash_table()
                                  existing_flow_group_write_lock();
                                  tree_put_node(fte)
                                    drop_ref_cnt(fte)
                                    del_sw_fte(fte)
                                    del_hash_table_entry();
                                    call_rcu();
                                  existing_flow_group_write_unlock();
  get_ref_cnt(fte) fails
  rcu_read_unlock();
                                  rcu grace period();
                                    [..]
                                    kmem_cache_free(fte);

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>

drivers/net/ethernet/mellanox/mlx5/core/fs_core.c

@@ -531,9 +531,16 @@ static void del_hw_fte(struct fs_node *node)
 	}
 }
 
+static void del_sw_fte_rcu(struct rcu_head *head)
+{
+	struct fs_fte *fte = container_of(head, struct fs_fte, rcu);
+	struct mlx5_flow_steering *steering = get_steering(&fte->node);
+
+	kmem_cache_free(steering->ftes_cache, fte);
+}
+
 static void del_sw_fte(struct fs_node *node)
 {
-	struct mlx5_flow_steering *steering = get_steering(node);
 	struct mlx5_flow_group *fg;
 	struct fs_fte *fte;
 	int err;
@@ -546,7 +553,8 @@ static void del_sw_fte(struct fs_node *node)
 				     rhash_fte);
 	WARN_ON(err);
 	ida_simple_remove(&fg->fte_allocator, fte->index - fg->start_index);
-	kmem_cache_free(steering->ftes_cache, fte);
+
+	call_rcu(&fte->rcu, del_sw_fte_rcu);
 }
 
 static void del_hw_flow_group(struct fs_node *node)
@@ -1623,22 +1631,47 @@ static u64 matched_fgs_get_version(struct list_head *match_head)
 }
 
 static struct fs_fte *
-lookup_fte_locked(struct mlx5_flow_group *g,
-		  const u32 *match_value,
-		  bool take_write)
+lookup_fte_for_write_locked(struct mlx5_flow_group *g, const u32 *match_value)
 {
 	struct fs_fte *fte_tmp;
 
-	if (take_write)
-		nested_down_write_ref_node(&g->node, FS_LOCK_PARENT);
-	else
-		nested_down_read_ref_node(&g->node, FS_LOCK_PARENT);
-	fte_tmp = rhashtable_lookup_fast(&g->ftes_hash, match_value,
-					 rhash_fte);
+	nested_down_write_ref_node(&g->node, FS_LOCK_PARENT);
+
+	fte_tmp = rhashtable_lookup_fast(&g->ftes_hash, match_value, rhash_fte);
 	if (!fte_tmp || !tree_get_node(&fte_tmp->node)) {
 		fte_tmp = NULL;
 		goto out;
 	}
+
+	if (!fte_tmp->node.active) {
+		tree_put_node(&fte_tmp->node, false);
+		fte_tmp = NULL;
+		goto out;
+	}
+	nested_down_write_ref_node(&fte_tmp->node, FS_LOCK_CHILD);
+
+out:
+	up_write_ref_node(&g->node, false);
+	return fte_tmp;
+}
+
+static struct fs_fte *
+lookup_fte_for_read_locked(struct mlx5_flow_group *g, const u32 *match_value)
+{
+	struct fs_fte *fte_tmp;
+
+	if (!tree_get_node(&g->node))
+		return NULL;
+
+	rcu_read_lock();
+	fte_tmp = rhashtable_lookup(&g->ftes_hash, match_value, rhash_fte);
+	if (!fte_tmp || !tree_get_node(&fte_tmp->node)) {
+		rcu_read_unlock();
+		fte_tmp = NULL;
+		goto out;
+	}
+	rcu_read_unlock();
+
 	if (!fte_tmp->node.active) {
 		tree_put_node(&fte_tmp->node, false);
 		fte_tmp = NULL;
@@ -1646,14 +1679,21 @@ lookup_fte_locked(struct mlx5_flow_group *g,
 	}
 
 	nested_down_write_ref_node(&fte_tmp->node, FS_LOCK_CHILD);
+
 out:
-	if (take_write)
-		up_write_ref_node(&g->node, false);
-	else
-		up_read_ref_node(&g->node);
+	tree_put_node(&g->node, false);
 	return fte_tmp;
 }
 
+static struct fs_fte *
+lookup_fte_locked(struct mlx5_flow_group *g, const u32 *match_value, bool write)
+{
+	if (write)
+		return lookup_fte_for_write_locked(g, match_value);
+	else
+		return lookup_fte_for_read_locked(g, match_value);
+}
+
 static struct mlx5_flow_handle *
 try_add_to_existing_fg(struct mlx5_flow_table *ft,
 		       struct list_head *match_head,

drivers/net/ethernet/mellanox/mlx5/core/fs_core.h

@@ -202,6 +202,7 @@ struct fs_fte {
 	enum fs_fte_status		status;
 	struct mlx5_fc			*counter;
 	struct rhash_head		hash;
+	struct rcu_head	rcu;
 	int				modify_mask;
 };