mlx5: use RCU lock in mlx5_eq_cq_get()

mlx5_eq_cq_get() is called in IRQ handler, the spinlock inside
gets a lot of contentions when we test some heavy workload
with 60 RX queues and 80 CPU's, and it is clearly shown in the
flame graph.

In fact, radix_tree_lookup() is perfectly fine with RCU read lock,
we don't have to take a spinlock on this hot path. This is pretty
much similar to commit 291c566a
("net/mlx4_core: Fix racy CQ (Completion Queue) free"). Slow paths
are still serialized with the spinlock, and with synchronize_irq()
it should be safe to just move the fast path to RCU read lock.

This patch itself reduces the latency by about 50% for our memcached
workload on a 4.14 kernel we test. In upstream, as pointed out by Saeed,
this spinlock gets some rework in commit 02d92f79
("net/mlx5: CQ Database per EQ"), so the difference could be smaller.

Cc: Saeed Mahameed <saeedm@mellanox.com>
Cc: Tariq Toukan <tariqt@mellanox.com>
Acked-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>

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

@@ -114,11 +114,11 @@ static struct mlx5_core_cq *mlx5_eq_cq_get(struct mlx5_eq *eq, u32 cqn)
 	struct mlx5_cq_table *table = &eq->cq_table;
 	struct mlx5_core_cq *cq = NULL;
 
-	spin_lock(&table->lock);
+	rcu_read_lock();
 	cq = radix_tree_lookup(&table->tree, cqn);
 	if (likely(cq))
 		mlx5_cq_hold(cq);
-	spin_unlock(&table->lock);
+	rcu_read_unlock();
 
 	return cq;
 }
@@ -371,9 +371,9 @@ int mlx5_eq_add_cq(struct mlx5_eq *eq, struct mlx5_core_cq *cq)
 	struct mlx5_cq_table *table = &eq->cq_table;
 	int err;
 
-	spin_lock_irq(&table->lock);
+	spin_lock(&table->lock);
 	err = radix_tree_insert(&table->tree, cq->cqn, cq);
-	spin_unlock_irq(&table->lock);
+	spin_unlock(&table->lock);
 
 	return err;
 }
@@ -383,9 +383,9 @@ int mlx5_eq_del_cq(struct mlx5_eq *eq, struct mlx5_core_cq *cq)
 	struct mlx5_cq_table *table = &eq->cq_table;
 	struct mlx5_core_cq *tmp;
 
-	spin_lock_irq(&table->lock);
+	spin_lock(&table->lock);
 	tmp = radix_tree_delete(&table->tree, cq->cqn);
-	spin_unlock_irq(&table->lock);
+	spin_unlock(&table->lock);
 
 	if (!tmp) {
 		mlx5_core_warn(eq->dev, "cq 0x%x not found in eq 0x%x tree\n", eq->eqn, cq->cqn);