mlx4: do not fire tasklet unless necessary

All rx and rx netdev interrupts are handled by respectively
by mlx4_en_rx_irq() and mlx4_en_tx_irq() which simply schedule a NAPI.

But mlx4_eq_int() also fires a tasklet to service all items that were
queued via mlx4_add_cq_to_tasklet(), but this handler was not called
unless user cqe was handled.

This is very confusing, as "mpstat -I SCPU ..." show huge number of
tasklet invocations.

This patch saves this overhead, by carefully firing the tasklet directly
from mlx4_add_cq_to_tasklet(), removing four atomic operations per IRQ.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Tariq Toukan <tariqt@mellanox.com>
Cc: Saeed Mahameed <saeedm@mellanox.com>
Acked-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/cq.c

@@ -81,8 +81,9 @@ void mlx4_cq_tasklet_cb(unsigned long data)
 
 static void mlx4_add_cq_to_tasklet(struct mlx4_cq *cq)
 {
-	unsigned long flags;
 	struct mlx4_eq_tasklet *tasklet_ctx = cq->tasklet_ctx.priv;
+	unsigned long flags;
+	bool kick;
 
 	spin_lock_irqsave(&tasklet_ctx->lock, flags);
 	/* When migrating CQs between EQs will be implemented, please note
@@ -92,7 +93,10 @@ static void mlx4_add_cq_to_tasklet(struct mlx4_cq *cq)
 	 */
 	if (list_empty_careful(&cq->tasklet_ctx.list)) {
 		atomic_inc(&cq->refcount);
+		kick = list_empty(&tasklet_ctx->list);
 		list_add_tail(&cq->tasklet_ctx.list, &tasklet_ctx->list);
+		if (kick)
+			tasklet_schedule(&tasklet_ctx->task);
 	}
 	spin_unlock_irqrestore(&tasklet_ctx->lock, flags);
 }

drivers/net/ethernet/mellanox/mlx4/eq.c

@@ -494,7 +494,7 @@ static int mlx4_eq_int(struct mlx4_dev *dev, struct mlx4_eq *eq)
 {
 	struct mlx4_priv *priv = mlx4_priv(dev);
 	struct mlx4_eqe *eqe;
-	int cqn = -1;
+	int cqn;
 	int eqes_found = 0;
 	int set_ci = 0;
 	int port;
@@ -840,13 +840,6 @@ static int mlx4_eq_int(struct mlx4_dev *dev, struct mlx4_eq *eq)
 
 	eq_set_ci(eq, 1);
 
-	/* cqn is 24bit wide but is initialized such that its higher bits
-	 * are ones too. Thus, if we got any event, cqn's high bits should be off
-	 * and we need to schedule the tasklet.
-	 */
-	if (!(cqn & ~0xffffff))
-		tasklet_schedule(&eq->tasklet_ctx.task);
-
 	return eqes_found;
 }