net: optimize ____napi_schedule() to avoid extra NET_RX_SOFTIRQ

____napi_schedule() adds a napi into current cpu softnet_data poll_list,
then raises NET_RX_SOFTIRQ to make sure net_rx_action() will process it.

Idea of this patch is to not raise NET_RX_SOFTIRQ when being called indirectly
from net_rx_action(), because we can process poll_list from this point,
without going to full softirq loop.

This needs a change in net_rx_action() to make sure we restart
its main loop if sd->poll_list was updated without NET_RX_SOFTIRQ
being raised.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Tested-by: Jason Xing <kerneljasonxing@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/core/dev.c

@@ -4360,7 +4360,11 @@ static inline void ____napi_schedule(struct softnet_data *sd,
 	}
 
 	list_add_tail(&napi->poll_list, &sd->poll_list);
-	__raise_softirq_irqoff(NET_RX_SOFTIRQ);
+	/* If not called from net_rx_action()
+	 * we have to raise NET_RX_SOFTIRQ.
+	 */
+	if (!sd->in_net_rx_action)
+		__raise_softirq_irqoff(NET_RX_SOFTIRQ);
 }
 
 #ifdef CONFIG_RPS
@@ -6648,6 +6652,7 @@ static __latent_entropy void net_rx_action(struct softirq_action *h)
 	LIST_HEAD(list);
 	LIST_HEAD(repoll);
 
+start:
 	sd->in_net_rx_action = true;
 	local_irq_disable();
 	list_splice_init(&sd->poll_list, &list);
@@ -6659,9 +6664,18 @@ static __latent_entropy void net_rx_action(struct softirq_action *h)
 		skb_defer_free_flush(sd);
 
 		if (list_empty(&list)) {
-			sd->in_net_rx_action = false;
-			if (!sd_has_rps_ipi_waiting(sd) && list_empty(&repoll))
-				goto end;
+			if (list_empty(&repoll)) {
+				sd->in_net_rx_action = false;
+				barrier();
+				/* We need to check if ____napi_schedule()
+				 * had refilled poll_list while
+				 * sd->in_net_rx_action was true.
+				 */
+				if (!list_empty(&sd->poll_list))
+					goto start;
+				if (!sd_has_rps_ipi_waiting(sd))
+					goto end;
+			}
 			break;
 		}