net: rps: add rps_input_queue_head_add() helper

process_backlog() can batch increments of sd->input_queue_head,
saving some memory bandwidth.

Also add READ_ONCE()/WRITE_ONCE() annotations around
sd->input_queue_head accesses.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/rps.h

@@ -138,11 +138,16 @@ static inline void rps_input_queue_tail_save(u32 *dest, u32 tail)
 #endif
 }
 
-static inline void rps_input_queue_head_incr(struct softnet_data *sd)
+static inline void rps_input_queue_head_add(struct softnet_data *sd, int val)
 {
 #ifdef CONFIG_RPS
-	sd->input_queue_head++;
+	WRITE_ONCE(sd->input_queue_head, sd->input_queue_head + val);
 #endif
 }
 
+static inline void rps_input_queue_head_incr(struct softnet_data *sd)
+{
+	rps_input_queue_head_add(sd, 1);
+}
+
 #endif /* _NET_RPS_H */

net/core/dev.c

@@ -4528,7 +4528,7 @@ set_rps_cpu(struct net_device *dev, struct sk_buff *skb,
 	out:
 #endif
 		rflow->last_qtail =
-			per_cpu(softnet_data, next_cpu).input_queue_head;
+			READ_ONCE(per_cpu(softnet_data, next_cpu).input_queue_head);
 	}
 
 	rflow->cpu = next_cpu;
@@ -4610,7 +4610,7 @@ static int get_rps_cpu(struct net_device *dev, struct sk_buff *skb,
 		 */
 		if (unlikely(tcpu != next_cpu) &&
 		    (tcpu >= nr_cpu_ids || !cpu_online(tcpu) ||
-		     ((int)(per_cpu(softnet_data, tcpu).input_queue_head -
+		     ((int)(READ_ONCE(per_cpu(softnet_data, tcpu).input_queue_head) -
 		      READ_ONCE(rflow->last_qtail))) >= 0)) {
 			tcpu = next_cpu;
 			rflow = set_rps_cpu(dev, skb, rflow, next_cpu);
@@ -4665,7 +4665,7 @@ bool rps_may_expire_flow(struct net_device *dev, u16 rxq_index,
 		rflow = &flow_table->flows[flow_id];
 		cpu = READ_ONCE(rflow->cpu);
 		if (rflow->filter == filter_id && cpu < nr_cpu_ids &&
-		    ((int)(per_cpu(softnet_data, cpu).input_queue_head -
+		    ((int)(READ_ONCE(per_cpu(softnet_data, cpu).input_queue_head) -
 			   READ_ONCE(rflow->last_qtail)) <
 		     (int)(10 * flow_table->mask)))
 			expire = false;
@@ -6045,9 +6045,10 @@ static int process_backlog(struct napi_struct *napi, int quota)
 			rcu_read_lock();
 			__netif_receive_skb(skb);
 			rcu_read_unlock();
-			rps_input_queue_head_incr(sd);
-			if (++work >= quota)
+			if (++work >= quota) {
+				rps_input_queue_head_add(sd, work);
 				return work;
+			}
 
 		}
 
@@ -6070,6 +6071,8 @@ static int process_backlog(struct napi_struct *napi, int quota)
 		backlog_unlock_irq_enable(sd);
 	}
 
+	if (work)
+		rps_input_queue_head_add(sd, work);
 	return work;
 }