[PATCH] rps: process the skb directly if rps cpu not changed

[<yang.yang29@zte.com.cn>]

From: xu xin <xu.xin16@zte.com.cn>

In the RPS procedure of NAPI receiving, regardless of whether the
rps-calculated CPU of the skb equals to the currently processing CPU, RPS
will always use enqueue_to_backlog to enqueue the skb to per-cpu backlog,
which will trigger a new NET_RX softirq.

Actually, it's not necessary to enqueue it to backlog when rps-calculated
CPU id equals to the current processing CPU, and we can call
__netif_receive_skb or __netif_receive_skb_list to process the skb directly.
The benefit is that it can reduce the number of softirqs of NET_RX and reduce
the processing delay of skb.

The measured result shows the patch brings 50% reduction of NET_RX softirqs.
The test was done on the QEMU environment with two-core CPU by iperf3.
taskset 01 iperf3 -c 192.168.2.250 -t 3 -u -R;
taskset 02 iperf3 -c 192.168.2.250 -t 3 -u -R;

Previous RPS:
		    	CPU0       CPU1
NET_RX:         45          0    (before iperf3 testing)
NET_RX:        1095         241   (after iperf3 testing)

Patched RPS:
                CPU0       CPU1
NET_RX:         28          4    (before iperf3 testing)
NET_RX:         573         32   (after iperf3 testing)

Signed-off-by: xu xin <xu.xin16@zte.com.cn>
Reviewed-by: Zhang Yunkai <zhang.yunkai@zte.com.cn>
Reviewed-by: Yang Yang <yang.yang29@zte.com.cn>
Cc: Xuexin Jiang <jiang.xuexin@zte.com.cn>
---
 net/core/dev.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index c7853192563d..c33ddac3c012 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5666,8 +5666,9 @@ static int netif_receive_skb_internal(struct sk_buff *skb)
 	if (static_branch_unlikely(&rps_needed)) {
 		struct rps_dev_flow voidflow, *rflow = &voidflow;
 		int cpu = get_rps_cpu(skb->dev, skb, &rflow);
+		int current_cpu = smp_processor_id();

-		if (cpu >= 0) {
+		if (cpu >= 0 && cpu != current_cpu) {
 			ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail);
 			rcu_read_unlock();
 			return ret;
@@ -5699,8 +5700,9 @@ void netif_receive_skb_list_internal(struct list_head *head)
 		list_for_each_entry_safe(skb, next, head, list) {
 			struct rps_dev_flow voidflow, *rflow = &voidflow;
 			int cpu = get_rps_cpu(skb->dev, skb, &rflow);
+			int current_cpu = smp_processor_id();

-			if (cpu >= 0) {
+			if (cpu >= 0 && cpu != current_cpu) {
 				/* Will be handled, remove from list */
 				skb_list_del_init(skb);
 				enqueue_to_backlog(skb, cpu, &rflow->last_qtail);
-- 
2.15.2