[PATCH net-next] net: save some cycles when doing skb_attempt_defer_free()

[Jason Xing <kerneljasonxing@gmail.com>]

From: Jason Xing <kernelxing@tencent.com>

Normally, we don't face these two exceptions very often meanwhile
we have some chance to meet the condition where the current cpu id
is the same as skb->alloc_cpu.

One simple test that can help us see the frequency of this statement
'cpu == raw_smp_processor_id()':
1. running iperf -s and iperf -c [ip] -P [MAX CPU]
2. using BPF to capture skb_attempt_defer_free()

I can see around 4% chance that happens to satisfy the statement.
So moving this statement at the beginning can save some cycles in
most cases.

Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
 net/core/skbuff.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index ab970ded8a7b..b4f252dc91fb 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -7002,9 +7002,9 @@ void skb_attempt_defer_free(struct sk_buff *skb)
 	unsigned int defer_max;
 	bool kick;
 
-	if (WARN_ON_ONCE(cpu >= nr_cpu_ids) ||
+	if (cpu == raw_smp_processor_id() ||
 	    !cpu_online(cpu) ||
-	    cpu == raw_smp_processor_id()) {
+	    WARN_ON_ONCE(cpu >= nr_cpu_ids)) {
 nodefer:	kfree_skb_napi_cache(skb);
 		return;
 	}
-- 
2.37.3