netdev - [PATCH net-next] tcp: add tcp_tx_skb_cache_key checking in sk_stream_alloc

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Date:   Wed, 1 Sep 2021 18:39:04 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     <davem@...emloft.net>, <kuba@...nel.org>
CC:     <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linuxarm@...neuler.org>, <edumazet@...gle.com>,
        <yoshfuji@...ux-ipv6.org>, <dsahern@...nel.org>
Subject: [PATCH net-next] tcp: add tcp_tx_skb_cache_key checking in sk_stream_alloc_skb()

Since tcp_tx_skb_cache is disabled by default in:
commit 0b7d7f6b2208 ("tcp: add tcp_tx_skb_cache sysctl")

Add tcp_tx_skb_cache_key checking in sk_stream_alloc_skb() to
avoid possible branch-misses.

Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
---
Also, the sk->sk_tx_skb_cache may be both changed by allocation
and freeing side, I assume there may be some implicit protection
here too, such as the NAPI protection for rx?
---
 net/ipv4/tcp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index e8b48df..226ddef 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -866,7 +866,7 @@ struct sk_buff *sk_stream_alloc_skb(struct sock *sk, int size, gfp_t gfp,
 {
 	struct sk_buff *skb;
 
-	if (likely(!size)) {
+	if (static_branch_unlikely(&tcp_tx_skb_cache_key) && likely(!size)) {
 		skb = sk->sk_tx_skb_cache;
 		if (skb) {
 			skb->truesize = SKB_TRUESIZE(skb_end_offset(skb));
-- 
2.7.4