netdev - [PATCH] tcp: use linear buffer for small frames

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Message-ID: <20220830123345.1909199-1-chenzhen126@huawei.com>
Date:   Tue, 30 Aug 2022 20:33:45 +0800
From:   Zhen Chen <chenzhen126@...wei.com>
To:     <edumazet@...gle.com>, <davem@...emloft.net>,
        <kuznet@....inr.ac.ru>, <yoshfuji@...ux-ipv6.org>,
        <netdev@...r.kernel.org>
CC:     <yanan@...wei.com>, <caowangbao@...wei.com>
Subject: [PATCH] tcp: use linear buffer for small frames

472c2e07eef0 ("tcp: add one skb cache for tx") and related patches added a
machanism to relax slab layer in tcp stack, by caching one skb per socket.
The feature is disabled by default and the patch also dropped linear payload
for small frames, which caused about 5% of performance regression for small
packets because nic drivers would bother to deal with fraglist than before.

As d8b81175e412 ("tcp: remove sk_{tr}x_skb_cache") reverted the whole
machanism but skipped the linear part, just make the revert complete.

Signed-off-by: Zhen Chen <chenzhen126@...wei.com>
---
 net/ipv4/tcp.c | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index e5011c136fdb..0b6010051598 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1154,6 +1154,30 @@ int tcp_sendpage(struct sock *sk, struct page *page, int offset,
 }
 EXPORT_SYMBOL(tcp_sendpage);
 
+/* Do not bother using a page frag for very small frames.
+ * But use this heuristic only for the first skb in write queue.
+ *
+ * Having no payload in skb->head allows better SACK shifting
+ * in tcp_shift_skb_data(), reducing sack/rack overhead, because
+ * write queue has less skbs.
+ * Each skb can hold up to MAX_SKB_FRAGS * 32Kbytes, or ~0.5 MB.
+ * This also speeds up tso_fragment(), since it wont fallback
+ * to tcp_fragment().
+ */
+static int linear_payload_sz(bool first_skb)
+{
+	if (first_skb)
+		return SKB_WITH_OVERHEAD(2048 - MAX_TCP_HEADER);
+	return 0;
+}
+
+static int select_size(bool first_skb, bool zc)
+{
+	if (zc)
+		return 0;
+	return linear_payload_sz(first_skb);
+}
+
 void tcp_free_fastopen_req(struct tcp_sock *tp)
 {
 	if (tp->fastopen_req) {
@@ -1311,6 +1335,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 
 		if (copy <= 0 || !tcp_skb_can_collapse_to(skb)) {
 			bool first_skb;
+			int linear;
 
 new_segment:
 			if (!sk_stream_memory_free(sk))
@@ -1322,7 +1347,8 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 					goto restart;
 			}
 			first_skb = tcp_rtx_and_write_queues_empty(sk);
-			skb = tcp_stream_alloc_skb(sk, 0, sk->sk_allocation,
+			linear = select_size(first_skb, zc);
+			skb = tcp_stream_alloc_skb(sk, linear, sk->sk_allocation,
 						   first_skb);
 			if (!skb)
 				goto wait_for_space;
-- 
2.23.0