netdev - [PATCH net-next 1/1] tcp: optimise skb_zerocopy_iter

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Message-Id: <a7e1690c00c5dfe700c30eb9a8a81ec59f6545dd.1650884401.git.asml.silence@gmail.com>
Date:   Thu, 28 Apr 2022 11:57:46 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     netdev@...r.kernel.org, "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     David Ahern <dsahern@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        linux-kernel@...r.kernel.org,
        Pavel Begunkov <asml.silence@...il.com>
Subject: [PATCH net-next 1/1] tcp: optimise skb_zerocopy_iter_stream()

It's expensive to make a copy of 40B struct iov_iter to the point it
was taking 0.2-0.5% of all cycles in my tests. iov_iter_revert() should
be fine as it's a simple case without nested reverts/truncates.

Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
---

split from a larger patchset, see
https://lore.kernel.org/netdev/cover.1648981570.git.asml.silence@gmail.com/

 net/core/skbuff.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 30b523fa4ad2..e51e53f8c200 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1350,7 +1350,6 @@ int skb_zerocopy_iter_stream(struct sock *sk, struct sk_buff *skb,
 			     struct ubuf_info *uarg)
 {
 	struct ubuf_info *orig_uarg = skb_zcopy(skb);
-	struct iov_iter orig_iter = msg->msg_iter;
 	int err, orig_len = skb->len;
 
 	/* An skb can only point to one uarg. This edge case happens when
@@ -1364,7 +1363,7 @@ int skb_zerocopy_iter_stream(struct sock *sk, struct sk_buff *skb,
 		struct sock *save_sk = skb->sk;
 
 		/* Streams do not free skb on error. Reset to prev state. */
-		msg->msg_iter = orig_iter;
+		iov_iter_revert(&msg->msg_iter, skb->len - orig_len);
 		skb->sk = sk;
 		___pskb_trim(skb, orig_len);
 		skb->sk = save_sk;
-- 
2.36.0