netdev - [PATCH netnext] mpls: Reduce skb re-allocations due to skb

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Message-id: <20240815161201.22021-1-cpaasch@apple.com>
Date: Thu, 15 Aug 2024 09:12:01 -0700
From: Christoph Paasch <cpaasch@...le.com>
To: netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
 Paolo Abeni <pabeni@...hat.com>, Jakub Kicinski <kuba@...nel.org>,
 David Miller <davem@...emloft.net>
Cc: Roopa Prabhu <roopa@...dia.com>, Craig Taylor <cmtaylor@...le.com>
Subject: [PATCH netnext] mpls: Reduce skb re-allocations due to skb_cow()

mpls_xmit() needs to prepend the MPLS-labels to the packet. That implies
one needs to make sure there is enough space for it in the headers.

Calling skb_cow() implies however that one wants to change even the
playload part of the packet (which is not true for MPLS). Thus, call
skb_cow_head() instead, which is what other tunnelling protocols do.

Running a server with this comm it entirely removed the calls to
pskb_expand_head() from the callstack in mpls_xmit() thus having
significant CPU-reduction, especially at peak times.

Cc: Roopa Prabhu <roopa@...dia.com>
Reported-by: Craig Taylor <cmtaylor@...le.com>
Signed-off-by: Christoph Paasch <cpaasch@...le.com>
---
 net/mpls/mpls_iptunnel.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/mpls/mpls_iptunnel.c b/net/mpls/mpls_iptunnel.c
index 4385fd3b13be..6e73da94af7f 100644
--- a/net/mpls/mpls_iptunnel.c
+++ b/net/mpls/mpls_iptunnel.c
@@ -106,7 +106,7 @@ static int mpls_xmit(struct sk_buff *skb)
 		hh_len = 0;

 	/* Ensure there is enough space for the headers in the skb */
-	if (skb_cow(skb, hh_len + new_header_size))
+	if (skb_cow_head(skb, hh_len + new_header_size))
 		goto drop;

 	skb_set_inner_protocol(skb, skb->protocol);
-- 
2.39.5 (Apple Git-154)