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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Yuya Kusakabe X-Mailer: b4 0.15.2 X-Developer-Signature: v=1; a=openpgp-sha256; l=1990; i=yuya.kusakabe@gmail.com; h=from:subject:message-id; bh=nQSMfLXooTdM45Aimk7uiT9RoDYsM7bG1cJP54C7pfM=; b=owEBbQKS/ZANAwAIASrX0XUqXRtNAcsmYgBql9+FlANzp9nxfyrPzdLUPJx3pqGmrdCXoEf+4 riHCgA2duKJAjMEAAEIAB0WIQTaB7usAfxNKMeqa6Yq19F1Kl0bTQUCapffhQAKCRAq19F1Kl0b TWa+D/4mWf99a/uWnnddkAbdfKxfimeoxmhbibYJ6ey7R67XQsLRX/ixjAbjAd0O2sTvdusSLZJ tkFR+vKXbc9mONJuiiu03A+LigfUAjgWU6N/hLrYMXH3XUpwNr21QBqxW2Hpg5OYUSTfFTJFzB9 b42plVdTTu7gWGpvNzrPABaGz5n699ULJZliilMYag2bPGKrt+StlDO2W9IHl2vVvuVig9452RP YQlxObylwpy5UgTtdHCmPs+2gIKSPoLnFVt02RMZnIZvezw7CXS8gOHVdwcZXvla9C2WrPbPPgF WV/AFIKRwBoMpsAljygK2rPnisqIRswwGAJyOkU08/5EI+91uSqraLpKfGfRw5o4YfGVd/or/Db c6tG9MR/hRG8WuHhzSlHBXAJTESbQtpeL/LtrIUh4uB8J6SaMjeoeQc5gwwa4qUG8eHau8uxOHu C+aBlQ4dMy6XcxTjXlOQd15MsbWrR4A0m7yv0pcGCLbGBfS1UW3xaI4yJngOHRv6fbA9LFH/++t LU8Z4yekS1NHl9r/kKpM++iKCCpK6GYUwpzefDQSp189+72BhNWJkW168GXFr/GqFaErqvkiYX8 vh75ZEL80UtyqdoObGxbAmj3Yd4r+1MOlEFu5hGYEbz2i3TRxBwCrBnhRNBYJS7OjXwWbpoSROQ PY30aMtKkEi/6nQ== X-Developer-Key: i=yuya.kusakabe@gmail.com; a=openpgp; fpr=DA07BBAC01FC4D28C7AA6BA62AD7D1752A5D1B4D The L2 encapsulation modes of the seg6 lwtunnel reallocate the skb head on every packet, where the IPv6 encapsulation modes reallocate only when they have to. This site only has to fit skb->mac_len before its skb_push(); the outer header that follows is already covered by __seg6_do_srh_encap()'s own skb_cow_head(). On a forwarding path neither reallocation is necessary: ixgbe leaves 206 bytes of headroom, against 14 (skb->mac_len) + 40 (ipv6hdr) + 24 (SRH with one segment) + 16 (LL_RESERVED_SPACE, via dst_dev_overhead()) = 94 for the whole encapsulation. The cost is amplified by CONFIG_INIT_ON_ALLOC_DEFAULT_ON, which many distributions enable: every new head is zeroed in full, and that memset alone accounts for 16% of the datapath profile. Use skb_cow_head() instead, matching the IPv6 encapsulation modes. Throughput at 0.5% packet loss, 64-byte frames forwarded through one 2.30 GHz core (Xeon E5-2650 v3, ixgbe 82599ES), offered by TRex and binary-searched over 10 runs of 10 s: Before: 660.7 kpps After: 991.3 kpps Assisted-by: Claude:claude-opus-5 Signed-off-by: Yuya Kusakabe --- net/ipv6/seg6_iptunnel.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/net/ipv6/seg6_iptunnel.c b/net/ipv6/seg6_iptunnel.c index 61c6a27bf202..a6556efd8e0b 100644 --- a/net/ipv6/seg6_iptunnel.c +++ b/net/ipv6/seg6_iptunnel.c @@ -446,8 +446,9 @@ static int seg6_do_srh(struct sk_buff *skb, struct dst_entry *cache_dst) if (!skb_mac_header_was_set(skb)) return -EINVAL; - if (pskb_expand_head(skb, skb->mac_len, 0, GFP_ATOMIC) < 0) - return -ENOMEM; + err = skb_cow_head(skb, skb->mac_len); + if (unlikely(err)) + return err; skb_mac_header_rebuild(skb); skb_push(skb, skb->mac_len); --- base-commit: c8ea08ba34f2a2e9bfb18ff3d69eb2d69b324f49 change-id: 20260902-seg6-l2cow-77dc3ba41232 Best regards, -- Yuya Kusakabe