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Date: Sat, 16 Nov 2019 13:01:01 -0800 (PST) From: David Miller <davem@...emloft.net> To: alobakin@...nk.ru Cc: ecree@...arflare.com, jiri@...lanox.com, edumazet@...gle.com, idosch@...lanox.com, pabeni@...hat.com, petrm@...lanox.com, sd@...asysnail.net, f.fainelli@...il.com, jaswinder.singh@...aro.org, manishc@...vell.com, GR-Linux-NIC-Dev@...vell.com, johannes.berg@...el.com, emmanuel.grumbach@...el.com, luciano.coelho@...el.com, linuxwifi@...el.com, kvalo@...eaurora.org, netdev@...r.kernel.org, linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH v2 net-next] net: core: allow fast GRO for skbs with Ethernet header in head From: Alexander Lobakin <alobakin@...nk.ru> Date: Fri, 15 Nov 2019 12:11:35 +0300 > Commit 78d3fd0b7de8 ("gro: Only use skb_gro_header for completely > non-linear packets") back in May'09 (v2.6.31-rc1) has changed the > original condition '!skb_headlen(skb)' to > 'skb->mac_header == skb->tail' in gro_reset_offset() saying: "Since > the drivers that need this optimisation all provide completely > non-linear packets" (note that this condition has become the current > 'skb_mac_header(skb) == skb_tail_pointer(skb)' later with commmit > ced14f6804a9 ("net: Correct comparisons and calculations using > skb->tail and skb-transport_header") without any functional changes). > > For now, we have the following rough statistics for v5.4-rc7: > 1) napi_gro_frags: 14 > 2) napi_gro_receive with skb->head containing (most of) payload: 83 > 3) napi_gro_receive with skb->head containing all the headers: 20 > 4) napi_gro_receive with skb->head containing only Ethernet header: 2 > > With the current condition, fast GRO with the usage of > NAPI_GRO_CB(skb)->frag0 is available only in the [1] case. > Packets pushed by [2] and [3] go through the 'slow' path, but > it's not a problem for them as they already contain all the needed > headers in skb->head, so pskb_may_pull() only moves skb->data. > > The layout of skbs in the fourth [4] case at the moment of > dev_gro_receive() is identical to skbs that have come through [1], > as napi_frags_skb() pulls Ethernet header to skb->head. The only > difference is that the mentioned condition is always false for them, > because skb_put() and friends irreversibly alter the tail pointer. > They also go through the 'slow' path, but now every single > pskb_may_pull() in every single .gro_receive() will call the *really* > slow __pskb_pull_tail() to pull headers to head. This significantly > decreases the overall performance for no visible reasons. ... > Signed-off-by: Alexander Lobakin <alobakin@...nk.ru> Applied to net-next, thanks.
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