Re: [PATCH v2 net-next] net: core: allow fast GRO for skbs with Ethernet header in head

[David Miller <davem@davemloft.net>]

From: Alexander Lobakin <alobakin@dlink.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@dlink.ru>

Applied to net-next, thanks.