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

[Alexander Lobakin]

David Miller wrote 17.11.2019 00:01:

> From: Alexander Lobakin <alobakin@xxxxxxxx>
> 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@xxxxxxxx>
>
> Applied to net-next, thanks.

Thank you!

Regards,
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