Re: [Intel-wired-lan] [PATCH RFC net-next v4 2/9] iavf: kill "legacy-rx" for good

[Przemek Kitszel]

On 7/5/23 17:55, Alexander Lobakin wrote:
> Ever since build_skb() became stable, the old way with allocating an skb
> for storing the headers separately, which will be then copied manually,
> was slower, less flexible and thus obsolete.
>
> * it had higher pressure on MM since it actually allocates new pages,
>    which then get split and refcount-biased (NAPI page cache);
> * it implies memcpy() of packet headers (40+ bytes per each frame);
> * the actual header length was calculated via eth_get_headlen(), which
>    invokes Flow Dissector and thus wastes a bunch of CPU cycles;
> * XDP makes it even more weird since it requires headroom for long and
>    also tailroom for some time (since mbuf landed). Take a look at the
>    ice driver, which is built around work-arounds to make XDP work with
>    it.
>
> Even on some quite low-end hardware (not a common case for 100G NICs) it
> was performing worse.
> The only advantage "legacy-rx" had is that it didn't require any
> reserved headroom and tailroom. But iavf didn't use this, as it always
> splits pages into two halves of 2k, while that save would only be useful
> when striding. And again, XDP effectively removes that sole pro.
>
> There's a train of features to land in IAVF soon: Page Pool, XDP, XSk,
> multi-buffer etc. Each new would require adding more and more Danse
> Macabre for absolutely no reason, besides making hotpath less and less
> effective.
> Remove the "feature" with all the related code. This includes at least
> one very hot branch (typically hit on each new frame), which was either
> always-true or always-false at least for a complete NAPI bulk of 64
> frames, the whole private flags cruft, and so on. Some stats:
>
> Function: add/remove: 0/4 grow/shrink: 0/7 up/down: 0/-757 (-757)
> RO Data: add/remove: 0/1 grow/shrink: 0/0 up/down: 0/-40 (-40)
>
> Reviewed-by: Alexander Duyck <alexanderduyck@xxxxxx>
> Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
> ---
>   drivers/net/ethernet/intel/iavf/iavf.h        |   2 +-
>   .../net/ethernet/intel/iavf/iavf_ethtool.c    | 140 ------------------
>   drivers/net/ethernet/intel/iavf/iavf_main.c   |  10 +-
>   drivers/net/ethernet/intel/iavf/iavf_txrx.c   |  84 +----------
>   drivers/net/ethernet/intel/iavf/iavf_txrx.h   |  27 +---
>   .../net/ethernet/intel/iavf/iavf_virtchnl.c   |   3 +-
>   6 files changed, 8 insertions(+), 258 deletions(-)

Good one, there were some random questions in my mind during the read,
but all are resolved by subsequent patches.
(It's a pity I have not found yet time to fully read them though)

Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@xxxxxxxxx>