Messages in this thread | | | Date | Fri, 14 Jul 2023 16:17:59 +0200 | Subject | Re: [Intel-wired-lan] [PATCH RFC net-next v4 2/9] iavf: kill "legacy-rx" for good | From | Przemek Kitszel <> |
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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@fb.com> > Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com> > --- > 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@intel.com>
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