On Wed, May 06, 2020 at 10:37:57AM +0200, Jesper Dangaard Brouer wrote: > On Wed, 6 May 2020 04:08:27 -0400 > "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > > > So for mergeable bufs, we use ewma machinery to guess the correct buffer > > size. If we don't guess correctly, XDP has to do aggressive copies. > > > > Problem is, xdp paths do not update the ewma at all, except > > sometimes with XDP_PASS. So whatever we happen to have > > before we attach XDP, will mostly stay around. > > > > The fix is probably to update ewma unconditionally. > > I personally find the code hard to follow, and (I admit) that it took > me some time to understand this code path (so I might still be wrong). > > In patch[1] I tried to explain (my understanding): > > In receive_mergeable() the frame size is more dynamic. There are two > basic cases: (1) buffer size is based on a exponentially weighted > moving average (see DECLARE_EWMA) of packet length. Or (2) in case > virtnet_get_headroom() have any headroom then buffer size is > PAGE_SIZE. The ctx pointer is this time used for encoding two values; > the buffer len "truesize" and headroom. In case (1) if the rx buffer > size is underestimated, the packet will have been split over more > buffers (num_buf info in virtio_net_hdr_mrg_rxbuf placed in top of > buffer area). If that happens the XDP path does a xdp_linearize_page > operation. > > > The EWMA code is not used when headroom is defined, which e.g. gets > enabled when running XDP. > > > [1] https://lore.kernel.org/netdev/158824572816.2172139.1358700000273697123.stgit@firesoul/ You are right. So I guess the problem is just inconsistency? When XDP program returns XDP_PASS, and it all fits in one page, then we trigger ewma_pkt_len_add(&rq->mrg_avg_pkt_len, head_skb->len); if it does not trigger XDP_PASS, or does not fit in one page, then we don't. Given XDP does not use ewma for sizing, let's not update the average either. > -- > Best regards, > Jesper Dangaard Brouer > MSc.CS, Principal Kernel Engineer at Red Hat > LinkedIn: http://www.linkedin.com/in/brouer
