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/ -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
