netdev - Re: performance bug in virtio net xdp

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Message-ID: <7d801479-5572-0031-b306-a735ca4ce0e4@redhat.com>
Date:   Wed, 6 May 2020 16:37:41 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eugenio Perez Martin <eperezma@...hat.com>
Subject: Re: performance bug in virtio net xdp


On 2020/5/6 下午4:08, Michael S. Tsirkin 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.


It looks ok to me since we always use PAGE_SIZE when XDP is enabled in 
get_mergeable_buf_len()?

Thanks


>
> The fix is probably to update ewma unconditionally.
>