linux-kernel - Re: performance bug in virtio net xdp

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Date:   Wed, 6 May 2020 07:57:41 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...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 Wed, May 06, 2020 at 04:37:41PM +0800, Jason Wang wrote:
> 
> 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
> 

Oh right. Good point! Answered in another thread.

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