netdev - Re: [PATCH net-next v5 1/4] virtio_ring: enable premapped mode whatever use_dma

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Message-ID: <1724134768.1456072-1-xuanzhuo@linux.alibaba.com>
Date: Tue, 20 Aug 2024 14:19:28 +0800
From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To: "Si-Wei Liu" <si-wei.liu@...cle.com>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>,
 Paolo Abeni <pabeni@...hat.com>,
 virtualization@...ts.linux.dev,
 Darren Kenny <darren.kenny@...cle.com>,
 Boris Ostrovsky <boris.ostrovsky@...cle.com>,
 netdev@...r.kernel.org,
 Jason Wang <jasowang@...hat.com>,
 Jakub Kicinski <kuba@...nel.org>,
 "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH net-next v5 1/4] virtio_ring: enable premapped mode whatever use_dma_api

On Mon, 19 Aug 2024 18:06:07 -0700, "Si-Wei Liu" <si-wei.liu@...cle.com> wrote:
> Hi,
>
> May I know if this is really an intended fix to post officially, or just
> a workaround/probe to make the offset in page_frag happy when
> net_high_order_alloc_disable is true? In case it's the former, even
> though this could fix the issue, I would assume clamping to a smaller
> page_frag than a regular page size for every buffer may have certain
> performance regression for the merge-able buffer case? Can you justify
> the performance impact with some benchmark runs with larger MTU and
> merge-able rx buffers to prove the regression is negligible? You would
> need to compare against where you don't have the inadvertent
> virtnet_rq_dma cost on any page i.e. getting all 4 patches of this
> series reverted. Both tests with net_high_order_alloc_disable set to on
> and off are needed.


I will post a PATCH, let we discuss under that.

Thanks.

>
> Thanks,
> -Siwei
>
> On 8/17/2024 6:20 AM, Xuan Zhuo wrote:
> > Hi, guys, I have a fix patch for this.
> > Could anybody test it?
> >
> > Thanks.
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index af474cc191d0..426d68c2d01d 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -2492,13 +2492,15 @@ static unsigned int get_mergeable_buf_len(struct receive_queue *rq,
> >   {
> >          struct virtnet_info *vi = rq->vq->vdev->priv;
> >          const size_t hdr_len = vi->hdr_len;
> > -       unsigned int len;
> > +       unsigned int len, max_len;
> > +
> > +       max_len = PAGE_SIZE - ALIGN(sizeof(struct virtnet_rq_dma), L1_CACHE_BYTES);
> >
> >          if (room)
> > -               return PAGE_SIZE - room;
> > +               return max_len - room;
> >
> >          len = hdr_len + clamp_t(unsigned int, ewma_pkt_len_read(avg_pkt_len),
> > -                               rq->min_buf_len, PAGE_SIZE - hdr_len);
> > +                               rq->min_buf_len, max_len - hdr_len);
> >
> >          return ALIGN(len, L1_CACHE_BYTES);
> >   }
>