Subject: Re: [PATCH net-next] virtio_net: migrate mergeable rx buffers to page frag allocators

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 29 Oct 2013 21:05:50 +0200

On Tue, Oct 29, 2013 at 06:44:40PM +0000, Eric Northup wrote:
> On Mon, Oct 28, 2013 at 10:44 PM, Michael Dalton <[email-masked]> wrote:
> > The virtio_net driver's mergeable receive buffer allocator
> > uses 4KB packet buffers. For MTU-sized traffic, SKB truesize
> > is > 4KB but only ~1500 bytes of the buffer is used to store
> > packet data, reducing the effective TCP window size
> > substantially. This patch addresses the performance concerns
> > with mergeable receive buffers by allocating MTU-sized packet
> > buffers using page frag allocators. If more than MAX_SKB_FRAGS
> > buffers are needed, the SKB frag_list is used.
> 
> This looks really good, just one nit below -- should we add MTU-sized
> packets, or add MTU+sizeof(virtnet_hdr)-sized packets?
> 
> Reviewed-by: Eric Northup <[email-masked]>

Yea, except it increases the typical number of packets per TSO
packet by a factor of 2.5, so it's almost sure to hurt performance
for some workloads.

Maybe the right thing to do is to pass a mix of small and big
buffers to the device. Device can pick the right size depending
on the size of the packet it has.

What to do for existing devices? I'm not sure. tcp rcv buf is at
least tunable.  I guess we could choose the buffer size depending on
default rcv buf. And/or maybe a module parameter ...

> >
> > Signed-off-by: Michael Dalton <[email-masked]>
> > ---
> >  drivers/net/virtio_net.c | 164 ++++++++++++++++++++++++++++++-----------------
> >  1 file changed, 106 insertions(+), 58 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 9fbdfcd..113ee93 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -124,6 +124,11 @@ struct virtnet_info {
> >         /* Lock for config space updates */
> >         struct mutex config_lock;
> >
> > +       /* Page_frag for GFP_KERNEL packet buffer allocation when we run
> > +        * low on memory.
> > +        */
> > +       struct page_frag alloc_frag;
> > +
> >         /* Does the affinity hint is set for virtqueues? */
> >         bool affinity_hint_set;
> >
> > @@ -217,33 +222,18 @@ static void skb_xmit_done(struct virtqueue *vq)
> >         netif_wake_subqueue(vi->dev, vq2txq(vq));
> >  }
> >
> > -static void set_skb_frag(struct sk_buff *skb, struct page *page,
> > -                        unsigned int offset, unsigned int *len)
> > -{
> > -       int size = min((unsigned)PAGE_SIZE - offset, *len);
> > -       int i = skb_shinfo(skb)->nr_frags;
> > -
> > -       __skb_fill_page_desc(skb, i, page, offset, size);
> > -
> > -       skb->data_len += size;
> > -       skb->len += size;
> > -       skb->truesize += PAGE_SIZE;
> > -       skb_shinfo(skb)->nr_frags++;
> > -       skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG;
> > -       *len -= size;
> > -}
> > -
> >  /* Called from bottom half context */
> >  static struct sk_buff *page_to_skb(struct receive_queue *rq,
> > -                                  struct page *page, unsigned int len)
> > +                                  struct page *page, unsigned int offset,
> > +                                  unsigned int len, unsigned int truesize)
> >  {
> >         struct virtnet_info *vi = rq->vq->vdev->priv;
> >         struct sk_buff *skb;
> >         struct skb_vnet_hdr *hdr;
> > -       unsigned int copy, hdr_len, offset;
> > +       unsigned int copy, hdr_len, hdr_padded_len;
> >         char *p;
> >
> > -       p = page_address(page);
> > +       p = page_address(page) + offset;
> >
> >         /* copy small packet so we can reuse these pages for small data */
> >         skb = netdev_alloc_skb_ip_align(vi->dev, GOOD_COPY_LEN);
> > @@ -254,16 +244,17 @@ static struct sk_buff *page_to_skb(struct receive_queue *rq,
> >
> >         if (vi->mergeable_rx_bufs) {
> >                 hdr_len = sizeof hdr->mhdr;
> > -               offset = hdr_len;
> > +               hdr_padded_len = sizeof hdr->mhdr;
> >         } else {
> >                 hdr_len = sizeof hdr->hdr;
> > -               offset = sizeof(struct padded_vnet_hdr);
> > +               hdr_padded_len = sizeof(struct padded_vnet_hdr);
> >         }
> >
> >         memcpy(hdr, p, hdr_len);
> >
> >         len -= hdr_len;
> > -       p += offset;
> > +       offset += hdr_padded_len;
> > +       p += hdr_padded_len;
> >
> >         copy = len;
> >         if (copy > skb_tailroom(skb))
> > @@ -273,6 +264,14 @@ static struct sk_buff *page_to_skb(struct receive_queue *rq,
> >         len -= copy;
> >         offset += copy;
> >
> > +       if (vi->mergeable_rx_bufs) {
> > +               if (len)
> > +                       skb_add_rx_frag(skb, 0, page, offset, len, truesize);
> > +               else
> > +                       put_page(page);
> > +               return skb;
> > +       }
> > +
> >         /*
> >          * Verify that we can indeed put this data into a skb.
> >          * This is here to handle cases when the device erroneously
> > @@ -284,9 +283,12 @@ static struct sk_buff *page_to_skb(struct receive_queue *rq,
> >                 dev_kfree_skb(skb);
> >                 return NULL;
> >         }
> > -
> > +       BUG_ON(offset >= PAGE_SIZE);
> >         while (len) {
> > -               set_skb_frag(skb, page, offset, &len);
> > +               unsigned int frag_size = min((unsigned)PAGE_SIZE - offset, len);
> > +               skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags, page, offset,
> > +                               frag_size, truesize);
> > +               len -= frag_size;
> >                 page = (struct page *)page->private;
> >                 offset = 0;
> >         }
> > @@ -297,33 +299,52 @@ static struct sk_buff *page_to_skb(struct receive_queue *rq,
> >         return skb;
> >  }
> >
> > -static int receive_mergeable(struct receive_queue *rq, struct sk_buff *skb)
> > +static int receive_mergeable(struct receive_queue *rq, struct sk_buff *head_skb)
> >  {
> > -       struct skb_vnet_hdr *hdr = skb_vnet_hdr(skb);
> > +       struct skb_vnet_hdr *hdr = skb_vnet_hdr(head_skb);
> > +       struct sk_buff *curr_skb = head_skb;
> > +       char *buf;
> >         struct page *page;
> > -       int num_buf, i, len;
> > +       int num_buf, len;
> >
> >         num_buf = hdr->mhdr.num_buffers;
> >         while (--num_buf) {
> > -               i = skb_shinfo(skb)->nr_frags;
> > -               if (i >= MAX_SKB_FRAGS) {
> > -                       pr_debug("%s: packet too long\n", skb->dev->name);
> > -                       skb->dev->stats.rx_length_errors++;
> > -                       return -EINVAL;
> > -               }
> > -               page = virtqueue_get_buf(rq->vq, &len);
> > -               if (!page) {
> > +               int num_skb_frags = skb_shinfo(curr_skb)->nr_frags;
> > +               buf = virtqueue_get_buf(rq->vq, &len);
> > +               if (unlikely(!buf)) {
> >                         pr_debug("%s: rx error: %d buffers missing\n",
> > -                                skb->dev->name, hdr->mhdr.num_buffers);
> > -                       skb->dev->stats.rx_length_errors++;
> > +                                head_skb->dev->name, hdr->mhdr.num_buffers);
> > +                       head_skb->dev->stats.rx_length_errors++;
> >                         return -EINVAL;
> >                 }
> > -
> > -               if (len > PAGE_SIZE)
> > -                       len = PAGE_SIZE;
> > -
> > -               set_skb_frag(skb, page, 0, &len);
> > -
> > +               if (unlikely(len > MAX_PACKET_LEN)) {
> > +                       pr_debug("%s: rx error: merge buffer too long\n",
> > +                                head_skb->dev->name);
> > +                       len = MAX_PACKET_LEN;
> > +               }
> > +               if (unlikely(num_skb_frags == MAX_SKB_FRAGS)) {
> > +                       struct sk_buff *nskb = alloc_skb(0, GFP_ATOMIC);
> > +                       if (unlikely(!nskb)) {
> > +                               head_skb->dev->stats.rx_dropped++;
> > +                               return -ENOMEM;
> > +                       }
> > +                       if (curr_skb == head_skb)
> > +                               skb_shinfo(curr_skb)->frag_list = nskb;
> > +                       else
> > +                               curr_skb->next = nskb;
> > +                       curr_skb = nskb;
> > +                       head_skb->truesize += nskb->truesize;
> > +                       num_skb_frags = 0;
> > +               }
> > +               if (curr_skb != head_skb) {
> > +                       head_skb->data_len += len;
> > +                       head_skb->len += len;
> > +                       head_skb->truesize += MAX_PACKET_LEN;
> > +               }
> > +               page = virt_to_head_page(buf);
> > +               skb_add_rx_frag(curr_skb, num_skb_frags, page,
> > +                               buf - (char *)page_address(page), len,
> > +                               MAX_PACKET_LEN);
> >                 --rq->num;
> >         }
> >         return 0;
> > @@ -341,8 +362,10 @@ static void receive_buf(struct receive_queue *rq, void *buf, unsigned int len)
> >         if (unlikely(len < sizeof(struct virtio_net_hdr) + ETH_HLEN)) {
> >                 pr_debug("%s: short packet %i\n", dev->name, len);
> >                 dev->stats.rx_length_errors++;
> > -               if (vi->mergeable_rx_bufs || vi->big_packets)
> > +               if (vi->big_packets)
> >                         give_pages(rq, buf);
> > +               else if (vi->mergeable_rx_bufs)
> > +                       put_page(virt_to_head_page(buf));
> >                 else
> >                         dev_kfree_skb(buf);
> >                 return;
> > @@ -352,19 +375,28 @@ static void receive_buf(struct receive_queue *rq, void *buf, unsigned int len)
> >                 skb = buf;
> >                 len -= sizeof(struct virtio_net_hdr);
> >                 skb_trim(skb, len);
> > +       } else if (vi->mergeable_rx_bufs) {
> > +               struct page *page = virt_to_head_page(buf);
> > +               skb = page_to_skb(rq, page,
> > +                                 (char *)buf - (char *)page_address(page),
> > +                                 len, MAX_PACKET_LEN);
> > +               if (unlikely(!skb)) {
> > +                       dev->stats.rx_dropped++;
> > +                       put_page(page);
> > +                       return;
> > +               }
> > +               if (receive_mergeable(rq, skb)) {
> > +                       dev_kfree_skb(skb);
> > +                       return;
> > +               }
> >         } else {
> >                 page = buf;
> > -               skb = page_to_skb(rq, page, len);
> > +               skb = page_to_skb(rq, page, 0, len, PAGE_SIZE);
> >                 if (unlikely(!skb)) {
> >                         dev->stats.rx_dropped++;
> >                         give_pages(rq, page);
> >                         return;
> >                 }
> > -               if (vi->mergeable_rx_bufs)
> > -                       if (receive_mergeable(rq, skb)) {
> > -                               dev_kfree_skb(skb);
> > -                               return;
> > -                       }
> >         }
> >
> >         hdr = skb_vnet_hdr(skb);
> > @@ -501,18 +533,28 @@ static int add_recvbuf_big(struct receive_queue *rq, gfp_t gfp)
> >
> >  static int add_recvbuf_mergeable(struct receive_queue *rq, gfp_t gfp)
> >  {
> > -       struct page *page;
> > +       struct virtnet_info *vi = rq->vq->vdev->priv;
> > +       char *buf = NULL;
> >         int err;
> >
> > -       page = get_a_page(rq, gfp);
> > -       if (!page)
> > +       if (gfp & __GFP_WAIT) {
> 
> Shouldn't this be MAX_PACKET_LEN + sizeof(virtio_net_hdr_mrg_rxbuf) ?
> 
> That way, we can receive an MTU-sized packet while only consuming a
> single queue entry.
> 
> > +               if (skb_page_frag_refill(MAX_PACKET_LEN, &vi->alloc_frag,
> > +                                        gfp)) {
> > +                       buf = (char *)page_address(vi->alloc_frag.page) +
> > +                             vi->alloc_frag.offset;
> > +                       get_page(vi->alloc_frag.page);
> > +                       vi->alloc_frag.offset += MAX_PACKET_LEN;
> > +               }
> > +       } else {
> > +               buf = netdev_alloc_frag(MAX_PACKET_LEN);
> > +       }
> > +       if (!buf)
> >                 return -ENOMEM;
> >
> > -       sg_init_one(rq->sg, page_address(page), PAGE_SIZE);
> > -
> > -       err = virtqueue_add_inbuf(rq->vq, rq->sg, 1, page, gfp);
> > +       sg_init_one(rq->sg, buf, MAX_PACKET_LEN);
> > +       err = virtqueue_add_inbuf(rq->vq, rq->sg, 1, buf, gfp);
> >         if (err < 0)
> > -               give_pages(rq, page);
> > +               put_page(virt_to_head_page(buf));
> >
> >         return err;
> >  }
> > @@ -1343,8 +1385,10 @@ static void free_unused_bufs(struct virtnet_info *vi)
> >                 struct virtqueue *vq = vi->rq[i].vq;
> >
> >                 while ((buf = virtqueue_detach_unused_buf(vq)) != NULL) {
> > -                       if (vi->mergeable_rx_bufs || vi->big_packets)
> > +                       if (vi->big_packets)
> >                                 give_pages(&vi->rq[i], buf);
> > +                       else if (vi->mergeable_rx_bufs)
> > +                               put_page(virt_to_head_page(buf));
> >                         else
> >                                 dev_kfree_skb(buf);
> >                         --vi->rq[i].num;
> > @@ -1650,6 +1694,8 @@ free_recv_bufs:
> >  free_vqs:
> >         cancel_delayed_work_sync(&vi->refill);
> >         virtnet_del_vqs(vi);
> > +       if (vi->alloc_frag.page)
> > +               put_page(vi->alloc_frag.page);
> >  free_index:
> >         free_percpu(vi->vq_index);
> >  free_stats:
> > @@ -1685,6 +1731,8 @@ static void virtnet_remove(struct virtio_device *vdev)
> >         unregister_netdev(vi->dev);
> >
> >         remove_vq_common(vi);
> > +       if (vi->alloc_frag.page)
> > +               put_page(vi->alloc_frag.page);
> >
> >         flush_work(&vi->config_work);
> >
> > --
> > 1.8.4.1
> >
> > _______________________________________________
> > Virtualization mailing list
> > [email-masked]
> > https://lists.linuxfoundation.org/mailman/listinfo/virtualization