Greg Kroah-Hartman
2021-Apr-12 08:39 UTC
[PATCH 5.10 055/188] virtio_net: Do not pull payload in skb->head
From: Eric Dumazet <edumazet at google.com>
commit 0f6925b3e8da0dbbb52447ca8a8b42b371aac7db upstream.
Xuan Zhuo reported that commit 3226b158e67c ("net: avoid 32 x truesize
under-estimation for tiny skbs") brought a ~10% performance drop.
The reason for the performance drop was that GRO was forced
to chain sk_buff (using skb_shinfo(skb)->frag_list), which
uses more memory but also cause packet consumers to go over
a lot of overhead handling all the tiny skbs.
It turns out that virtio_net page_to_skb() has a wrong strategy :
It allocates skbs with GOOD_COPY_LEN (128) bytes in skb->head, then
copies 128 bytes from the page, before feeding the packet to GRO stack.
This was suboptimal before commit 3226b158e67c ("net: avoid 32 x truesize
under-estimation for tiny skbs") because GRO was using 2 frags per MSS,
meaning we were not packing MSS with 100% efficiency.
Fix is to pull only the ethernet header in page_to_skb()
Then, we change virtio_net_hdr_to_skb() to pull the missing
headers, instead of assuming they were already pulled by callers.
This fixes the performance regression, but could also allow virtio_net
to accept packets with more than 128bytes of headers.
Many thanks to Xuan Zhuo for his report, and his tests/help.
Fixes: 3226b158e67c ("net: avoid 32 x truesize under-estimation for tiny
skbs")
Reported-by: Xuan Zhuo <xuanzhuo at linux.alibaba.com>
Link: https://www.spinics.net/lists/netdev/msg731397.html
Co-Developed-by: Xuan Zhuo <xuanzhuo at linux.alibaba.com>
Signed-off-by: Xuan Zhuo <xuanzhuo at linux.alibaba.com>
Signed-off-by: Eric Dumazet <edumazet at google.com>
Cc: "Michael S. Tsirkin" <mst at redhat.com>
Cc: Jason Wang <jasowang at redhat.com>
Cc: virtualization at lists.linux-foundation.org
Acked-by: Jason Wang <jasowang at redhat.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
---
drivers/net/virtio_net.c | 10 +++++++---
include/linux/virtio_net.h | 14 +++++++++-----
2 files changed, 16 insertions(+), 8 deletions(-)
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -406,9 +406,13 @@ static struct sk_buff *page_to_skb(struc
offset += hdr_padded_len;
p += hdr_padded_len;
- copy = len;
- if (copy > skb_tailroom(skb))
- copy = skb_tailroom(skb);
+ /* Copy all frame if it fits skb->head, otherwise
+ * we let virtio_net_hdr_to_skb() and GRO pull headers as needed.
+ */
+ if (len <= skb_tailroom(skb))
+ copy = len;
+ else
+ copy = ETH_HLEN + metasize;
skb_put_data(skb, p, copy);
if (metasize) {
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -65,14 +65,18 @@ static inline int virtio_net_hdr_to_skb(
skb_reset_mac_header(skb);
if (hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) {
- u16 start = __virtio16_to_cpu(little_endian, hdr->csum_start);
- u16 off = __virtio16_to_cpu(little_endian, hdr->csum_offset);
+ u32 start = __virtio16_to_cpu(little_endian, hdr->csum_start);
+ u32 off = __virtio16_to_cpu(little_endian, hdr->csum_offset);
+ u32 needed = start + max_t(u32, thlen, off + sizeof(__sum16));
+
+ if (!pskb_may_pull(skb, needed))
+ return -EINVAL;
if (!skb_partial_csum_set(skb, start, off))
return -EINVAL;
p_off = skb_transport_offset(skb) + thlen;
- if (p_off > skb_headlen(skb))
+ if (!pskb_may_pull(skb, p_off))
return -EINVAL;
} else {
/* gso packets without NEEDS_CSUM do not set transport_offset.
@@ -102,14 +106,14 @@ retry:
}
p_off = keys.control.thoff + thlen;
- if (p_off > skb_headlen(skb) ||
+ if (!pskb_may_pull(skb, p_off) ||
keys.basic.ip_proto != ip_proto)
return -EINVAL;
skb_set_transport_header(skb, keys.control.thoff);
} else if (gso_type) {
p_off = thlen;
- if (p_off > skb_headlen(skb))
+ if (!pskb_may_pull(skb, p_off))
return -EINVAL;
}
}
Michael S. Tsirkin
2021-Apr-12 09:11 UTC
[PATCH 5.10 055/188] virtio_net: Do not pull payload in skb->head
On Mon, Apr 12, 2021 at 10:39:29AM +0200, Greg Kroah-Hartman wrote:> From: Eric Dumazet <edumazet at google.com> > > commit 0f6925b3e8da0dbbb52447ca8a8b42b371aac7db upstream. > > Xuan Zhuo reported that commit 3226b158e67c ("net: avoid 32 x truesize > under-estimation for tiny skbs") brought a ~10% performance drop. > > The reason for the performance drop was that GRO was forced > to chain sk_buff (using skb_shinfo(skb)->frag_list), which > uses more memory but also cause packet consumers to go over > a lot of overhead handling all the tiny skbs. > > It turns out that virtio_net page_to_skb() has a wrong strategy : > It allocates skbs with GOOD_COPY_LEN (128) bytes in skb->head, then > copies 128 bytes from the page, before feeding the packet to GRO stack. > > This was suboptimal before commit 3226b158e67c ("net: avoid 32 x truesize > under-estimation for tiny skbs") because GRO was using 2 frags per MSS, > meaning we were not packing MSS with 100% efficiency. > > Fix is to pull only the ethernet header in page_to_skb() > > Then, we change virtio_net_hdr_to_skb() to pull the missing > headers, instead of assuming they were already pulled by callers. > > This fixes the performance regression, but could also allow virtio_net > to accept packets with more than 128bytes of headers. > > Many thanks to Xuan Zhuo for his report, and his tests/help. > > Fixes: 3226b158e67c ("net: avoid 32 x truesize under-estimation for tiny skbs") > Reported-by: Xuan Zhuo <xuanzhuo at linux.alibaba.com> > Link: https://www.spinics.net/lists/netdev/msg731397.html > Co-Developed-by: Xuan Zhuo <xuanzhuo at linux.alibaba.com> > Signed-off-by: Xuan Zhuo <xuanzhuo at linux.alibaba.com> > Signed-off-by: Eric Dumazet <edumazet at google.com> > Cc: "Michael S. Tsirkin" <mst at redhat.com> > Cc: Jason Wang <jasowang at redhat.com> > Cc: virtualization at lists.linux-foundation.org > Acked-by: Jason Wang <jasowang at redhat.com> > Signed-off-by: David S. Miller <davem at davemloft.net> > Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>Note that an issue related to this patch was recently reported. It's quite possible that the root cause is a bug elsewhere in the kernel, but it probably makes sense to defer the backport until we know more ...> --- > drivers/net/virtio_net.c | 10 +++++++--- > include/linux/virtio_net.h | 14 +++++++++----- > 2 files changed, 16 insertions(+), 8 deletions(-) > > --- a/drivers/net/virtio_net.c > +++ b/drivers/net/virtio_net.c > @@ -406,9 +406,13 @@ static struct sk_buff *page_to_skb(struc > offset += hdr_padded_len; > p += hdr_padded_len; > > - copy = len; > - if (copy > skb_tailroom(skb)) > - copy = skb_tailroom(skb); > + /* Copy all frame if it fits skb->head, otherwise > + * we let virtio_net_hdr_to_skb() and GRO pull headers as needed. > + */ > + if (len <= skb_tailroom(skb)) > + copy = len; > + else > + copy = ETH_HLEN + metasize; > skb_put_data(skb, p, copy); > > if (metasize) { > --- a/include/linux/virtio_net.h > +++ b/include/linux/virtio_net.h > @@ -65,14 +65,18 @@ static inline int virtio_net_hdr_to_skb( > skb_reset_mac_header(skb); > > if (hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) { > - u16 start = __virtio16_to_cpu(little_endian, hdr->csum_start); > - u16 off = __virtio16_to_cpu(little_endian, hdr->csum_offset); > + u32 start = __virtio16_to_cpu(little_endian, hdr->csum_start); > + u32 off = __virtio16_to_cpu(little_endian, hdr->csum_offset); > + u32 needed = start + max_t(u32, thlen, off + sizeof(__sum16)); > + > + if (!pskb_may_pull(skb, needed)) > + return -EINVAL; > > if (!skb_partial_csum_set(skb, start, off)) > return -EINVAL; > > p_off = skb_transport_offset(skb) + thlen; > - if (p_off > skb_headlen(skb)) > + if (!pskb_may_pull(skb, p_off)) > return -EINVAL; > } else { > /* gso packets without NEEDS_CSUM do not set transport_offset. > @@ -102,14 +106,14 @@ retry: > } > > p_off = keys.control.thoff + thlen; > - if (p_off > skb_headlen(skb) || > + if (!pskb_may_pull(skb, p_off) || > keys.basic.ip_proto != ip_proto) > return -EINVAL; > > skb_set_transport_header(skb, keys.control.thoff); > } else if (gso_type) { > p_off = thlen; > - if (p_off > skb_headlen(skb)) > + if (!pskb_may_pull(skb, p_off)) > return -EINVAL; > } > } >