On 17-01-23 12:09 PM, Michael S. Tsirkin wrote: > On Mon, Jan 23, 2017 at 09:22:36PM +0200, Michael S. Tsirkin wrote: >> On Tue, Jan 17, 2017 at 02:22:59PM -0800, John Fastabend wrote: >>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c >>> index 62dbf4b..3b129b4 100644 >>> --- a/drivers/net/virtio_net.c >>> +++ b/drivers/net/virtio_net.c >>> @@ -41,6 +41,9 @@ >>> #define GOOD_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN) >>> #define GOOD_COPY_LEN 128 >>> >>> +/* Amount of XDP headroom to prepend to packets for use by xdp_adjust_head >>> */ >>> +#define VIRTIO_XDP_HEADROOM 256 >>> + >>> /* RX packet size EWMA. The average packet size is used to determine the >>> packet >>> * buffer size when refilling RX rings. As the entire RX ring may be >>> refilled >>> * at once, the weight is chosen so that the EWMA will be insensitive to >>> short- >> >> I wonder where does this number come from? This is quite a lot and >> means that using XDP_PASS will slow down any sockets on top of it. >> Which in turn means people will try to remove XDP when not in use, >> causing resets. E.g. build_skb (which I have a patch to switch to) uses >> a much more reasonable NET_SKB_PAD.
I just used the value Alexei (or someone?) came up with. I think it needs to be large enough to avoid copy in header encap cases. So minimum VXLAN_HDR + OUTER_UDP + OUTER_IPV6_HDR + OUTER_MAC = 8 + 8 + 40 + 14 = 70 The choice of VXLAN hdr was sort of arbitrary but seems good for estimates. For what its worth there is also a ndo_set_rx_headroom could we use that to set it and choose a reasonable default. >> >> -- >> MST > > > Let me show you a patch that I've been cooking. What is missing there > is handling corner cases like e.g. when ring size is ~4 entries so > using smaller buffers might mean we no longer have enough space to store > a full packet. So it looks like I have to maintain the skb copy path > for this hardware. > > With this patch, standard configuration has NET_SKB_PAD + NET_IP_ALIGN > bytes head padding. Would this be enough for XDP? If yes we do not > need the resets. Based on above seems a bit small (L1_CACHE_BYTES + 2)? How tricky would it be to add support for ndo_set_rx_headroom. > > Thoughts? I'll take a look at the patch this afternoon. Thanks. > > ---> > > virtio_net: switch to build_skb for mrg_rxbuf > > For small packets data copy was observed to > take up about 15% CPU time. Switch to build_skb > and avoid the copy when using mergeable rx buffers. > > As a bonus, medium-size skbs that fit in a page will be > completely linear. > > Of course, we now need to lower the lower bound on packet size, > to make sure a sane number of skbs fits in rx socket buffer. > By how much? I don't know yet. > > It might also be useful to prefetch the packet buffer since > net stack will likely use it soon. > > Lightly tested, in particular, I didn't yet test what this > actually does to performance - sending this out for early > feedback/flames. > > TODO: it appears that Linux won't handle correctly the case of first > buffer being very small (or consisting exclusively of virtio header). > This is already the case for current code, need to fix. > TODO: might be unfair to the last packet in a fragment as we include > remaining space if any in its truesize. > > Signed-off-by: Michael S. Tsirkin <m...@redhat.com> > > ---- > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c > index b425fa1..a6b996f 100644 > --- a/drivers/net/virtio_net.c > +++ b/drivers/net/virtio_net.c > @@ -38,6 +38,8 @@ module_param(gso, bool, 0444); > > /* FIXME: MTU in config. */ > #define GOOD_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN) > +//#define MIN_PACKET_ALLOC GOOD_PACKET_LEN > +#define MIN_PACKET_ALLOC 128 > #define GOOD_COPY_LEN 128 > > /* RX packet size EWMA. The average packet size is used to determine the > packet > @@ -246,6 +248,9 @@ static void *mergeable_ctx_to_buf_address(unsigned long > mrg_ctx) > static unsigned long mergeable_buf_to_ctx(void *buf, unsigned int truesize) > { > unsigned int size = truesize / MERGEABLE_BUFFER_ALIGN; > + > + BUG_ON((unsigned long)buf & (MERGEABLE_BUFFER_ALIGN - 1)); > + BUG_ON(size - 1 >= MERGEABLE_BUFFER_ALIGN); > return (unsigned long)buf | (size - 1); > } > > @@ -354,25 +359,54 @@ static struct sk_buff *receive_big(struct net_device > *dev, > return NULL; > } > > +#define VNET_SKB_PAD (NET_SKB_PAD + NET_IP_ALIGN) > +#define VNET_SKB_BUG (VNET_SKB_PAD < sizeof(struct virtio_net_hdr_mrg_rxbuf)) > +#define VNET_SKB_LEN(len) ((len) - sizeof(struct virtio_net_hdr_mrg_rxbuf)) > +#define VNET_SKB_OFF VNET_SKB_LEN(VNET_SKB_PAD) > + > +static struct sk_buff *vnet_build_skb(struct virtnet_info *vi, > + void *buf, > + unsigned int len, unsigned int truesize) > +{ > + struct sk_buff *skb = build_skb(buf, truesize); > + > + if (!skb) > + return NULL; > + > + skb_reserve(skb, VNET_SKB_PAD); > + skb_put(skb, VNET_SKB_LEN(len)); > + > + return skb; > +} > + > static struct sk_buff *receive_mergeable(struct net_device *dev, > struct virtnet_info *vi, > struct receive_queue *rq, > unsigned long ctx, > - unsigned int len) > + unsigned int len, > + struct virtio_net_hdr_mrg_rxbuf *hdr) > { > void *buf = mergeable_ctx_to_buf_address(ctx); > - struct virtio_net_hdr_mrg_rxbuf *hdr = buf; > - u16 num_buf = virtio16_to_cpu(vi->vdev, hdr->num_buffers); > + u16 num_buf; > struct page *page = virt_to_head_page(buf); > - int offset = buf - page_address(page); > - unsigned int truesize = max(len, mergeable_ctx_to_buf_truesize(ctx)); > + unsigned int truesize = mergeable_ctx_to_buf_truesize(ctx); > + int offset; > + struct sk_buff *head_skb; > + struct sk_buff *curr_skb; > + > + BUG_ON(len > truesize); > > - struct sk_buff *head_skb = page_to_skb(vi, rq, page, offset, len, > - truesize); > - struct sk_buff *curr_skb = head_skb; > + /* copy header: build_skb will overwrite it */ > + memcpy(hdr, buf + VNET_SKB_OFF, sizeof *hdr); > + > + head_skb = vnet_build_skb(vi, buf, len, truesize); > + curr_skb = head_skb; > + > + num_buf = virtio16_to_cpu(vi->vdev, hdr->num_buffers); > > if (unlikely(!curr_skb)) > goto err_skb; > + > while (--num_buf) { > int num_skb_frags; > > @@ -386,7 +420,7 @@ static struct sk_buff *receive_mergeable(struct > net_device *dev, > goto err_buf; > } > > - buf = mergeable_ctx_to_buf_address(ctx); > + buf = mergeable_ctx_to_buf_address(ctx) + VNET_SKB_OFF; > page = virt_to_head_page(buf); > > num_skb_frags = skb_shinfo(curr_skb)->nr_frags; > @@ -403,7 +437,8 @@ static struct sk_buff *receive_mergeable(struct > net_device *dev, > head_skb->truesize += nskb->truesize; > num_skb_frags = 0; > } > - truesize = max(len, mergeable_ctx_to_buf_truesize(ctx)); > + truesize = mergeable_ctx_to_buf_truesize(ctx); > + BUG_ON(len > truesize); > if (curr_skb != head_skb) { > head_skb->data_len += len; > head_skb->len += len; > @@ -449,6 +484,7 @@ static void receive_buf(struct virtnet_info *vi, struct > receive_queue *rq, > struct virtnet_stats *stats = this_cpu_ptr(vi->stats); > struct sk_buff *skb; > struct virtio_net_hdr_mrg_rxbuf *hdr; > + struct virtio_net_hdr_mrg_rxbuf hdr0; > > if (unlikely(len < vi->hdr_len + ETH_HLEN)) { > pr_debug("%s: short packet %i\n", dev->name, len); > @@ -465,17 +501,24 @@ static void receive_buf(struct virtnet_info *vi, struct > receive_queue *rq, > return; > } > > - if (vi->mergeable_rx_bufs) > - skb = receive_mergeable(dev, vi, rq, (unsigned long)buf, len); > - else if (vi->big_packets) > + if (vi->mergeable_rx_bufs) { > + skb = receive_mergeable(dev, vi, rq, (unsigned long)buf, len, > + &hdr0); > + if (unlikely(!skb)) > + return; > + hdr = &hdr0; > + } else if (vi->big_packets) { > skb = receive_big(dev, vi, rq, buf, len); > - else > + if (unlikely(!skb)) > + return; > + hdr = skb_vnet_hdr(skb); > + } else { > skb = receive_small(vi, buf, len); > + if (unlikely(!skb)) > + return; > + hdr = skb_vnet_hdr(skb); > + } > > - if (unlikely(!skb)) > - return; > - > - hdr = skb_vnet_hdr(skb); > > u64_stats_update_begin(&stats->rx_syncp); > stats->rx_bytes += skb->len; > @@ -581,11 +624,14 @@ static int add_recvbuf_big(struct virtnet_info *vi, > struct receive_queue *rq, > > static unsigned int get_mergeable_buf_len(struct ewma_pkt_len *avg_pkt_len) > { > - const size_t hdr_len = sizeof(struct virtio_net_hdr_mrg_rxbuf); > + unsigned int hdr; > unsigned int len; > > - len = hdr_len + clamp_t(unsigned int, ewma_pkt_len_read(avg_pkt_len), > - GOOD_PACKET_LEN, PAGE_SIZE - hdr_len); > + hdr = ALIGN(VNET_SKB_PAD + SKB_DATA_ALIGN(sizeof(struct > skb_shared_info)), > + MERGEABLE_BUFFER_ALIGN); > + > + len = hdr + clamp_t(unsigned int, ewma_pkt_len_read(avg_pkt_len), > + MIN_PACKET_ALLOC, PAGE_SIZE - hdr); > return ALIGN(len, MERGEABLE_BUFFER_ALIGN); > } > > @@ -601,8 +647,11 @@ static int add_recvbuf_mergeable(struct receive_queue > *rq, gfp_t gfp) > if (unlikely(!skb_page_frag_refill(len, alloc_frag, gfp))) > return -ENOMEM; > > - buf = (char *)page_address(alloc_frag->page) + alloc_frag->offset; > - ctx = mergeable_buf_to_ctx(buf, len); > + BUILD_BUG_ON(VNET_SKB_BUG); > + > + buf = (char *)page_address(alloc_frag->page) + alloc_frag->offset + > + VNET_SKB_OFF; > + //ctx = mergeable_buf_to_ctx(buf - VNET_SKB_OFF, len); > get_page(alloc_frag->page); > alloc_frag->offset += len; > hole = alloc_frag->size - alloc_frag->offset; > @@ -615,8 +664,10 @@ static int add_recvbuf_mergeable(struct receive_queue > *rq, gfp_t gfp) > len += hole; > alloc_frag->offset += hole; > } > + ctx = mergeable_buf_to_ctx(buf - VNET_SKB_OFF, len); > > - sg_init_one(rq->sg, buf, len); > + sg_init_one(rq->sg, buf, > + len - VNET_SKB_OFF - SKB_DATA_ALIGN(sizeof(struct > skb_shared_info))); > err = virtqueue_add_inbuf(rq->vq, rq->sg, 1, (void *)ctx, gfp); > if (err < 0) > put_page(virt_to_head_page(buf)); >