Re: [RFC] defer skb allocation in virtio_net -- mergable buff part
From: Michael S. Tsirkin
Date: Tue Aug 25 2009 - 07:43:24 EST
Wanted to try this awhile now, good to see this worked on!
Some comments inline.
On Wed, Aug 12, 2009 at 11:33:51PM -0700, Shirley Ma wrote:
> Guest virtio_net receives packets from its pre-allocated vring
> buffers, then it delivers these packets to upper layer protocols
> as skb buffs. So it's not necessary to pre-allocate skb for each
> mergable buffer, then frees it when it's useless.
>
> This patch has deferred skb allocation to when receiving packets,
> it reduces skb pre-allocations and skb_frees. And it induces two
> page list: freed_pages and used_page list, used_pages is used to
> track pages pre-allocated, it is only useful when removing virtio_net.
>
> This patch has tested and measured against 2.6.31-rc4 git,
> I thought this patch will improve large packet performance, but I saw
> netperf TCP_STREAM performance improved for small packet for both
> local guest to host and host to local guest cases. It also reduces
> UDP packets drop rate from host to local guest. I am not fully understand
> why.
>
> The netperf results from my laptop are:
>
> mtu=1500
> netperf -H xxx -l 120
>
> w/o patch w/i patch (two runs)
> guest to host: 3336.84Mb/s 3730.14Mb/s ~ 3582.88Mb/s
>
> host to guest: 3165.10Mb/s 3370.39Mb/s ~ 3407.96Mb/s
>
> Here is the patch for your review. The same approach can apply to non-mergable
> buffs too, so we can use code in common.
Yes, we should do that. The way to test is to hack virtio to ignore
host support for mergeable buffers.
> If there is no objection, I will
> submit the non-mergable buffs patch later.
>
The whole page cache management in virtio net is too complex.
Since it seems you bypass it for some cases, this might be where
savings come from?
> Signed-off-by: Shirley Ma <xma@xxxxxxxxxx>
> ---
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 2a6e81d..e31ebc9 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -17,6 +17,7 @@
> * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
> */
> //#define DEBUG
> +#include <linux/list.h>
> #include <linux/netdevice.h>
> #include <linux/etherdevice.h>
> #include <linux/ethtool.h>
> @@ -39,6 +40,12 @@ module_param(gso, bool, 0444);
>
> #define VIRTNET_SEND_COMMAND_SG_MAX 2
>
> +struct page_list
> +{
Kernel style is "struct page_list {".
Also, prefix with virtnet_?
> + struct page *page;
> + struct list_head list;
> +};
> +
> struct virtnet_info
> {
> struct virtio_device *vdev;
> @@ -72,6 +79,8 @@ struct virtnet_info
>
> /* Chain pages by the private ptr. */
> struct page *pages;
Do we need the pages list now? Can we do without?
Pls document fields below.
> + struct list_head used_pages;
Seems a waste to have this list just for dev down.
Extend virtio to give us all buffers from vq
on shutdown?
> + struct list_head freed_pages;
> };
>
> static inline void *skb_vnet_hdr(struct sk_buff *skb)
> @@ -106,6 +115,26 @@ static struct page *get_a_page(struct virtnet_info *vi, gfp_t gfp_mask)
> return p;
> }
>
> +static struct page_list *get_a_free_page(struct virtnet_info *vi, gfp_t gfp_mask)
always called with gfp_mask GFP_ATOMIC?
Also - line too long here and elsewere. Run checkpatch?
> +{
> + struct page_list *plist;
> +
> + if (list_empty(&vi->freed_pages)) {
special handling for empty list looks a bit ugly
really necessary?
> + plist = kmalloc(sizeof(struct page_list), gfp_mask);
sizeof *plist
> + if (!plist)
> + return NULL;
> + list_add_tail(&plist->list, &vi->freed_pages);
> + plist->page = alloc_page(gfp_mask);
> + } else {
> + plist = list_first_entry(&vi->freed_pages, struct page_list, list);
is the first entry special? pls document in struct definition
> + if (!plist->page)
> + plist->page = alloc_page(gfp_mask);
what does it mean plist->page == NULL? Please document this
in struct definition.
> + }
> + if (plist->page)
> + list_move_tail(&plist->list, &vi->used_pages);
> + return plist;
> +}
> +
> static void skb_xmit_done(struct virtqueue *svq)
> {
> struct virtnet_info *vi = svq->vdev->priv;
> @@ -121,14 +150,14 @@ static void skb_xmit_done(struct virtqueue *svq)
> tasklet_schedule(&vi->tasklet);
> }
>
> -static void receive_skb(struct net_device *dev, struct sk_buff *skb,
> +static void receive_skb(struct net_device *dev, void *buf,
what is buf, here? can it have proper type and not void*?
> unsigned len)
> {
> struct virtnet_info *vi = netdev_priv(dev);
> - struct virtio_net_hdr *hdr = skb_vnet_hdr(skb);
> int err;
> int i;
> -
> + struct sk_buff *skb = NULL;
> + struct virtio_net_hdr *hdr = NULL;
do we have to init these to NULL?
> 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++;
> @@ -136,15 +165,30 @@ static void receive_skb(struct net_device *dev, struct sk_buff *skb,
> }
>
> if (vi->mergeable_rx_bufs) {
> - struct virtio_net_hdr_mrg_rxbuf *mhdr = skb_vnet_hdr(skb);
> + struct virtio_net_hdr_mrg_rxbuf *mhdr;
> unsigned int copy;
> - char *p = page_address(skb_shinfo(skb)->frags[0].page);
> + skb_frag_t *f;
> + struct page_list *plist = (struct page_list *)buf;
cast not necessary
> + char *p = page_address(plist->page);
> +
> + skb = netdev_alloc_skb(vi->dev, GOOD_COPY_LEN + NET_IP_ALIGN);
> + if (unlikely(!skb)) {
> + /* drop the packet */
obvious, but okay ...
> + dev->stats.rx_dropped++;
but what does the below do? Maybe add a comment ...
> + list_move_tail(&plist->list, &vi->freed_pages);
> + return;
> + }
> +
> + skb_reserve(skb, NET_IP_ALIGN);
>
> if (len > PAGE_SIZE)
> len = PAGE_SIZE;
> len -= sizeof(struct virtio_net_hdr_mrg_rxbuf);
>
> - memcpy(hdr, p, sizeof(*mhdr));
> + mhdr = skb_vnet_hdr(skb);
> + memcpy(mhdr, p, sizeof(*mhdr));
> + hdr = (struct virtio_net_hdr *)mhdr;
> +
> p += sizeof(*mhdr);
>
> copy = len;
> @@ -155,20 +199,20 @@ static void receive_skb(struct net_device *dev, struct sk_buff *skb,
>
> len -= copy;
>
> - if (!len) {
> - give_a_page(vi, skb_shinfo(skb)->frags[0].page);
> - skb_shinfo(skb)->nr_frags--;
> - } else {
> - skb_shinfo(skb)->frags[0].page_offset +=
> + if (len) {
> + f = &skb_shinfo(skb)->frags[0];
> + f->page = plist->page;
> + skb_shinfo(skb)->frags[0].page_offset =
> sizeof(*mhdr) + copy;
> skb_shinfo(skb)->frags[0].size = len;
> skb->data_len += len;
> skb->len += len;
> + skb_shinfo(skb)->nr_frags++;
> + plist->page = NULL;
> }
> + list_move_tail(&plist->list, &vi->freed_pages);
>
> while (--mhdr->num_buffers) {
> - struct sk_buff *nskb;
> -
> i = skb_shinfo(skb)->nr_frags;
> if (i >= MAX_SKB_FRAGS) {
> pr_debug("%s: packet too long %d\n", dev->name,
> @@ -177,30 +221,30 @@ static void receive_skb(struct net_device *dev, struct sk_buff *skb,
> goto drop;
> }
>
> - nskb = vi->rvq->vq_ops->get_buf(vi->rvq, &len);
> - if (!nskb) {
> + plist = vi->rvq->vq_ops->get_buf(vi->rvq, &len);
> + if (!plist) {
> pr_debug("%s: rx error: %d buffers missing\n",
> dev->name, mhdr->num_buffers);
> dev->stats.rx_length_errors++;
> goto drop;
> }
> -
> - __skb_unlink(nskb, &vi->recv);
> vi->num--;
> -
> - skb_shinfo(skb)->frags[i] = skb_shinfo(nskb)->frags[0];
> - skb_shinfo(nskb)->nr_frags = 0;
> - kfree_skb(nskb);
> -
> + f = &skb_shinfo(skb)->frags[i];
> + f->page = plist->page;
> + f->page_offset = 0;
I though skb is pre-zeroed anyway. No?
> +
> if (len > PAGE_SIZE)
> len = PAGE_SIZE;
> -
please don't change empty lines arbitrarily
> skb_shinfo(skb)->frags[i].size = len;
> skb_shinfo(skb)->nr_frags++;
> skb->data_len += len;
> skb->len += len;
> + plist->page = NULL;
> + list_move_tail(&plist->list, &vi->freed_pages);
did we set all fields here? how about truesize?
Does skb_add_rx_frag do what we want by any chance?
> }
> } else {
> + skb = (struct sk_buff *)buf;
don't cast away void
> + hdr = skb_vnet_hdr(skb);
> len -= sizeof(struct virtio_net_hdr);
>
> if (len <= MAX_PACKET_LEN)
> @@ -329,7 +373,6 @@ static void try_fill_recv_maxbufs(struct virtnet_info *vi)
>
> static void try_fill_recv(struct virtnet_info *vi)
> {
> - struct sk_buff *skb;
> struct scatterlist sg[1];
> int err;
>
> @@ -339,39 +382,21 @@ static void try_fill_recv(struct virtnet_info *vi)
> }
>
> for (;;) {
> - skb_frag_t *f;
> -
> - skb = netdev_alloc_skb(vi->dev, GOOD_COPY_LEN + NET_IP_ALIGN);
> - if (unlikely(!skb))
> - break;
> -
> - skb_reserve(skb, NET_IP_ALIGN);
> -
> - f = &skb_shinfo(skb)->frags[0];
> - f->page = get_a_page(vi, GFP_ATOMIC);
> - if (!f->page) {
> - kfree_skb(skb);
> + struct page_list *plist = get_a_free_page(vi, GFP_ATOMIC);
> + if (!plist || !plist->page)
> break;
> - }
> -
> - f->page_offset = 0;
> - f->size = PAGE_SIZE;
> -
> - skb_shinfo(skb)->nr_frags++;
> -
> - sg_init_one(sg, page_address(f->page), PAGE_SIZE);
> - skb_queue_head(&vi->recv, skb);
> -
> - err = vi->rvq->vq_ops->add_buf(vi->rvq, sg, 0, 1, skb);
> + sg_init_one(sg, page_address(plist->page), PAGE_SIZE);
> + err = vi->rvq->vq_ops->add_buf(vi->rvq, sg, 0, 1, plist);
> if (err) {
> - skb_unlink(skb, &vi->recv);
> - kfree_skb(skb);
> + list_move_tail(&plist->list, &vi->freed_pages);
> break;
> }
> vi->num++;
> }
> +
> if (unlikely(vi->num > vi->max))
> vi->max = vi->num;
> +
unnecessary empty lines - if you like them, make a
separate patch. and seems to have trailing whitespace.
> vi->rvq->vq_ops->kick(vi->rvq);
> }
>
> @@ -388,14 +413,15 @@ static void skb_recv_done(struct virtqueue *rvq)
> static int virtnet_poll(struct napi_struct *napi, int budget)
> {
> struct virtnet_info *vi = container_of(napi, struct virtnet_info, napi);
> - struct sk_buff *skb = NULL;
> + void *buf = NULL;
Does it need to be initialized?
> unsigned int len, received = 0;
>
> again:
> while (received < budget &&
> - (skb = vi->rvq->vq_ops->get_buf(vi->rvq, &len)) != NULL) {
> - __skb_unlink(skb, &vi->recv);
> - receive_skb(vi->dev, skb, len);
> + (buf = vi->rvq->vq_ops->get_buf(vi->rvq, &len)) != NULL) {
> + if (!vi->mergeable_rx_bufs)
> + __skb_unlink((struct sk_buff *)buf, &vi->recv);
cast not necessary
> + receive_skb(vi->dev, buf, len);
> vi->num--;
> received++;
> }
> @@ -893,6 +919,8 @@ static int virtnet_probe(struct virtio_device *vdev)
> vi->vdev = vdev;
> vdev->priv = vi;
> vi->pages = NULL;
> + INIT_LIST_HEAD(&vi->used_pages);
> + INIT_LIST_HEAD(&vi->freed_pages);
>
> /* If they give us a callback when all buffers are done, we don't need
> * the timer. */
> @@ -969,6 +997,7 @@ static void virtnet_remove(struct virtio_device *vdev)
> {
> struct virtnet_info *vi = vdev->priv;
> struct sk_buff *skb;
> + struct page_list *plist, *tp;
>
> /* Stop all the virtqueues. */
> vdev->config->reset(vdev);
> @@ -977,14 +1006,23 @@ static void virtnet_remove(struct virtio_device *vdev)
> del_timer_sync(&vi->xmit_free_timer);
>
> /* Free our skbs in send and recv queues, if any. */
> - while ((skb = __skb_dequeue(&vi->recv)) != NULL) {
> - kfree_skb(skb);
> - vi->num--;
> + if (!vi->mergeable_rx_bufs) {
> + while ((skb = __skb_dequeue(&vi->recv)) != NULL) {
> + kfree_skb(skb);
> + vi->num--;
> + }
> + BUG_ON(vi->num != 0);
> + } else {
> + list_splice_init(&vi->used_pages, &vi->freed_pages);
> + list_for_each_entry_safe(plist, tp, &vi->freed_pages, list) {
> + vi->num--;
> + if (plist->page)
> + __free_pages(plist->page, 0);
> + kfree(plist);
> + }
BUG_ON here as well?
> }
So, you are fixing this to share code. Good.
> __skb_queue_purge(&vi->send);
>
> - BUG_ON(vi->num != 0);
> -
> unregister_netdev(vi->dev);
>
> vdev->config->del_vqs(vi->vdev);
>
>
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