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Date: Wed, 15 Oct 2014 14:57:07 +0300
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Jason Wang <jasowang@...hat.com>
Cc: linux-kernel@...r.kernel.org,
Rusty Russell <rusty@...tcorp.com.au>,
virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH RFC] virtio_net: enable tx interrupt
On Wed, Oct 15, 2014 at 07:04:20PM +0800, Jason Wang wrote:
> On 10/15/2014 05:53 AM, Michael S. Tsirkin wrote:
> > On newer hosts that support delayed tx interrupts,
> > we probably don't have much to gain from orphaning
> > packets early.
> >
> > Based on patch by Jason Wang.
> >
> > Note: this will likely degrade performance for hosts without event idx
> > support. Various fallback options are available, including
> > orphaning conditionally.
> > Testing TBD.
> >
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > ---
> > drivers/net/virtio_net.c | 119 +++++++++++++++++++++++++++++++++--------------
> > 1 file changed, 83 insertions(+), 36 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 6b6e136..62c059d 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -72,6 +72,8 @@ struct send_queue {
> >
> > /* Name of the send queue: output.$index */
> > char name[40];
> > +
> > + struct napi_struct napi;
> > };
> >
> > /* Internal representation of a receive virtqueue */
> > @@ -211,15 +213,38 @@ static struct page *get_a_page(struct receive_queue *rq, gfp_t gfp_mask)
> > return p;
> > }
> >
> > +static int free_old_xmit_skbs(struct send_queue *sq, int budget)
> > +{
> > + struct sk_buff *skb;
> > + unsigned int len;
> > + struct virtnet_info *vi = sq->vq->vdev->priv;
> > + struct virtnet_stats *stats = this_cpu_ptr(vi->stats);
> > + int sent = 0;
> > +
> > + while (sent < budget &&
> > + (skb = virtqueue_get_buf(sq->vq, &len)) != NULL) {
> > + pr_debug("Sent skb %p\n", skb);
> > +
> > + u64_stats_update_begin(&stats->tx_syncp);
> > + stats->tx_bytes += skb->len;
> > + stats->tx_packets++;
> > + u64_stats_update_end(&stats->tx_syncp);
> > +
> > + dev_kfree_skb_any(skb);
> > + sent++;
> > + }
> > +
> > + return sent;
> > +}
> > +
> > static void skb_xmit_done(struct virtqueue *vq)
> > {
> > struct virtnet_info *vi = vq->vdev->priv;
> > + struct send_queue *sq = &vi->sq[vq2txq(vq)];
> >
> > - /* Suppress further interrupts. */
> > - virtqueue_disable_cb(vq);
> > -
> > - /* We were probably waiting for more output buffers. */
> > - netif_wake_subqueue(vi->dev, vq2txq(vq));
> > + if (napi_schedule_prep(&sq->napi)) {
> > + __napi_schedule(&sq->napi);
> > + }
> > }
> >
> > static unsigned int mergeable_ctx_to_buf_truesize(unsigned long mrg_ctx)
> > @@ -766,6 +791,37 @@ again:
> > return received;
> > }
> >
> > +static int virtnet_poll_tx(struct napi_struct *napi, int budget)
> > +{
> > + struct send_queue *sq =
> > + container_of(napi, struct send_queue, napi);
> > + struct virtnet_info *vi = sq->vq->vdev->priv;
> > + struct netdev_queue *txq = netdev_get_tx_queue(vi->dev, vq2txq(sq->vq));
> > + unsigned int r, sent = 0;
> > +
> > +again:
> > + __netif_tx_lock(txq, smp_processor_id());
> > + virtqueue_disable_cb(sq->vq);
> > + sent += free_old_xmit_skbs(sq, budget - sent);
> > +
> > + if (sent < budget) {
> > + r = virtqueue_enable_cb_prepare(sq->vq);
>
> So even virtqueue_enable_cb_delayed() was used in start_xmit(). This can
> move used index backwards to trigger unnecessary interrupts.
Good point. I'll rework this to use virtqueue_enable_cb_delayed.
virtqueue_enable_cb_delayed_prepare might be nice to
reduce lock contention, but that needs to be benchmarked.
> > + napi_complete(napi);
> > + __netif_tx_unlock(txq);
> > + if (unlikely(virtqueue_poll(sq->vq, r)) &&
> > + napi_schedule_prep(napi)) {
> > + virtqueue_disable_cb(sq->vq);
> > + __napi_schedule(napi);
> > + goto again;
> > + }
> > + } else {
> > + __netif_tx_unlock(txq);
> > + }
> > +
> > + netif_wake_subqueue(vi->dev, vq2txq(sq->vq));
> > + return sent;
> > +}
> > +
> > #ifdef CONFIG_NET_RX_BUSY_POLL
> > /* must be called with local_bh_disable()d */
> > static int virtnet_busy_poll(struct napi_struct *napi)
> > @@ -814,30 +870,12 @@ static int virtnet_open(struct net_device *dev)
> > if (!try_fill_recv(&vi->rq[i], GFP_KERNEL))
> > schedule_delayed_work(&vi->refill, 0);
> > virtnet_napi_enable(&vi->rq[i]);
> > + napi_enable(&vi->sq[i].napi);
> > }
> >
> > return 0;
> > }
> >
> > -static void free_old_xmit_skbs(struct send_queue *sq)
> > -{
> > - struct sk_buff *skb;
> > - unsigned int len;
> > - struct virtnet_info *vi = sq->vq->vdev->priv;
> > - struct virtnet_stats *stats = this_cpu_ptr(vi->stats);
> > -
> > - while ((skb = virtqueue_get_buf(sq->vq, &len)) != NULL) {
> > - pr_debug("Sent skb %p\n", skb);
> > -
> > - u64_stats_update_begin(&stats->tx_syncp);
> > - stats->tx_bytes += skb->len;
> > - stats->tx_packets++;
> > - u64_stats_update_end(&stats->tx_syncp);
> > -
> > - dev_kfree_skb_any(skb);
> > - }
> > -}
> > -
> > static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
> > {
> > struct skb_vnet_hdr *hdr;
> > @@ -902,7 +940,9 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
> > sg_set_buf(sq->sg, hdr, hdr_len);
> > num_sg = skb_to_sgvec(skb, sq->sg + 1, 0, skb->len) + 1;
> > }
> > - return virtqueue_add_outbuf(sq->vq, sq->sg, num_sg, skb, GFP_ATOMIC);
> > +
> > + return virtqueue_add_outbuf(sq->vq, sq->sg, num_sg, skb,
> > + GFP_ATOMIC);
> > }
> >
> > static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> > @@ -910,10 +950,9 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> > struct virtnet_info *vi = netdev_priv(dev);
> > int qnum = skb_get_queue_mapping(skb);
> > struct send_queue *sq = &vi->sq[qnum];
> > - int err;
> > + int err, qsize = virtqueue_get_vring_size(sq->vq);
> >
> > - /* Free up any pending old buffers before queueing new ones. */
> > - free_old_xmit_skbs(sq);
>
> I think we'd better keep this. Since it may the tx skb freeing not
> totally depends on the tx interrupt, delayed interrupt may work better
> without damage the latency.
Hmm ok but I think it's best to do it at the end,
after we have sent the packet.
Will update the patch.
> > + virtqueue_disable_cb(sq->vq);
> >
> > /* Try to transmit */
> > err = xmit_skb(sq, skb);
> > @@ -930,22 +969,20 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> > }
> > virtqueue_kick(sq->vq);
> >
> > - /* Don't wait up for transmitted skbs to be freed. */
> > - skb_orphan(skb);
> > - nf_reset(skb);
> > -
> > /* Apparently nice girls don't return TX_BUSY; stop the queue
> > * before it gets out of hand. Naturally, this wastes entries. */
> > if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
> > netif_stop_subqueue(dev, qnum);
> > if (unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
> > /* More just got used, free them then recheck. */
> > - free_old_xmit_skbs(sq);
> > + free_old_xmit_skbs(sq, qsize);
> > if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
> > netif_start_subqueue(dev, qnum);
> > virtqueue_disable_cb(sq->vq);
> > }
> > }
> > + } else if (virtqueue_enable_cb_delayed(sq->vq)) {
> > + free_old_xmit_skbs(sq, qsize);
> > }
> >
> > return NETDEV_TX_OK;
> > @@ -1124,8 +1161,10 @@ static int virtnet_close(struct net_device *dev)
> > /* Make sure refill_work doesn't re-enable napi! */
> > cancel_delayed_work_sync(&vi->refill);
> >
> > - for (i = 0; i < vi->max_queue_pairs; i++)
> > + for (i = 0; i < vi->max_queue_pairs; i++) {
> > napi_disable(&vi->rq[i].napi);
> > + napi_disable(&vi->sq[i].napi);
> > + }
> >
> > return 0;
> > }
> > @@ -1438,8 +1477,10 @@ static void virtnet_free_queues(struct virtnet_info *vi)
> > {
> > int i;
> >
> > - for (i = 0; i < vi->max_queue_pairs; i++)
> > + for (i = 0; i < vi->max_queue_pairs; i++) {
> > netif_napi_del(&vi->rq[i].napi);
> > + netif_napi_del(&vi->sq[i].napi);
> > + }
> >
> > kfree(vi->rq);
> > kfree(vi->sq);
> > @@ -1593,6 +1634,8 @@ static int virtnet_alloc_queues(struct virtnet_info *vi)
> > netif_napi_add(vi->dev, &vi->rq[i].napi, virtnet_poll,
> > napi_weight);
> > napi_hash_add(&vi->rq[i].napi);
> > + netif_napi_add(vi->dev, &vi->sq[i].napi, virtnet_poll_tx,
> > + napi_weight);
> >
> > sg_init_table(vi->rq[i].sg, ARRAY_SIZE(vi->rq[i].sg));
> > ewma_init(&vi->rq[i].mrg_avg_pkt_len, 1, RECEIVE_AVG_WEIGHT);
> > @@ -1893,8 +1936,10 @@ static int virtnet_freeze(struct virtio_device *vdev)
> > if (netif_running(vi->dev)) {
> > for (i = 0; i < vi->max_queue_pairs; i++) {
> > napi_disable(&vi->rq[i].napi);
> > + napi_disable(&vi->sq[i].napi);
> > napi_hash_del(&vi->rq[i].napi);
> > netif_napi_del(&vi->rq[i].napi);
> > + netif_napi_del(&vi->sq[i].napi);
> > }
> > }
> >
> > @@ -1919,8 +1964,10 @@ static int virtnet_restore(struct virtio_device *vdev)
> > if (!try_fill_recv(&vi->rq[i], GFP_KERNEL))
> > schedule_delayed_work(&vi->refill, 0);
> >
> > - for (i = 0; i < vi->max_queue_pairs; i++)
> > + for (i = 0; i < vi->max_queue_pairs; i++) {
> > virtnet_napi_enable(&vi->rq[i]);
> > + napi_enable(&vi->sq[i].napi);
> > + }
> > }
> >
> > netif_device_attach(vi->dev);
--
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