Subject: Re: [PATCH RFC] virtio_net: enable tx interrupt

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 15 Oct 2014 14:57:07 +0300

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 <[email-masked]>
> > ---
> >  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);