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Date:	Fri, 17 Oct 2014 13:11:33 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH RFC v2 1/3] virtio_net: enable tx interrupt

On 10/15/2014 10:32 PM, 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 might degrade performance for
> hosts without event idx support.
> Should be addressed by the next patch.
>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
>  drivers/net/virtio_net.c | 137 ++++++++++++++++++++++++++++++++---------------
>  1 file changed, 94 insertions(+), 43 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 13d0a8b..a9bf178 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 */
> @@ -217,15 +219,37 @@ static struct page *get_a_page(struct receive_queue *rq, gfp_t gfp_mask)
>  	return p;
>  }
>  
> +static unsigned 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);
> +	unsigned int packets = 0;
> +
> +	while (packets < 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);
> +		packets++;
> +	}
> +
> +	return packets;
> +}
> +
>  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)
> @@ -774,6 +798,37 @@ 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 sent = 0;
> +	bool enable_done;
> +
> +again:
> +	__netif_tx_lock(txq, smp_processor_id());
> +	virtqueue_disable_cb(sq->vq);
> +	sent += free_old_xmit_skbs(sq, budget - sent);
> +
> +	if (sent < budget) {
> +		enable_done = virtqueue_enable_cb_delayed(sq->vq);
> +		napi_complete(napi);
> +		__netif_tx_unlock(txq);
> +		if (unlikely(enable_done) && napi_schedule_prep(napi)) {

I think you mean unlikely(!enable_done) here?
> +			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)
> @@ -822,30 +877,12 @@ 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;
> @@ -911,7 +948,9 @@ 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)
> @@ -919,12 +958,16 @@ 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);
>  	struct netdev_queue *txq = netdev_get_tx_queue(dev, qnum);
>  	bool kick = !skb->xmit_more;
> +	bool stopped;
> +
> +	virtqueue_disable_cb(sq->vq);
>  
> -	/* Free up any pending old buffers before queueing new ones. */
> -	free_old_xmit_skbs(sq);
> +	/* We are going to push one skb.
> +	 * Try to pop one off to free space for it. */
> +	free_old_xmit_skbs(sq, 1);

Looks like qsize instead of 1 is better? The more skbs freed in
ndo_start_xmit() the less chance tx napi will be scheduled.
>  
>  	/* Try to transmit */
>  	err = xmit_skb(sq, skb);
> @@ -940,27 +983,25 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
>  		return NETDEV_TX_OK;
>  	}
>  
> -	/* 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);
> -			if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
> -				netif_start_subqueue(dev, qnum);
> -				virtqueue_disable_cb(sq->vq);
> -			}
> -		}
> +		stopped = true;
> +	} else {
> +		stopped = false;
>  	}
>  
>  	if (kick || netif_xmit_stopped(txq))
>  		virtqueue_kick(sq->vq);

Looks like we'd better move this in the end in case the queue could be
restarted by following lines?
>  
> +	if (unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
> +		/* More just got used, free them then recheck. */
> +		free_old_xmit_skbs(sq, qsize);
> +		if (stopped && sq->vq->num_free >= 2+MAX_SKB_FRAGS)
> +			netif_start_subqueue(dev, qnum);

Why drop virtqueue_disable_cb() here?
> +	}
> +
>  	return NETDEV_TX_OK;
>  }
>  
> @@ -1137,8 +1178,10 @@ 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;
>  }
> @@ -1457,8 +1500,10 @@ 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);
> @@ -1612,6 +1657,8 @@ 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);
> @@ -1916,8 +1963,10 @@ 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);
>  		}
>  	}
>  
> @@ -1942,8 +1991,10 @@ 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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