[PATCH net-next RFC 2/4] virtio-net: transmit napi

[PATCH net-next RFC 2/4] virtio-net: transmit napi

[Willem de Bruijn]

From: Willem de Bruijn <willemb@google.com>

Convert virtio-net to a standard napi tx completion path. This enables
better TCP pacing using TCP small queues and increases single stream
throughput.

The virtio-net driver currently cleans tx descriptors on transmission
of new packets in ndo_start_xmit. Latency depends on new traffic, so
is unbounded. To avoid deadlock when a socket reaches its snd limit,
packets are orphaned on tranmission. This breaks socket backpressure,
including TSQ.

Napi increases the number of interrupts generated compared to the
current model, which keeps interrupts disabled as long as the ring
has enough free descriptors. Keep tx napi optional for now. Follow-on
patches will reduce the interrupt cost.

Signed-off-by: Willem de Bruijn <willemb@google.com>
---
 drivers/net/virtio_net.c | 73 ++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 61 insertions(+), 12 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 8c21e9a4adc7..9a9031640179 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -33,6 +33,8 @@
 static int napi_weight = NAPI_POLL_WEIGHT;
 module_param(napi_weight, int, 0444);
 
+static int napi_tx_weight = NAPI_POLL_WEIGHT;
+
 static bool csum = true, gso = true;
 module_param(csum, bool, 0444);
 module_param(gso, bool, 0444);
@@ -86,6 +88,8 @@ struct send_queue {
 
 	/* Name of the send queue: output.$index */
 	char name[40];
+
+	struct napi_struct napi;
 };
 
 /* Internal representation of a receive virtqueue */
@@ -262,12 +266,16 @@ static void virtqueue_napi_complete(struct napi_struct *napi,
 static void skb_xmit_done(struct virtqueue *vq)
 {
 	struct virtnet_info *vi = vq->vdev->priv;
+	struct napi_struct *napi = &vi->sq[vq2txq(vq)].napi;
 
 	/* Suppress further interrupts. */
 	virtqueue_disable_cb(vq);
 
-	/* We were probably waiting for more output buffers. */
-	netif_wake_subqueue(vi->dev, vq2txq(vq));
+	if (napi->weight)
+		virtqueue_napi_schedule(napi, vq);
+	else
+		/* We were probably waiting for more output buffers. */
+		netif_wake_subqueue(vi->dev, vq2txq(vq));
 }
 
 static unsigned int mergeable_ctx_to_buf_truesize(unsigned long mrg_ctx)
@@ -961,6 +969,9 @@ static void skb_recv_done(struct virtqueue *rvq)
 
 static void virtnet_napi_enable(struct virtqueue *vq, struct napi_struct *napi)
 {
+	if (!napi->weight)
+		return;
+
 	napi_enable(napi);
 
 	/* If all buffers were filled by other side before we napi_enabled, we
@@ -1046,12 +1057,13 @@ static int virtnet_open(struct net_device *dev)
 			if (!try_fill_recv(vi, &vi->rq[i], GFP_KERNEL))
 				schedule_delayed_work(&vi->refill, 0);
 		virtnet_napi_enable(vi->rq[i].vq, &vi->rq[i].napi);
+		virtnet_napi_enable(vi->sq[i].vq, &vi->sq[i].napi);
 	}
 
 	return 0;
 }
 
-static void free_old_xmit_skbs(struct send_queue *sq)
+static unsigned int free_old_xmit_skbs(struct send_queue *sq, int budget)
 {
 	struct sk_buff *skb;
 	unsigned int len;
@@ -1060,7 +1072,8 @@ static void free_old_xmit_skbs(struct send_queue *sq)
 	unsigned int packets = 0;
 	unsigned int bytes = 0;
 
-	while ((skb = virtqueue_get_buf(sq->vq, &len)) != NULL) {
+	while (packets < budget &&
+	       (skb = virtqueue_get_buf(sq->vq, &len)) != NULL) {
 		pr_debug("Sent skb %p\n", skb);
 
 		bytes += skb->len;
@@ -1073,12 +1086,35 @@ static void free_old_xmit_skbs(struct send_queue *sq)
 	 * happens when called speculatively from start_xmit.
 	 */
 	if (!packets)
-		return;
+		return 0;
 
 	u64_stats_update_begin(&stats->tx_syncp);
 	stats->tx_bytes += bytes;
 	stats->tx_packets += packets;
 	u64_stats_update_end(&stats->tx_syncp);
+
+	return packets;
+}
+
+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));
+	bool complete = false;
+
+	__netif_tx_lock(txq, smp_processor_id());
+	if (free_old_xmit_skbs(sq, budget) < budget)
+		complete = true;
+	__netif_tx_unlock(txq);
+
+	if (complete)
+		virtqueue_napi_complete(napi, sq->vq, 0);
+
+	if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
+		netif_wake_subqueue(vi->dev, vq2txq(sq->vq));
+
+	return complete ? 0 : budget;
 }
 
 static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
@@ -1130,9 +1166,11 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 	int err;
 	struct netdev_queue *txq = netdev_get_tx_queue(dev, qnum);
 	bool kick = !skb->xmit_more;
+	bool use_napi = sq->napi.weight;
 
 	/* Free up any pending old buffers before queueing new ones. */
-	free_old_xmit_skbs(sq);
+	if (!use_napi)
+		free_old_xmit_skbs(sq, napi_weight);
 
 	/* timestamp packet in software */
 	skb_tx_timestamp(skb);
@@ -1152,8 +1190,10 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 	}
 
 	/* Don't wait up for transmitted skbs to be freed. */
-	skb_orphan(skb);
-	nf_reset(skb);
+	if (!use_napi) {
+		skb_orphan(skb);
+		nf_reset(skb);
+	}
 
 	/* If running out of space, stop queue to avoid getting packets that we
 	 * are then unable to transmit.
@@ -1167,9 +1207,10 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 	 */
 	if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
 		netif_stop_subqueue(dev, qnum);
-		if (unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
+		if (!use_napi &&
+		    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, virtqueue_get_vring_size(sq->vq));
 			if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
 				netif_start_subqueue(dev, qnum);
 				virtqueue_disable_cb(sq->vq);
@@ -1371,8 +1412,10 @@ 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;
 }
@@ -1719,6 +1762,7 @@ static void virtnet_freeze_down(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);
 	}
 }
 
@@ -1741,8 +1785,10 @@ static int virtnet_restore_up(struct virtio_device *vdev)
 			if (!try_fill_recv(vi, &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].vq, &vi->rq[i].napi);
+			virtnet_napi_enable(vi->sq[i].vq, &vi->sq[i].napi);
+		}
 	}
 
 	netif_device_attach(vi->dev);
@@ -1947,6 +1993,7 @@ static void virtnet_free_queues(struct virtnet_info *vi)
 	for (i = 0; i < vi->max_queue_pairs; i++) {
 		napi_hash_del(&vi->rq[i].napi);
 		netif_napi_del(&vi->rq[i].napi);
+		netif_napi_del(&vi->sq[i].napi);
 	}
 
 	/* We called napi_hash_del() before netif_napi_del(),
@@ -2132,6 +2179,8 @@ static int virtnet_alloc_queues(struct virtnet_info *vi)
 		vi->rq[i].pages = NULL;
 		netif_napi_add(vi->dev, &vi->rq[i].napi, virtnet_poll,
 			       napi_weight);
+		netif_napi_add(vi->dev, &vi->sq[i].napi, virtnet_poll_tx,
+			       napi_tx_weight);
 
 		sg_init_table(vi->rq[i].sg, ARRAY_SIZE(vi->rq[i].sg));
 		ewma_pkt_len_init(&vi->rq[i].mrg_avg_pkt_len);
-- 
2.12.0.rc1.440.g5b76565f74-goog

[PATCH net-next RFC 0/4] virtio-net tx napi

[Willem de Bruijn]

From: Willem de Bruijn <willemb@google.com>

Add napi for virtio-net transmit completion processing. Based on
previous patchsets by Jason Wang:

  [RFC V7 PATCH 0/7] enable tx interrupts for virtio-net
  http://lkml.iu.edu/hypermail/linux/kernel/1505.3/00245.html

This patchset is not ready for submission yet, but it is time for
another checkpoint. Among others, it requires more testing with
more diverse workloads.


Before commit b0c39dbdc204 ("virtio_net: don't free buffers in xmit
ring") the virtio-net driver would free transmitted packets on
transmission of new packets in ndo_start_xmit and, to catch the edge
case when no new packet is sent, also in a timer at 10HZ.

A timer can cause long stalls. VIRTIO_F_NOTIFY_ON_EMPTY avoids stalls
due to low free descriptor count. It does not address a stalls due to
low socket SO_SNDBUF. Increasing timer frequency decreases that stall
time, but increases interrupt rate and, thus, cycle count.

Currently, with no timer, packets are freed only at ndo_start_xmit.
Latency of consume_skb is now unbounded. To avoid a deadlock if a sock
reaches SO_SNDBUF, packets are orphaned on tx. This breaks TCP small
queues.

Reenable TCP small queues by removing the orphan. Instead of using a
timer, convert the driver to regular tx napi. This does not have the
unresolved stall issue and does not have any frequency to tune.

By keeping interrupts enabled by default, napi increases tx
interrupt rate. VIRTIO_F_EVENT_IDX avoids sending an interrupt if
one is already unacknowledged, so makes this more feasible today.
Combine that with two optimizations that bring interrupt rate
back in line with the existing code:

Interrupt coalescing delays interrupts until a number of events
accrue or a timer fires.

Tx completion cleaning on rx interrupts elides most explicit tx
interrupts by relying on the fact that many rx interrupts fire.

Tested by running {1, 10, 100} TCP_STREAM and TCP_RR tests from a
guest to a server on the host, on an x86_64 Haswell. The guest
runs 4 vCPUs pinned to 4 cores. vhost and the test server are
pinned to a core each.

All results are the median of 5 runs, with variance well < 10%.
Used neper (github.com/google/neper) as test process. Tests used
experimental_zcopy=0. This is likely no longer needed.

Napi increases single stream throughput, but increases cycle cost
across the board. Interrupt moderation ("+vhost") reverts both, if
not fully. For this workload with ACKs in the return path, the
last optimization ("at-rx") is more effective. For UDP this is
likely not true.

             upstream     napi   +vhost   +at-rx +v+at-rx
Stream:
  1x:
  Mbps          30182    38782    30106    38002    32842
  Gcycles         405      499      386      403      417        

  10x:
  Mbps          40441    40575    41638    40260    41299
  Gcycles         438      545      430      416      416

  100x:
  Mbps          34049    34697    34763    34637    34259
  Gcycles         441      545      433      415      422

Latency (us):
  1x:
  p50              24       24       24       21       24
  p99              27       27       27       26       27
  Gcycles         299      430      432      312      297

10x:
  p50              30       31       31       42       31
  p99              40       46       48       52       42
  Gcycles         347      423      471      322      463

100x:
  p50             155      151      163      306      161
  p99             337      329      352      361      349
  Gcycles         340      421      463      306      441


Lower throughput at 100x vs 10x can be (at least in part)
explained by looking at bytes per packet sent (nstat). It likely
also explains the lower throughput of 1x for some variants.

upstream:

 N=1   bytes/pkt=16581
 N=10  bytes/pkt=61513
 N=100 bytes/pkt=51558

at_rx:

 N=1   bytes/pkt=65204
 N=10  bytes/pkt=65148
 N=100 bytes/pkt=56840

For this experiment, vhost has 64 frames and usecs thresholds.
Configuring this from the guest requires additional patches to qemu.
Temporary patch:

  @@ -846,9 +845,6 @@ static int vhost_net_open(struct inode *inode, struct file *f)
          vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT, dev);
          vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN, dev);
 
  -       vqs[VHOST_NET_VQ_TX]->max_coalesce_ktime = ktime_set(0, 64 * NSEC_PER_USEC);
  -       vqs[VHOST_NET_VQ_TX]->max_coalesce_frames = 64;
  -
          f->private_data = n;

TODO
 - restart timer if trylock failed and lock not held by hande_tx
   - start timer only at end of handle_tx and kill at start
 - make napi_tx configurable
 - increase test coverage
   - 4KB TCP_RR
   - UDP
   - multithreaded sender
   - with experimental_zcopytx


Willem de Bruijn (4):
  virtio-net: napi helper functions
  virtio-net: transmit napi
  vhost: interrupt coalescing support
  virtio-net: clean tx descriptors from rx napi

 drivers/net/virtio_net.c   | 157 +++++++++++++++++++++++++++++++++------------
 drivers/vhost/vhost.c      |  74 ++++++++++++++++++++-
 drivers/vhost/vhost.h      |  12 ++++
 include/uapi/linux/vhost.h |  11 ++++
 4 files changed, 211 insertions(+), 43 deletions(-)

-- 
2.12.0.rc1.440.g5b76565f74-goog

[PATCH net-next RFC 2/4] virtio-net: transmit napi

[Willem de Bruijn]

From: Willem de Bruijn <willemb@google.com>

Convert virtio-net to a standard napi tx completion path. This enables
better TCP pacing using TCP small queues and increases single stream
throughput.

The virtio-net driver currently cleans tx descriptors on transmission
of new packets in ndo_start_xmit. Latency depends on new traffic, so
is unbounded. To avoid deadlock when a socket reaches its snd limit,
packets are orphaned on tranmission. This breaks socket backpressure,
including TSQ.

Napi increases the number of interrupts generated compared to the
current model, which keeps interrupts disabled as long as the ring
has enough free descriptors. Keep tx napi optional for now. Follow-on
patches will reduce the interrupt cost.

Signed-off-by: Willem de Bruijn <willemb@google.com>
---
 drivers/net/virtio_net.c | 73 ++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 61 insertions(+), 12 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 8c21e9a4adc7..9a9031640179 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -33,6 +33,8 @@
 static int napi_weight = NAPI_POLL_WEIGHT;
 module_param(napi_weight, int, 0444);
 
+static int napi_tx_weight = NAPI_POLL_WEIGHT;
+
 static bool csum = true, gso = true;
 module_param(csum, bool, 0444);
 module_param(gso, bool, 0444);
@@ -86,6 +88,8 @@ struct send_queue {
 
 	/* Name of the send queue: output.$index */
 	char name[40];
+
+	struct napi_struct napi;
 };
 
 /* Internal representation of a receive virtqueue */
@@ -262,12 +266,16 @@ static void virtqueue_napi_complete(struct napi_struct *napi,
 static void skb_xmit_done(struct virtqueue *vq)
 {
 	struct virtnet_info *vi = vq->vdev->priv;
+	struct napi_struct *napi = &vi->sq[vq2txq(vq)].napi;
 
 	/* Suppress further interrupts. */
 	virtqueue_disable_cb(vq);
 
-	/* We were probably waiting for more output buffers. */
-	netif_wake_subqueue(vi->dev, vq2txq(vq));
+	if (napi->weight)
+		virtqueue_napi_schedule(napi, vq);
+	else
+		/* We were probably waiting for more output buffers. */
+		netif_wake_subqueue(vi->dev, vq2txq(vq));
 }
 
 static unsigned int mergeable_ctx_to_buf_truesize(unsigned long mrg_ctx)
@@ -961,6 +969,9 @@ static void skb_recv_done(struct virtqueue *rvq)
 
 static void virtnet_napi_enable(struct virtqueue *vq, struct napi_struct *napi)
 {
+	if (!napi->weight)
+		return;
+
 	napi_enable(napi);
 
 	/* If all buffers were filled by other side before we napi_enabled, we
@@ -1046,12 +1057,13 @@ static int virtnet_open(struct net_device *dev)
 			if (!try_fill_recv(vi, &vi->rq[i], GFP_KERNEL))
 				schedule_delayed_work(&vi->refill, 0);
 		virtnet_napi_enable(vi->rq[i].vq, &vi->rq[i].napi);
+		virtnet_napi_enable(vi->sq[i].vq, &vi->sq[i].napi);
 	}
 
 	return 0;
 }
 
-static void free_old_xmit_skbs(struct send_queue *sq)
+static unsigned int free_old_xmit_skbs(struct send_queue *sq, int budget)
 {
 	struct sk_buff *skb;
 	unsigned int len;
@@ -1060,7 +1072,8 @@ static void free_old_xmit_skbs(struct send_queue *sq)
 	unsigned int packets = 0;
 	unsigned int bytes = 0;
 
-	while ((skb = virtqueue_get_buf(sq->vq, &len)) != NULL) {
+	while (packets < budget &&
+	       (skb = virtqueue_get_buf(sq->vq, &len)) != NULL) {
 		pr_debug("Sent skb %p\n", skb);
 
 		bytes += skb->len;
@@ -1073,12 +1086,35 @@ static void free_old_xmit_skbs(struct send_queue *sq)
 	 * happens when called speculatively from start_xmit.
 	 */
 	if (!packets)
-		return;
+		return 0;
 
 	u64_stats_update_begin(&stats->tx_syncp);
 	stats->tx_bytes += bytes;
 	stats->tx_packets += packets;
 	u64_stats_update_end(&stats->tx_syncp);
+
+	return packets;
+}
+
+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));
+	bool complete = false;
+
+	__netif_tx_lock(txq, smp_processor_id());
+	if (free_old_xmit_skbs(sq, budget) < budget)
+		complete = true;
+	__netif_tx_unlock(txq);
+
+	if (complete)
+		virtqueue_napi_complete(napi, sq->vq, 0);
+
+	if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
+		netif_wake_subqueue(vi->dev, vq2txq(sq->vq));
+
+	return complete ? 0 : budget;
 }
 
 static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
@@ -1130,9 +1166,11 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 	int err;
 	struct netdev_queue *txq = netdev_get_tx_queue(dev, qnum);
 	bool kick = !skb->xmit_more;
+	bool use_napi = sq->napi.weight;
 
 	/* Free up any pending old buffers before queueing new ones. */
-	free_old_xmit_skbs(sq);
+	if (!use_napi)
+		free_old_xmit_skbs(sq, napi_weight);
 
 	/* timestamp packet in software */
 	skb_tx_timestamp(skb);
@@ -1152,8 +1190,10 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 	}
 
 	/* Don't wait up for transmitted skbs to be freed. */
-	skb_orphan(skb);
-	nf_reset(skb);
+	if (!use_napi) {
+		skb_orphan(skb);
+		nf_reset(skb);
+	}
 
 	/* If running out of space, stop queue to avoid getting packets that we
 	 * are then unable to transmit.
@@ -1167,9 +1207,10 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 	 */
 	if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
 		netif_stop_subqueue(dev, qnum);
-		if (unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
+		if (!use_napi &&
+		    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, virtqueue_get_vring_size(sq->vq));
 			if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
 				netif_start_subqueue(dev, qnum);
 				virtqueue_disable_cb(sq->vq);
@@ -1371,8 +1412,10 @@ 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;
 }
@@ -1719,6 +1762,7 @@ static void virtnet_freeze_down(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);
 	}
 }
 
@@ -1741,8 +1785,10 @@ static int virtnet_restore_up(struct virtio_device *vdev)
 			if (!try_fill_recv(vi, &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].vq, &vi->rq[i].napi);
+			virtnet_napi_enable(vi->sq[i].vq, &vi->sq[i].napi);
+		}
 	}
 
 	netif_device_attach(vi->dev);
@@ -1947,6 +1993,7 @@ static void virtnet_free_queues(struct virtnet_info *vi)
 	for (i = 0; i < vi->max_queue_pairs; i++) {
 		napi_hash_del(&vi->rq[i].napi);
 		netif_napi_del(&vi->rq[i].napi);
+		netif_napi_del(&vi->sq[i].napi);
 	}
 
 	/* We called napi_hash_del() before netif_napi_del(),
@@ -2132,6 +2179,8 @@ static int virtnet_alloc_queues(struct virtnet_info *vi)
 		vi->rq[i].pages = NULL;
 		netif_napi_add(vi->dev, &vi->rq[i].napi, virtnet_poll,
 			       napi_weight);
+		netif_napi_add(vi->dev, &vi->sq[i].napi, virtnet_poll_tx,
+			       napi_tx_weight);
 
 		sg_init_table(vi->rq[i].sg, ARRAY_SIZE(vi->rq[i].sg));
 		ewma_pkt_len_init(&vi->rq[i].mrg_avg_pkt_len);
-- 
2.12.0.rc1.440.g5b76565f74-goog

Re: [PATCH net-next RFC 2/4] virtio-net: transmit napi

[Jason Wang]

On 2017年03月03日 22:39, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@google.com>
>
> Convert virtio-net to a standard napi tx completion path. This enables
> better TCP pacing using TCP small queues and increases single stream
> throughput.
>
> The virtio-net driver currently cleans tx descriptors on transmission
> of new packets in ndo_start_xmit. Latency depends on new traffic, so
> is unbounded. To avoid deadlock when a socket reaches its snd limit,
> packets are orphaned on tranmission. This breaks socket backpressure,
> including TSQ.
>
> Napi increases the number of interrupts generated compared to the
> current model, which keeps interrupts disabled as long as the ring
> has enough free descriptors. Keep tx napi optional for now. Follow-on
> patches will reduce the interrupt cost.
>
> Signed-off-by: Willem de Bruijn <willemb@google.com>
> ---
>   drivers/net/virtio_net.c | 73 ++++++++++++++++++++++++++++++++++++++++--------
>   1 file changed, 61 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 8c21e9a4adc7..9a9031640179 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -33,6 +33,8 @@
>   static int napi_weight = NAPI_POLL_WEIGHT;
>   module_param(napi_weight, int, 0444);
>   
> +static int napi_tx_weight = NAPI_POLL_WEIGHT;
> +

Maybe we should use module_param for this? Or in the future, use 
tx-frames-irq for a per-device configuration.

Thanks

Re: [PATCH net-next RFC 2/4] virtio-net: transmit napi

[Willem de Bruijn]

>>   drivers/net/virtio_net.c | 73
>> ++++++++++++++++++++++++++++++++++++++++--------
>>   1 file changed, 61 insertions(+), 12 deletions(-)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index 8c21e9a4adc7..9a9031640179 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -33,6 +33,8 @@
>>   static int napi_weight = NAPI_POLL_WEIGHT;
>>   module_param(napi_weight, int, 0444);
>>   +static int napi_tx_weight = NAPI_POLL_WEIGHT;
>> +
>
>
> Maybe we should use module_param for this? Or in the future, use
> tx-frames-irq for a per-device configuration.

This option should eventually just go away, and napi tx become the
standard mode.

In the short term, while we evaluate it on varied workloads, a
module_param sounds good to me. In general that is frowned
upon, as it leads to different configuration interfaces for each
device driver. But that should not be a concern in this limited
case.

Re: [PATCH net-next RFC 2/4] virtio-net: transmit napi

[David Miller]

From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: Mon, 6 Mar 2017 12:50:19 -0500

>>>   drivers/net/virtio_net.c | 73
>>> ++++++++++++++++++++++++++++++++++++++++--------
>>>   1 file changed, 61 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>> index 8c21e9a4adc7..9a9031640179 100644
>>> --- a/drivers/net/virtio_net.c
>>> +++ b/drivers/net/virtio_net.c
>>> @@ -33,6 +33,8 @@
>>>   static int napi_weight = NAPI_POLL_WEIGHT;
>>>   module_param(napi_weight, int, 0444);
>>>   +static int napi_tx_weight = NAPI_POLL_WEIGHT;
>>> +
>>
>>
>> Maybe we should use module_param for this? Or in the future, use
>> tx-frames-irq for a per-device configuration.
> 
> This option should eventually just go away, and napi tx become the
> standard mode.
> 
> In the short term, while we evaluate it on varied workloads, a
> module_param sounds good to me. In general that is frowned
> upon, as it leads to different configuration interfaces for each
> device driver. But that should not be a concern in this limited
> case.

In any event, do we really need a TX weight at all?

I guess you tried this, but why doesn't it not work to just do
all TX work unconditionally in a NAPI poll pass?  This is how
we encourage all NIC drivers to handle this.

Re: [PATCH net-next RFC 2/4] virtio-net: transmit napi

[Michael S. Tsirkin]

On Mon, Mar 06, 2017 at 10:55:22AM -0800, David Miller wrote:
> From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> Date: Mon, 6 Mar 2017 12:50:19 -0500
> 
> >>>   drivers/net/virtio_net.c | 73
> >>> ++++++++++++++++++++++++++++++++++++++++--------
> >>>   1 file changed, 61 insertions(+), 12 deletions(-)
> >>>
> >>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> >>> index 8c21e9a4adc7..9a9031640179 100644
> >>> --- a/drivers/net/virtio_net.c
> >>> +++ b/drivers/net/virtio_net.c
> >>> @@ -33,6 +33,8 @@
> >>>   static int napi_weight = NAPI_POLL_WEIGHT;
> >>>   module_param(napi_weight, int, 0444);
> >>>   +static int napi_tx_weight = NAPI_POLL_WEIGHT;
> >>> +
> >>
> >>
> >> Maybe we should use module_param for this? Or in the future, use
> >> tx-frames-irq for a per-device configuration.
> > 
> > This option should eventually just go away, and napi tx become the
> > standard mode.
> > 
> > In the short term, while we evaluate it on varied workloads, a
> > module_param sounds good to me. In general that is frowned
> > upon, as it leads to different configuration interfaces for each
> > device driver. But that should not be a concern in this limited
> > case.
> 
> In any event, do we really need a TX weight at all?
> 
> I guess you tried this, but why doesn't it not work to just do
> all TX work unconditionally in a NAPI poll pass?  This is how
> we encourage all NIC drivers to handle this.

This seems to be more or less what this driver does already.
So I suspect it can just ignore the weight.

-- 
MST

Re: [PATCH net-next RFC 2/4] virtio-net: transmit napi

[Willem de Bruijn]

On Mon, Mar 6, 2017 at 2:33 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Mon, Mar 06, 2017 at 10:55:22AM -0800, David Miller wrote:
>> From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
>> Date: Mon, 6 Mar 2017 12:50:19 -0500
>>
>> >>>   drivers/net/virtio_net.c | 73
>> >>> ++++++++++++++++++++++++++++++++++++++++--------
>> >>>   1 file changed, 61 insertions(+), 12 deletions(-)
>> >>>
>> >>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> >>> index 8c21e9a4adc7..9a9031640179 100644
>> >>> --- a/drivers/net/virtio_net.c
>> >>> +++ b/drivers/net/virtio_net.c
>> >>> @@ -33,6 +33,8 @@
>> >>>   static int napi_weight = NAPI_POLL_WEIGHT;
>> >>>   module_param(napi_weight, int, 0444);
>> >>>   +static int napi_tx_weight = NAPI_POLL_WEIGHT;
>> >>> +
>> >>
>> >>
>> >> Maybe we should use module_param for this? Or in the future, use
>> >> tx-frames-irq for a per-device configuration.
>> >
>> > This option should eventually just go away, and napi tx become the
>> > standard mode.
>> >
>> > In the short term, while we evaluate it on varied workloads, a
>> > module_param sounds good to me. In general that is frowned
>> > upon, as it leads to different configuration interfaces for each
>> > device driver. But that should not be a concern in this limited
>> > case.
>>
>> In any event, do we really need a TX weight at all?
>>
>> I guess you tried this, but why doesn't it not work to just do
>> all TX work unconditionally in a NAPI poll pass?  This is how
>> we encourage all NIC drivers to handle this.
>
> This seems to be more or less what this driver does already.
> So I suspect it can just ignore the weight.

Okay. Then we still need a boolean to toggle tx napi until we're
sure that the old path can be deprecated.