Linux Virtualization - Apr 2017 - [PATCH net-next 3/3] virtio-net: clean tx descriptors from rx napi

From: Willem de Bruijn <willemb at 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

Changes:
  RFC -> v1:
    - dropped vhost interrupt moderation patch:
        not needed and likely expensive at light load
    - remove tx napi weight
      - always clean all tx completions
      - use boolean to toggle tx-napi, instead
    - only clean tx in rx if tx-napi is enabled
      - then clean tx before rx
    - fix: add missing braces in virtnet_freeze_down
    - testing: add 4KB TCP_RR + UDP test results


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 an optimization that brings interrupt rate
back in line with the existing version for most workloads:

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, UDP} STREAM, RR, 4K_RR benchmarks
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.

Napi increases single stream throughput, but increases cycle cost.
Processing completions on rx interrupts optimization brings this down,
especially for bi-directional workloads. UDP_STREAM is unidirectional
and continues to see a ~10% lower throughput.

Not showing number for only the optimization patch. That showed no
significant difference with upstream.

             upstream     napi   +at-rx
TCP_STREAM:
1x:
  Mbps          30537    37666    37910
  Gcycles         400      540      405

10x:
  Mbps          41012    39954    40245
  Gcycles         434      546      421

100x:
  Mbps          34088    34172    34245
  Gcycles         435      546      418

TCP_RR Latency (us):
1x:
  p50              24       24       21
  p99              27       27       27
  Gcycles         299      432      308

10x:
  p50              31       31       41
  p99              40       46       52
  Gcycles         346      428      322

100x:
  p50             155      151      310
  p99             334      329      362
  Gcycles         336      421      308

TCP_RR 4K:
1x:
  p50              30       30       27
  p99              34       33       34
  Gcycles         307      437      305

10x:
  p50              63       67       65
  p99              76       77       87
  Gcycles         334      425      315

100x:
  p50             421      497      511
  p99             510      571      773
  Gcycles         350      430      321

UDP_STREAM:
1x:
  Mbps          29802    26360    26608
  Gcycles         305      363      362

10x:
  Mbps          29901    26801    27078
  Gcycles         287      363      360

100x:
  Mbps          29952    26822    27054
  Gcycles         336      351      354

UDP_RR:
1x:
  p50              24       21       19
  p99              27       24       23
  Gcycles         299      431      309

10x:
  p50              31       27       35
  p99              40       35       54
  Gcycles         346      421      325

100x:
  p50             155      153      240
  p99             334      323      462
  Gcycles         336      421      311

UDP_RR 4K:
1x:
  p50              24       25       23
  p99              27       28       30
  Gcycles         299      435      321

10x:
  p50              31       35       48
  p99              40       54       66
  Gcycles         346      451      308

100x:
  p50             155      210      307
  p99             334      451      519
  Gcycles         336      440      297

Note that GSO is enabled, so 4K RR still translates to one packet
per request.

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

Willem de Bruijn (3):
  virtio-net: napi helper functions
  virtio-net: transmit napi
  virtio-net: clean tx descriptors from rx napi

 drivers/net/virtio_net.c | 150 ++++++++++++++++++++++++++++++++++-------------
 1 file changed, 110 insertions(+), 40 deletions(-)

-- 
2.12.2.564.g063fe858b8-goog

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

Prepare virtio-net for tx napi by converting existing napi code to
use helper functions. This also deduplicates some logic.

Signed-off-by: Willem de Bruijn <willemb at google.com>
Signed-off-by: Jason Wang <jasowang at redhat.com>
---
 drivers/net/virtio_net.c | 65 ++++++++++++++++++++++++++----------------------
 1 file changed, 35 insertions(+), 30 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index b0d241d110ec..6aac0ad0d9b2 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -239,6 +239,26 @@ static struct page *get_a_page(struct receive_queue *rq,
gfp_t gfp_mask)
 	return p;
 }
 
+static void virtqueue_napi_schedule(struct napi_struct *napi,
+				    struct virtqueue *vq)
+{
+	if (napi_schedule_prep(napi)) {
+		virtqueue_disable_cb(vq);
+		__napi_schedule(napi);
+	}
+}
+
+static void virtqueue_napi_complete(struct napi_struct *napi,
+				    struct virtqueue *vq, int processed)
+{
+	int opaque;
+
+	opaque = virtqueue_enable_cb_prepare(vq);
+	if (napi_complete_done(napi, processed) &&
+	    unlikely(virtqueue_poll(vq, opaque)))
+		virtqueue_napi_schedule(napi, vq);
+}
+
 static void skb_xmit_done(struct virtqueue *vq)
 {
 	struct virtnet_info *vi = vq->vdev->priv;
@@ -936,27 +956,20 @@ static void skb_recv_done(struct virtqueue *rvq)
 	struct virtnet_info *vi = rvq->vdev->priv;
 	struct receive_queue *rq = &vi->rq[vq2rxq(rvq)];
 
-	/* Schedule NAPI, Suppress further interrupts if successful. */
-	if (napi_schedule_prep(&rq->napi)) {
-		virtqueue_disable_cb(rvq);
-		__napi_schedule(&rq->napi);
-	}
+	virtqueue_napi_schedule(&rq->napi, rvq);
 }
 
-static void virtnet_napi_enable(struct receive_queue *rq)
+static void virtnet_napi_enable(struct virtqueue *vq, struct napi_struct *napi)
 {
-	napi_enable(&rq->napi);
+	napi_enable(napi);
 
 	/* If all buffers were filled by other side before we napi_enabled, we
-	 * won't get another interrupt, so process any outstanding packets
-	 * now.  virtnet_poll wants re-enable the queue, so we disable here.
-	 * We synchronize against interrupts via NAPI_STATE_SCHED */
-	if (napi_schedule_prep(&rq->napi)) {
-		virtqueue_disable_cb(rq->vq);
-		local_bh_disable();
-		__napi_schedule(&rq->napi);
-		local_bh_enable();
-	}
+	 * won't get another interrupt, so process any outstanding packets now.
+	 * Call local_bh_enable after to trigger softIRQ processing.
+	 */
+	local_bh_disable();
+	virtqueue_napi_schedule(napi, vq);
+	local_bh_enable();
 }
 
 static void refill_work(struct work_struct *work)
@@ -971,7 +984,7 @@ static void refill_work(struct work_struct *work)
 
 		napi_disable(&rq->napi);
 		still_empty = !try_fill_recv(vi, rq, GFP_KERNEL);
-		virtnet_napi_enable(rq);
+		virtnet_napi_enable(rq->vq, &rq->napi);
 
 		/* In theory, this can happen: if we don't get any buffers in
 		 * we will *never* try to fill again.
@@ -1011,21 +1024,13 @@ static int virtnet_poll(struct napi_struct *napi, int
budget)
 {
 	struct receive_queue *rq  		container_of(napi, struct receive_queue, napi);
-	unsigned int r, received;
+	unsigned int received;
 
 	received = virtnet_receive(rq, budget);
 
 	/* Out of packets? */
-	if (received < budget) {
-		r = virtqueue_enable_cb_prepare(rq->vq);
-		if (napi_complete_done(napi, received)) {
-			if (unlikely(virtqueue_poll(rq->vq, r)) &&
-			    napi_schedule_prep(napi)) {
-				virtqueue_disable_cb(rq->vq);
-				__napi_schedule(napi);
-			}
-		}
-	}
+	if (received < budget)
+		virtqueue_napi_complete(napi, rq->vq, received);
 
 	return received;
 }
@@ -1040,7 +1045,7 @@ static int virtnet_open(struct net_device *dev)
 			/* Make sure we have some buffers: if oom use wq. */
 			if (!try_fill_recv(vi, &vi->rq[i], GFP_KERNEL))
 				schedule_delayed_work(&vi->refill, 0);
-		virtnet_napi_enable(&vi->rq[i]);
+		virtnet_napi_enable(vi->rq[i].vq, &vi->rq[i].napi);
 	}
 
 	return 0;
@@ -1747,7 +1752,7 @@ static int virtnet_restore_up(struct virtio_device *vdev)
 				schedule_delayed_work(&vi->refill, 0);
 
 		for (i = 0; i < vi->max_queue_pairs; i++)
-			virtnet_napi_enable(&vi->rq[i]);
+			virtnet_napi_enable(vi->rq[i].vq, &vi->rq[i].napi);
 	}
 
 	netif_device_attach(vi->dev);
-- 
2.12.2.564.g063fe858b8-goog

From: Willem de Bruijn <willemb at 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 at google.com>
Signed-off-by: Jason Wang <jasowang at redhat.com>
---
 drivers/net/virtio_net.c | 63 ++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 53 insertions(+), 10 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 6aac0ad0d9b2..95d938e82080 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -33,9 +33,10 @@
 static int napi_weight = NAPI_POLL_WEIGHT;
 module_param(napi_weight, int, 0444);
 
-static bool csum = true, gso = true;
+static bool csum = true, gso = true, napi_tx = true;
 module_param(csum, bool, 0444);
 module_param(gso, bool, 0444);
+module_param(napi_tx, bool, 0644);
 
 /* FIXME: MTU in config. */
 #define GOOD_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
@@ -86,6 +87,8 @@ 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 +265,16 @@ 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 +968,9 @@ 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
@@ -1046,6 +1056,7 @@ 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;
@@ -1081,6 +1092,24 @@ static void free_old_xmit_skbs(struct send_queue *sq)
 	u64_stats_update_end(&stats->tx_syncp);
 }
 
+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));
+
+	__netif_tx_lock(txq, smp_processor_id());
+	free_old_xmit_skbs(sq);
+	__netif_tx_unlock(txq);
+
+	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 0;
+}
+
 static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
 {
 	struct virtio_net_hdr_mrg_rxbuf *hdr;
@@ -1130,9 +1159,11 @@ 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);
 
 	/* timestamp packet in software */
 	skb_tx_timestamp(skb);
@@ -1152,8 +1183,10 @@ 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.
@@ -1167,7 +1200,8 @@ 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);
 			if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
@@ -1371,8 +1405,10 @@ 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;
 }
@@ -1727,8 +1763,10 @@ static void virtnet_freeze_down(struct virtio_device
*vdev)
 	cancel_delayed_work_sync(&vi->refill);
 
 	if (netif_running(vi->dev)) {
-		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);
+		}
 	}
 }
 
@@ -1751,8 +1789,10 @@ 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);
@@ -1957,6 +1997,7 @@ 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(),
@@ -2142,6 +2183,8 @@ 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 ? napi_weight : 0);
 
 		sg_init_table(vi->rq[i].sg, ARRAY_SIZE(vi->rq[i].sg));
 		ewma_pkt_len_init(&vi->rq[i].mrg_avg_pkt_len);
-- 
2.12.2.564.g063fe858b8-goog

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

Amortize the cost of virtual interrupts by doing both rx and tx work
on reception of a receive interrupt if tx napi is enabled. With
VIRTIO_F_EVENT_IDX, this suppresses most explicit tx completion
interrupts for bidirectional workloads.

Signed-off-by: Willem de Bruijn <willemb at google.com>
---
 drivers/net/virtio_net.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 95d938e82080..af830eb212bf 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1030,12 +1030,34 @@ static int virtnet_receive(struct receive_queue *rq, int
budget)
 	return received;
 }
 
+static void free_old_xmit_skbs(struct send_queue *sq);
+
+static void virtnet_poll_cleantx(struct receive_queue *rq)
+{
+	struct virtnet_info *vi = rq->vq->vdev->priv;
+	unsigned int index = vq2rxq(rq->vq);
+	struct send_queue *sq = &vi->sq[index];
+	struct netdev_queue *txq = netdev_get_tx_queue(vi->dev, index);
+
+	if (!sq->napi.weight)
+		return;
+
+	__netif_tx_lock(txq, smp_processor_id());
+	free_old_xmit_skbs(sq);
+	__netif_tx_unlock(txq);
+
+	if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
+		netif_wake_subqueue(vi->dev, vq2txq(sq->vq));
+}
+
 static int virtnet_poll(struct napi_struct *napi, int budget)
 {
 	struct receive_queue *rq  		container_of(napi, struct receive_queue, napi);
 	unsigned int received;
 
+	virtnet_poll_cleantx(rq);
+
 	received = virtnet_receive(rq, budget);
 
 	/* Out of packets? */
-- 
2.12.2.564.g063fe858b8-goog

On Sun, Apr 02, 2017 at 04:10:11PM -0400, Willem de Bruijn
wrote:
> From: Willem de Bruijn <willemb at 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 at google.com>
> Signed-off-by: Jason Wang <jasowang at redhat.com>
> ---
>  drivers/net/virtio_net.c | 63
++++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 53 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 6aac0ad0d9b2..95d938e82080 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -33,9 +33,10 @@
>  static int napi_weight = NAPI_POLL_WEIGHT;
>  module_param(napi_weight, int, 0444);
>  
> -static bool csum = true, gso = true;
> +static bool csum = true, gso = true, napi_tx = true;
>  module_param(csum, bool, 0444);
>  module_param(gso, bool, 0444);
> +module_param(napi_tx, bool, 0644);
>  
>  /* FIXME: MTU in config. */
>  #define GOOD_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
Off by default seems safer until we can find better ways
to reduce the overhead, esp for UDP.
> @@ -86,6 +87,8 @@ 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 +265,16 @@ 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 +968,9 @@ 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
> @@ -1046,6 +1056,7 @@ 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;
> @@ -1081,6 +1092,24 @@ static void free_old_xmit_skbs(struct send_queue
*sq)
>  	u64_stats_update_end(&stats->tx_syncp);
>  }
>  
> +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));
> +
> +	__netif_tx_lock(txq, smp_processor_id());
> +	free_old_xmit_skbs(sq);
> +	__netif_tx_unlock(txq);
> +
> +	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 0;
> +}
> +
>  static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
>  {
>  	struct virtio_net_hdr_mrg_rxbuf *hdr;
> @@ -1130,9 +1159,11 @@ 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);
>  
>  	/* timestamp packet in software */
>  	skb_tx_timestamp(skb);
> @@ -1152,8 +1183,10 @@ 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.
> @@ -1167,7 +1200,8 @@ 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);
>  			if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
> @@ -1371,8 +1405,10 @@ 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;
>  }
> @@ -1727,8 +1763,10 @@ static void virtnet_freeze_down(struct virtio_device
*vdev)
>  	cancel_delayed_work_sync(&vi->refill);
>  
>  	if (netif_running(vi->dev)) {
> -		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);
> +		}
>  	}
>  }
>  
> @@ -1751,8 +1789,10 @@ 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);
> @@ -1957,6 +1997,7 @@ 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(),
> @@ -2142,6 +2183,8 @@ 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 ? napi_weight : 0);
>  
>  		sg_init_table(vi->rq[i].sg, ARRAY_SIZE(vi->rq[i].sg));
>  		ewma_pkt_len_init(&vi->rq[i].mrg_avg_pkt_len);
> -- 
> 2.12.2.564.g063fe858b8-goog

On Sun, Apr 02, 2017 at 04:10:12PM -0400, Willem de Bruijn
wrote:
> From: Willem de Bruijn <willemb at google.com>
> 
> Amortize the cost of virtual interrupts by doing both rx and tx work
> on reception of a receive interrupt if tx napi is enabled. With
> VIRTIO_F_EVENT_IDX, this suppresses most explicit tx completion
> interrupts for bidirectional workloads.
> 
> Signed-off-by: Willem de Bruijn <willemb at google.com>
> ---
>  drivers/net/virtio_net.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 95d938e82080..af830eb212bf 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1030,12 +1030,34 @@ static int virtnet_receive(struct receive_queue
*rq, int budget)
>  	return received;
>  }
>  
> +static void free_old_xmit_skbs(struct send_queue *sq);
> +
Could you pls re-arrange code to avoid forward declarations?
> +static void virtnet_poll_cleantx(struct receive_queue *rq)
> +{
> +	struct virtnet_info *vi = rq->vq->vdev->priv;
> +	unsigned int index = vq2rxq(rq->vq);
> +	struct send_queue *sq = &vi->sq[index];
> +	struct netdev_queue *txq = netdev_get_tx_queue(vi->dev, index);
> +
> +	if (!sq->napi.weight)
> +		return;
> +
> +	__netif_tx_lock(txq, smp_processor_id());
> +	free_old_xmit_skbs(sq);
> +	__netif_tx_unlock(txq);
> +
> +	if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
> +		netif_wake_subqueue(vi->dev, vq2txq(sq->vq));
> +}
> +
Looks very similar to virtnet_poll_tx.


I think this might be waking the tx queue too early, so
it will tend to stay almost full for long periods of time.
Why not defer wakeup until queue is at least half empty?

I wonder whether it's worth it to handle very short queues
correctly - they previously made very slow progress,
not they are never woken up.

I'm a bit concerned about the cost of these wakeups
and locking. I note that this wake is called basically
every time queue is not full.

Would it make sense to limit the amount of tx polling?
Maybe use trylock to reduce the conflict with xmit?
>  static int virtnet_poll(struct napi_struct *napi, int budget)
>  {
>  	struct receive_queue *rq >  		container_of(napi, struct receive_queue,
napi);
>  	unsigned int received;
>  
> +	virtnet_poll_cleantx(rq);
> +
>  	received = virtnet_receive(rq, budget);
>  
>  	/* Out of packets? */
> -- 
> 2.12.2.564.g063fe858b8-goog