Re: [PATCH RFC 1/2] virtio-net: bql support

From: Jason Wang
Date: Thu Dec 27 2018 - 04:57:33 EST

On 2018/12/26 äå11:15, Michael S. Tsirkin wrote:
On Thu, Dec 06, 2018 at 04:17:36PM +0800, Jason Wang wrote:
On 2018/12/6 äå6:54, Michael S. Tsirkin wrote:
When use_napi is set, let's enable BQLs. Note: some of the issues are
similar to wifi. It's worth considering whether something similar to
commit 36148c2bbfbe ("mac80211: Adjust TSQ pacing shift") might be

I've played a similar patch several days before. The tricky part is the mode
switching between napi and no napi. We should make sure when the packet is
sent and trakced by BQL, it should be consumed by BQL as well. I did it by
tracking it through skb->cb. And deal with the freeze by reset the BQL
status. Patch attached.

But when testing with vhost-net, I don't very a stable performance, it was
probably because we batch the used ring updating so tx interrupt may come
randomly. We probably need to implement time bounded coalescing mechanism
which could be configured from userspace.

Btw, maybe it's time just enable napi TX by default. I get ~10% TCP_RR
regression on machine without APICv, (haven't found time to test APICv
machine). But consider it was for correctness, I think it's acceptable? Then
we can do optimization on top?

I don't see how it's for correctness to be frank.

Socket accounting is wrong in the case. This should be a bug in fact.

What if we just do the bulk free? Does that fix the regression?

I can test it.

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
drivers/net/virtio_net.c | 27 +++++++++++++++++++--------
1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index cecfd77c9f3c..b657bde6b94b 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1325,7 +1325,8 @@ static int virtnet_receive(struct receive_queue *rq, int budget,
return stats.packets;
-static void free_old_xmit_skbs(struct send_queue *sq)
+static void free_old_xmit_skbs(struct send_queue *sq, struct netdev_queue *txq,
+ bool use_napi)
struct sk_buff *skb;
unsigned int len;
@@ -1347,6 +1348,9 @@ static void free_old_xmit_skbs(struct send_queue *sq)
if (!packets)
+ if (use_napi)
+ netdev_tx_completed_queue(txq, packets, bytes);
sq->stats.bytes += bytes;
sq->stats.packets += packets;
@@ -1364,7 +1368,7 @@ static void virtnet_poll_cleantx(struct receive_queue *rq)
if (__netif_tx_trylock(txq)) {
- free_old_xmit_skbs(sq);
+ free_old_xmit_skbs(sq, txq, true);
@@ -1440,7 +1444,7 @@ static int virtnet_poll_tx(struct napi_struct *napi, int budget)
struct netdev_queue *txq = netdev_get_tx_queue(vi->dev, vq2txq(sq->vq));
__netif_tx_lock(txq, raw_smp_processor_id());
- free_old_xmit_skbs(sq);
+ free_old_xmit_skbs(sq, txq, true);
virtqueue_napi_complete(napi, sq->vq, 0);
@@ -1505,13 +1509,15 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
struct send_queue *sq = &vi->sq[qnum];
int err;
struct netdev_queue *txq = netdev_get_tx_queue(dev, qnum);
- bool kick = !skb->xmit_more;
+ bool more = skb->xmit_more;
bool use_napi = sq->napi.weight;
+ unsigned int bytes = skb->len;
+ bool kick;
/* Free up any pending old buffers before queueing new ones. */
- free_old_xmit_skbs(sq);
+ free_old_xmit_skbs(sq, txq, use_napi);
- if (use_napi && kick)
+ if (use_napi && !more)
/* timestamp packet in software */
@@ -1552,7 +1558,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
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, txq, false);
if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
netif_start_subqueue(dev, qnum);
@@ -1560,7 +1566,12 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
- if (kick || netif_xmit_stopped(txq)) {
+ if (use_napi)
+ kick = __netdev_tx_sent_queue(txq, bytes, more);
+ else
+ kick = !more || netif_xmit_stopped(txq);
+ if (kick) {
if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq)) {