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

[Jason Wang <jasowang@xxxxxxxxxx>]

On 2018/12/6 上午6:54, 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
> benefitial.


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?


Thanks


> 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)
>   		return;
>   
> +	if (use_napi)
> +		netdev_tx_completed_queue(txq, packets, bytes);
> +
>   	u64_stats_update_begin(&sq->stats.syncp);
>   	sq->stats.bytes += bytes;
>   	sq->stats.packets += packets;
> @@ -1364,7 +1368,7 @@ static void virtnet_poll_cleantx(struct receive_queue *rq)
>   		return;
>   
>   	if (__netif_tx_trylock(txq)) {
> -		free_old_xmit_skbs(sq);
> +		free_old_xmit_skbs(sq, txq, true);
>   		__netif_tx_unlock(txq);
>   	}
>   
> @@ -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);
>   	__netif_tx_unlock(txq);
>   
>   	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)
>   		virtqueue_enable_cb_delayed(sq->vq);
>   
>   	/* 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);
>   				virtqueue_disable_cb(sq->vq);
> @@ -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)) {
>   			u64_stats_update_begin(&sq->stats.syncp);
>   			sq->stats.kicks++;
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