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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 30 Dec 2018 13:48:12 -0500

On Thu, Dec 27, 2018 at 06:04:53PM +0800, Jason Wang wrote:
> 
> On 2018/12/26 下午11:22, Michael S. Tsirkin wrote:
> > On Thu, Dec 06, 2018 at 04:17:36PM +0800, Jason Wang wrote:
> > > 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 just went over the patch again and I don't understand this comment.
> > This patch only enabled BQL with tx napi.
> > 
> > Thus there's no mode switching.
> > 
> > What did I miss?
> 
> 
> Consider the case:
> 
> 
> TX NAPI is disabled:
> 
> send N packets
> 
> turn TX NAPI on:
> 
> get tx interrupt
> 
> BQL try to consume those packets when lead WARN for dql.
> 
> 
> Thanks

Can one really switch tx napi on and off? How?
While root can change the napi_tx module parameter, I don't think
that has any effect outside device probe time. What did I miss?

> 
> 
> > 
> > 
> > > 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 <[email-masked]>
> > > > ---
> > > >    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++;