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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 2 Jan 2019 08:59:30 -0500

On Wed, Jan 02, 2019 at 11:28:43AM +0800, Jason Wang wrote:
> 
> On 2018/12/31 上午2:45, Michael S. Tsirkin wrote:
> > On Thu, Dec 27, 2018 at 06:00:36PM +0800, Jason Wang wrote:
> > > On 2018/12/26 下午11:19, 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 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,
> > > > So how about increasing TSQ pacing shift then?
> > > 
> > > I can test this. But changing default TCP value is much more than a
> > > virtio-net specific thing.
> > Well same logic as wifi applies. Unpredictable latencies related
> > to radio in one case, to host scheduler in the other.
> > 
> > > > > 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.
> > > > I don't think it's reasonable to expect userspace to be that smart ...
> > > > Why do we need time bounded? used ring is always updated when ring
> > > > becomes empty.
> > > 
> > > We don't add used when means BQL may not see the consumed packet in time.
> > > And the delay varies based on the workload since we count packets not bytes
> > > or time before doing the batched updating.
> > > 
> > > Thanks
> > Sorry I still don't get it.
> > When nothing is outstanding then we do update the used.
> > So if BQL stops userspace from sending packets then
> > we get an interrupt and packets start flowing again.
> 
> 
> Yes, but how about the cases of multiple flows. That's where I see unstable
> results.
> 
> 
> > 
> > It might be suboptimal, we might need to tune it but I doubt running
> > timers is a solution, timer interrupts cause VM exits.
> 
> 
> Probably not a timer but a time counter (or event byte counter) in vhost to
> add used and signal guest if it exceeds a value instead of waiting the
> number of packets.
> 
> 
> Thanks

Well we already have VHOST_NET_WEIGHT - is it too big then?

And maybe we should expose the "MORE" flag in the descriptor -
do you think that will help?

> 
> > 
> > > > > 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++;
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