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

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Date:   Wed, 26 Dec 2018 10:22:30 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     linux-kernel@...r.kernel.org, maxime.coquelin@...hat.com,
        tiwei.bie@...el.com, wexu@...hat.com, jfreimann@...hat.com,
        "David S. Miller" <davem@...emloft.net>,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH RFC 1/2] virtio-net: bql support

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?


> 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@...hat.com>
> > ---
> >   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++;