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Date: Sun, 30 Dec 2018 13:48:12 -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 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 <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++;
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