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Date: Wed, 15 Oct 2014 13:43:09 +0300
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Jason Wang <jasowang@...hat.com>
Cc: rusty@...tcorp.com.au, virtualization@...ts.linux-foundation.org,
linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
davem@...emloft.net, eric.dumazet@...il.com
Subject: Re: [RFC PATCH net-next 5/6] virtio-net: enable tx interrupt
On Wed, Oct 15, 2014 at 06:25:25PM +0800, Jason Wang wrote:
> On 10/15/2014 06:18 PM, Michael S. Tsirkin wrote:
> > On Wed, Oct 15, 2014 at 03:25:29PM +0800, Jason Wang wrote:
> >> > Orphan skb in ndo_start_xmit() breaks socket accounting and packet
> >> > queuing. This in fact breaks lots of things such as pktgen and several
> >> > TCP optimizations. And also make BQL can't be implemented for
> >> > virtio-net.
> >> >
> >> > This patch tries to solve this issue by enabling tx interrupt. To
> >> > avoid introducing extra spinlocks, a tx napi was scheduled to free
> >> > those packets.
> >> >
> >> > More tx interrupt mitigation method could be used on top.
> >> >
> >> > Cc: Rusty Russell <rusty@...tcorp.com.au>
> >> > Cc: Michael S. Tsirkin <mst@...hat.com>
> >> > Signed-off-by: Jason Wang <jasowang@...hat.com>
> >> > ---
> >> > drivers/net/virtio_net.c | 125 +++++++++++++++++++++++++++++++---------------
> >> > 1 files changed, 85 insertions(+), 40 deletions(-)
> >> >
> >> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> >> > index ccf98f9..2afc2e2 100644
> >> > --- a/drivers/net/virtio_net.c
> >> > +++ b/drivers/net/virtio_net.c
> >> > @@ -72,6 +72,8 @@ struct send_queue {
> >> >
> >> > /* Name of the send queue: output.$index */
> >> > char name[40];
> >> > +
> >> > + struct napi_struct napi;
> >> > };
> >> >
> >> > /* Internal representation of a receive virtqueue */
> >> > @@ -217,15 +219,40 @@ static struct page *get_a_page(struct receive_queue *rq, gfp_t gfp_mask)
> >> > return p;
> >> > }
> >> >
> >> > +static int free_old_xmit_skbs(struct send_queue *sq, int budget)
> >> > +{
> >> > + struct sk_buff *skb;
> >> > + unsigned int len;
> >> > + struct virtnet_info *vi = sq->vq->vdev->priv;
> >> > + struct virtnet_stats *stats = this_cpu_ptr(vi->stats);
> >> > + u64 tx_bytes = 0, tx_packets = 0;
> >> > +
> >> > + while (tx_packets < budget &&
> >> > + (skb = virtqueue_get_buf(sq->vq, &len)) != NULL) {
> >> > + pr_debug("Sent skb %p\n", skb);
> >> > +
> >> > + tx_bytes += skb->len;
> >> > + tx_packets++;
> >> > +
> >> > + dev_kfree_skb_any(skb);
> >> > + }
> >> > +
> >> > + u64_stats_update_begin(&stats->tx_syncp);
> >> > + stats->tx_bytes += tx_bytes;
> >> > + stats->tx_packets =+ tx_packets;
> >> > + u64_stats_update_end(&stats->tx_syncp);
> >> > +
> >> > + return tx_packets;
> >> > +}
> >> > +
> >> > static void skb_xmit_done(struct virtqueue *vq)
> >> > {
> >> > struct virtnet_info *vi = vq->vdev->priv;
> >> > + struct send_queue *sq = &vi->sq[vq2txq(vq)];
> >> >
> >> > - /* Suppress further interrupts. */
> >> > - virtqueue_disable_cb(vq);
> >> > -
> >> > - /* We were probably waiting for more output buffers. */
> >> > - netif_wake_subqueue(vi->dev, vq2txq(vq));
> >> > + if (napi_schedule_prep(&sq->napi)) {
> >> > + __napi_schedule(&sq->napi);
> >> > + }
> >> > }
> >> >
> >> > static unsigned int mergeable_ctx_to_buf_truesize(unsigned long mrg_ctx)
> >> > @@ -774,7 +801,39 @@ again:
> >> > return received;
> >> > }
> >> >
> >> > +static int virtnet_poll_tx(struct napi_struct *napi, int budget)
> >> > +{
> >> > + struct send_queue *sq =
> >> > + container_of(napi, struct send_queue, napi);
> >> > + struct virtnet_info *vi = sq->vq->vdev->priv;
> >> > + struct netdev_queue *txq = netdev_get_tx_queue(vi->dev, vq2txq(sq->vq));
> >> > + unsigned int r, sent = 0;
> >> > +
> >> > +again:
> >> > + __netif_tx_lock(txq, smp_processor_id());
> >> > + virtqueue_disable_cb(sq->vq);
> >> > + sent += free_old_xmit_skbs(sq, budget - sent);
> >> > +
> >> > + if (sent < budget) {
> >> > + r = virtqueue_enable_cb_prepare(sq->vq);
> >> > + napi_complete(napi);
> >> > + __netif_tx_unlock(txq);
> >> > + if (unlikely(virtqueue_poll(sq->vq, r)) &&
> > So you are enabling callback on the next packet,
> > which is almost sure to cause an interrupt storm
> > on the guest.
> >
> >
> > I think it's a bad idea, this is why I used
> > enable_cb_delayed in my patch.
>
> Right, will do this, but may also need to make sure used event never
> goes back since we may call virtqueue_enable_cb_avail().
That's why my patch always calls virtqueue_enable_cb_delayed.
So no need for hacks.
Maybe you can review my patch and comment?
> >
> >
> >> > + napi_schedule_prep(napi)) {
> >> > + virtqueue_disable_cb(sq->vq);
> >> > + __napi_schedule(napi);
> >> > + goto again;
> >> > + }
> >> > + } else {
> >> > + __netif_tx_unlock(txq);
> >> > + }
> >> > +
> >> > + netif_wake_subqueue(vi->dev, vq2txq(sq->vq));
> >> > + return sent;
> >> > +}
> >> > +
> >> > #ifdef CONFIG_NET_RX_BUSY_POLL
> >> > +
> >> > /* must be called with local_bh_disable()d */
> >> > static int virtnet_busy_poll(struct napi_struct *napi)
> >> > {
> >> > @@ -822,36 +881,12 @@ static int virtnet_open(struct net_device *dev)
> >> > if (!try_fill_recv(&vi->rq[i], GFP_KERNEL))
> >> > schedule_delayed_work(&vi->refill, 0);
> >> > virtnet_napi_enable(&vi->rq[i]);
> >> > + napi_enable(&vi->sq[i].napi);
> >> > }
> >> >
> >> > return 0;
> >> > }
> >> >
> >> > -static int free_old_xmit_skbs(struct send_queue *sq)
> >> > -{
> >> > - struct sk_buff *skb;
> >> > - unsigned int len;
> >> > - struct virtnet_info *vi = sq->vq->vdev->priv;
> >> > - struct virtnet_stats *stats = this_cpu_ptr(vi->stats);
> >> > - u64 tx_bytes = 0, tx_packets = 0;
> >> > -
> >> > - while ((skb = virtqueue_get_buf(sq->vq, &len)) != NULL) {
> >> > - pr_debug("Sent skb %p\n", skb);
> >> > -
> >> > - tx_bytes += skb->len;
> >> > - tx_packets++;
> >> > -
> >> > - dev_kfree_skb_any(skb);
> >> > - }
> >> > -
> >> > - u64_stats_update_begin(&stats->tx_syncp);
> >> > - stats->tx_bytes += tx_bytes;
> >> > - stats->tx_packets =+ tx_packets;
> >> > - u64_stats_update_end(&stats->tx_syncp);
> >> > -
> >> > - return tx_packets;
> >> > -}
> >> > -
> > So you end up moving it all anyway, why bother splitting out
> > minor changes in previous patches?
>
> To make review easier, but if you think this complicated it in fact,
> will pack them into a single patch.
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