Subject: Re: [RFC PATCH net-next 5/6] virtio-net: enable tx interrupt

From: Jason Wang <[email-masked]>
Date: Wed, 15 Oct 2014 19:00:22 +0800

On 10/15/2014 06:43 PM, Michael S. Tsirkin wrote:
> 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 <[email-masked]>
>>>>> > >> > Cc: Michael S. Tsirkin <[email-masked]>
>>>>> > >> > Signed-off-by: Jason Wang <[email-masked]>
>>>>> > >> > ---
>>>>> > >> >  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?

I think I miss the virtqueue_enable_cb_delayed() in your patch when I'm
reviewing it. Will do.