Re: [PATCH net-next] virtio-net: switch napi_tx without downing nic

[Heng Qi <hengqi@xxxxxxxxxxxxxxxxx>]

在 2023/12/20 下午10:45, Willem de Bruijn 写道:
> Heng Qi wrote:
> > virtio-net has two ways to switch napi_tx: one is through the
> > module parameter, and the other is through coalescing parameter
> > settings (provided that the nic status is down).
> >
> > Sometimes we face performance regression caused by napi_tx,
> > then we need to switch napi_tx when debugging. However, the
> > existing methods are a bit troublesome, such as needing to
> > reload the driver or turn off the network card. So try to make
> > this update.
> >
> > Signed-off-by: Heng Qi <hengqi@xxxxxxxxxxxxxxxxx>
> > Reviewed-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> The commit does not explain why it is safe to do so.

virtnet_napi_tx_disable ensures that already scheduled tx napi ends and 
no new tx napi will be scheduled.

Afterwards, if the __netif_tx_lock_bh lock is held, the stack cannot 
send the packet.

Then we can safely toggle the weight to indicate where to clear the buffers.

> The tx-napi weights are not really weights: it is a boolean whether
> napi is used for transmit cleaning, or whether packets are cleaned
> in ndo_start_xmit.

Right.

> There certainly are some subtle issues with regard to pausing/waking
> queues when switching between modes.

What are "subtle issues" and if there are any, we find them.
So far my test results show it's working fine.

> Calling napi_enable/napi_disable without bringing down the device is
> allowed. The actually napi.weight field is only updated when neither
> napi nor ndo_start_xmit is running.

YES.

> So I don't see an immediate issue.

Switching napi_tx requires reloading the driver or downing the nic, 
which is troublesome.
I think it would be better if we could find a better way.

Thanks!

> > ---
> >   drivers/net/virtio_net.c | 81 ++++++++++++++++++----------------------
> >   1 file changed, 37 insertions(+), 44 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 10614e9f7cad..12f8e1f9971c 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -3559,16 +3559,37 @@ static int virtnet_coal_params_supported(struct ethtool_coalesce *ec)
> >   	return 0;
> >   }
> >   
> > -static int virtnet_should_update_vq_weight(int dev_flags, int weight,
> > -					   int vq_weight, bool *should_update)
> > +static void virtnet_switch_napi_tx(struct virtnet_info *vi, u32 qstart,
> > +				   u32 qend, u32 tx_frames)
> >   {
> > -	if (weight ^ vq_weight) {
> > -		if (dev_flags & IFF_UP)
> > -			return -EBUSY;
> > -		*should_update = true;
> > -	}
> > +	struct net_device *dev = vi->dev;
> > +	int new_weight, cur_weight;
> > +	struct netdev_queue *txq;
> > +	struct send_queue *sq;
> >   
> > -	return 0;
> > +	new_weight = tx_frames ? NAPI_POLL_WEIGHT : 0;
> > +	for (; qstart < qend; qstart++) {
> > +		sq = &vi->sq[qstart];
> > +		cur_weight = sq->napi.weight;
> > +		if (!(new_weight ^ cur_weight))
> > +			continue;
> > +
> > +		if (!(dev->flags & IFF_UP)) {
> > +			sq->napi.weight = new_weight;
> > +			continue;
> > +		}
> > +
> > +		if (cur_weight)
> > +			virtnet_napi_tx_disable(&sq->napi);
> > +
> > +		txq = netdev_get_tx_queue(dev, qstart);
> > +		__netif_tx_lock_bh(txq);
> > +		sq->napi.weight = new_weight;
> > +		__netif_tx_unlock_bh(txq);
> > +
> > +		if (!cur_weight)
> > +			virtnet_napi_tx_enable(vi, sq->vq, &sq->napi);
> > +	}
> >   }
> >   
> >   static int virtnet_set_coalesce(struct net_device *dev,
> > @@ -3577,25 +3598,11 @@ static int virtnet_set_coalesce(struct net_device *dev,
> >   				struct netlink_ext_ack *extack)
> >   {
> >   	struct virtnet_info *vi = netdev_priv(dev);
> > -	int ret, queue_number, napi_weight;
> > -	bool update_napi = false;
> > -
> > -	/* Can't change NAPI weight if the link is up */
> > -	napi_weight = ec->tx_max_coalesced_frames ? NAPI_POLL_WEIGHT : 0;
> > -	for (queue_number = 0; queue_number < vi->max_queue_pairs; queue_number++) {
> > -		ret = virtnet_should_update_vq_weight(dev->flags, napi_weight,
> > -						      vi->sq[queue_number].napi.weight,
> > -						      &update_napi);
> > -		if (ret)
> > -			return ret;
> > -
> > -		if (update_napi) {
> > -			/* All queues that belong to [queue_number, vi->max_queue_pairs] will be
> > -			 * updated for the sake of simplicity, which might not be necessary
> > -			 */
> > -			break;
> > -		}
> > -	}
> > +	int ret;
> > +
> > +	/* Param tx_frames can be used to switch napi_tx */
> > +	virtnet_switch_napi_tx(vi, 0, vi->max_queue_pairs,
> > +			       ec->tx_max_coalesced_frames);
> >   
> >   	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_NOTF_COAL))
> >   		ret = virtnet_send_notf_coal_cmds(vi, ec);
> > @@ -3605,11 +3612,6 @@ static int virtnet_set_coalesce(struct net_device *dev,
> >   	if (ret)
> >   		return ret;
> >   
> > -	if (update_napi) {
> > -		for (; queue_number < vi->max_queue_pairs; queue_number++)
> > -			vi->sq[queue_number].napi.weight = napi_weight;
> > -	}
> > -
> >   	return ret;
> >   }
> >   
> > @@ -3641,19 +3643,13 @@ static int virtnet_set_per_queue_coalesce(struct net_device *dev,
> >   					  struct ethtool_coalesce *ec)
> >   {
> >   	struct virtnet_info *vi = netdev_priv(dev);
> > -	int ret, napi_weight;
> > -	bool update_napi = false;
> > +	int ret;
> >   
> >   	if (queue >= vi->max_queue_pairs)
> >   		return -EINVAL;
> >   
> > -	/* Can't change NAPI weight if the link is up */
> > -	napi_weight = ec->tx_max_coalesced_frames ? NAPI_POLL_WEIGHT : 0;
> > -	ret = virtnet_should_update_vq_weight(dev->flags, napi_weight,
> > -					      vi->sq[queue].napi.weight,
> > -					      &update_napi);
> > -	if (ret)
> > -		return ret;
> > +	/* Param tx_frames can be used to switch napi_tx */
> > +	virtnet_switch_napi_tx(vi, queue, queue, ec->tx_max_coalesced_frames);
> >   
> >   	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_VQ_NOTF_COAL))
> >   		ret = virtnet_send_notf_coal_vq_cmds(vi, ec, queue);
> > @@ -3663,9 +3659,6 @@ static int virtnet_set_per_queue_coalesce(struct net_device *dev,
> >   	if (ret)
> >   		return ret;
> >   
> > -	if (update_napi)
> > -		vi->sq[queue].napi.weight = napi_weight;
> > -
> >   	return 0;
> >   }
> >   
> > --
> > 2.19.1.6.gb485710b