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Date: Wed, 20 Dec 2023 09:45:25 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Heng Qi <hengqi@...ux.alibaba.com>,
netdev@...r.kernel.org,
virtualization@...ts.linux-foundation.org
Cc: Jason Wang <jasowang@...hat.com>,
"Michael S. Tsirkin" <mst@...hat.com>,
"David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>,
Alexei Starovoitov <ast@...nel.org>,
Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Subject: Re: [PATCH net-next] virtio-net: switch napi_tx without downing nic
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@...ux.alibaba.com>
> Reviewed-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
The commit does not explain why it is safe to do so.
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.
There certainly are some subtle issues with regard to pausing/waking
queues when switching between modes.
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. So I don't see an immediate issue.
> ---
> 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
>
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