netdev - Re: [PATCH net 1/2] virtio_net: separate the logic of checking whether sq is full

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Message-ID: <CACGkMEsyDvtTnUAUsv4Mg9sNnjthUuPaHsXbSs0vxGuQTAJPrQ@mail.gmail.com>
Date:   Tue, 7 Mar 2023 16:34:25 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc:     netdev@...r.kernel.org, "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>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        virtualization@...ts.linux-foundation.org, bpf@...r.kernel.org
Subject: Re: [PATCH net 1/2] virtio_net: separate the logic of checking
 whether sq is full

On Mon, Mar 6, 2023 at 12:15 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>
> Separate the logic of checking whether sq is full. The subsequent patch
> will reuse this func.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> ---
>  drivers/net/virtio_net.c | 59 ++++++++++++++++++++++++----------------
>  1 file changed, 35 insertions(+), 24 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index fb5e68ed3ec2..777de0ec0b1b 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1750,6 +1750,40 @@ static void free_old_xmit_skbs(struct send_queue *sq, bool in_napi)
>         u64_stats_update_end(&sq->stats.syncp);
>  }
>
> +static void check_sq_full(struct virtnet_info *vi, struct net_device *dev,
> +                         struct send_queue *sq)
> +{

Nit: we need a better name, it's not only the check but also can disable TX.

Thanks

> +       bool use_napi = sq->napi.weight;
> +       int qnum;
> +
> +       qnum = sq - vi->sq;
> +
> +       /* If running out of space, stop queue to avoid getting packets that we
> +        * are then unable to transmit.
> +        * An alternative would be to force queuing layer to requeue the skb by
> +        * returning NETDEV_TX_BUSY. However, NETDEV_TX_BUSY should not be
> +        * returned in a normal path of operation: it means that driver is not
> +        * maintaining the TX queue stop/start state properly, and causes
> +        * the stack to do a non-trivial amount of useless work.
> +        * Since most packets only take 1 or 2 ring slots, stopping the queue
> +        * early means 16 slots are typically wasted.
> +        */
> +       if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
> +               netif_stop_subqueue(dev, qnum);
> +               if (use_napi) {
> +                       if (unlikely(!virtqueue_enable_cb_delayed(sq->vq)))
> +                               virtqueue_napi_schedule(&sq->napi, sq->vq);
> +               } else if (unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
> +                       /* More just got used, free them then recheck. */
> +                       free_old_xmit_skbs(sq, false);
> +                       if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
> +                               netif_start_subqueue(dev, qnum);
> +                               virtqueue_disable_cb(sq->vq);
> +                       }
> +               }
> +       }
> +}
> +
>  static bool is_xdp_raw_buffer_queue(struct virtnet_info *vi, int q)
>  {
>         if (q < (vi->curr_queue_pairs - vi->xdp_queue_pairs))
> @@ -1989,30 +2023,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
>                 nf_reset_ct(skb);
>         }
>
> -       /* If running out of space, stop queue to avoid getting packets that we
> -        * are then unable to transmit.
> -        * An alternative would be to force queuing layer to requeue the skb by
> -        * returning NETDEV_TX_BUSY. However, NETDEV_TX_BUSY should not be
> -        * returned in a normal path of operation: it means that driver is not
> -        * maintaining the TX queue stop/start state properly, and causes
> -        * the stack to do a non-trivial amount of useless work.
> -        * Since most packets only take 1 or 2 ring slots, stopping the queue
> -        * early means 16 slots are typically wasted.
> -        */
> -       if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
> -               netif_stop_subqueue(dev, qnum);
> -               if (use_napi) {
> -                       if (unlikely(!virtqueue_enable_cb_delayed(sq->vq)))
> -                               virtqueue_napi_schedule(&sq->napi, sq->vq);
> -               } else if (unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
> -                       /* More just got used, free them then recheck. */
> -                       free_old_xmit_skbs(sq, false);
> -                       if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
> -                               netif_start_subqueue(dev, qnum);
> -                               virtqueue_disable_cb(sq->vq);
> -                       }
> -               }
> -       }
> +       check_sq_full(vi, dev, sq);
>
>         if (kick || netif_xmit_stopped(txq)) {
>                 if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq)) {
> --
> 2.32.0.3.g01195cf9f
>