netdev - Re: [PATCH net-next 1/3] virtio_net: enable irq for the control vq

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Message-ID: <CACGkMEtpy0ZDcGQReaPVtJy4hDcqdKQwEF2Uhf5W4+7g=jts-Q@mail.gmail.com>
Date: Tue, 7 May 2024 11:15:22 +0800
From: Jason Wang <jasowang@...hat.com>
To: Heng Qi <hengqi@...ux.alibaba.com>
Cc: netdev@...r.kernel.org, virtualization@...ts.linux.dev, 
	"Michael S . Tsirkin" <mst@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
	Eric Dumazet <edumazet@...gle.com>, "David S . Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next 1/3] virtio_net: enable irq for the control vq

On Thu, Apr 25, 2024 at 8:59 PM Heng Qi <hengqi@...ux.alibaba.com> wrote:
>
> Control vq polling request results consume more CPU.
> Especially when dim issues more control requests to the device,
> it's beneficial to the guest to enable control vq's irq.
>
> Suggested-by: Jason Wang <jasowang@...hat.com>
> Signed-off-by: Heng Qi <hengqi@...ux.alibaba.com>
> ---
>  drivers/net/virtio_net.c | 45 ++++++++++++++++++++++++++++++----------
>  1 file changed, 34 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index a4d3c76654a4..79a1b30c173c 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -287,6 +287,12 @@ struct virtnet_info {
>         bool has_cvq;
>         struct mutex cvq_lock;
>
> +       /* Wait for the device to complete the request */
> +       struct completion completion;
> +
> +       /* Work struct for acquisition of cvq processing results. */
> +       struct work_struct get_cvq;
> +
>         /* Host can handle any s/g split between our header and packet data */
>         bool any_header_sg;
>
> @@ -520,6 +526,13 @@ static bool virtqueue_napi_complete(struct napi_struct *napi,
>         return false;
>  }
>
> +static void virtnet_cvq_done(struct virtqueue *cvq)
> +{
> +       struct virtnet_info *vi = cvq->vdev->priv;
> +
> +       schedule_work(&vi->get_cvq);
> +}
> +
>  static void skb_xmit_done(struct virtqueue *vq)
>  {
>         struct virtnet_info *vi = vq->vdev->priv;
> @@ -2036,6 +2049,20 @@ static bool try_fill_recv(struct virtnet_info *vi, struct receive_queue *rq,
>         return !oom;
>  }
>
> +static void virtnet_get_cvq_work(struct work_struct *work)
> +{
> +       struct virtnet_info *vi =
> +               container_of(work, struct virtnet_info, get_cvq);
> +       unsigned int tmp;
> +       void *res;
> +
> +       mutex_lock(&vi->cvq_lock);
> +       res = virtqueue_get_buf(vi->cvq, &tmp);
> +       if (res)
> +               complete(&vi->completion);
> +       mutex_unlock(&vi->cvq_lock);
> +}
> +
>  static void skb_recv_done(struct virtqueue *rvq)
>  {
>         struct virtnet_info *vi = rvq->vdev->priv;
> @@ -2531,7 +2558,7 @@ static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,
>                                  struct scatterlist *out)
>  {
>         struct scatterlist *sgs[4], hdr, stat;
> -       unsigned out_num = 0, tmp;
> +       unsigned out_num = 0;
>         int ret;
>
>         /* Caller should know better */
> @@ -2566,16 +2593,10 @@ static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,
>                 return vi->ctrl->status == VIRTIO_NET_OK;
>         }
>
> -       /* Spin for a response, the kick causes an ioport write, trapping
> -        * into the hypervisor, so the request should be handled immediately.
> -        */
> -       while (!virtqueue_get_buf(vi->cvq, &tmp) &&
> -              !virtqueue_is_broken(vi->cvq)) {
> -               cond_resched();
> -               cpu_relax();
> -       }
> -
>         mutex_unlock(&vi->cvq_lock);
> +
> +       wait_for_completion(&vi->completion);
> +

A question here, can multiple cvq requests be submitted to the device?
If yes, what happens if the device completes them out of order?

Thanks

>         return vi->ctrl->status == VIRTIO_NET_OK;
>  }
>
> @@ -4433,7 +4454,7 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
>
>         /* Parameters for control virtqueue, if any */
>         if (vi->has_cvq) {
> -               callbacks[total_vqs - 1] = NULL;
> +               callbacks[total_vqs - 1] = virtnet_cvq_done;
>                 names[total_vqs - 1] = "control";
>         }
>
> @@ -4952,6 +4973,8 @@ static int virtnet_probe(struct virtio_device *vdev)
>         if (vi->has_rss || vi->has_rss_hash_report)
>                 virtnet_init_default_rss(vi);
>
> +       INIT_WORK(&vi->get_cvq, virtnet_get_cvq_work);
> +       init_completion(&vi->completion);
>         enable_rx_mode_work(vi);
>
>         /* serialize netdev register + virtio_device_ready() with ndo_open() */
> --
> 2.32.0.3.g01195cf9f
>