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

From: Heng Qi <[email-masked]>
Date: Tue, 7 May 2024 14:27:11 +0800

On Tue, 7 May 2024 14:24:16 +0800, Jason Wang <[email-masked]> wrote:
> On Tue, May 7, 2024 at 12:03 PM Heng Qi <[email-masked]> wrote:
> >
> > On Tue, 7 May 2024 11:15:22 +0800, Jason Wang <[email-masked]> wrote:
> > > On Thu, Apr 25, 2024 at 8:59 PM Heng Qi <[email-masked]> 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 <[email-masked]>
> > > > Signed-off-by: Heng Qi <[email-masked]>
> > > > ---
> > > >  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?
> >
> > For user commands (such as ethtool cmds), multiple cvq requests is not allowed.
> > because it holds the netlink lock when waiting for the response.
> >
> > For multiple dim commands and a user command allowed to be sent simultaneously
> > , the corresponding command-specific information(desc_state) will be used to
> > distinguish different responses.
> 
> Just to make sure we are on the same page. I meant at least we are
> still use the global completion which seems to be problematic.
> 
> wait_for_completion(&vi->completion);

This completion is only used by the ethtool command, so it is the global one.

dim commands use specific coal_free_list and coal_wait_list to complete
multiple command issuance (please see patch 3).

Thanks.

> 
> Thanks
> 
> 
> >
> > Thanks.
> >
> > >
> > > 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
> > > >
> > >
> >
>