Re: [PATCH 1/2] vdpa: mlx5: prevent cvq work from hogging CPU

[Jason Wang <jasowang@redhat.com>]

On Mon, Mar 21, 2022 at 4:53 PM Hillf Danton <hdanton@sina.com> wrote:
>
> On Mon, 21 Mar 2022 14:04:28 +0800 Jason Wang wrote:
> > A userspace triggerable infinite loop could happen in
> > mlx5_cvq_kick_handler() if userspace keeps sending a huge amount of
> > cvq requests.
> >
> > Fixing this by introducing a quota and re-queue the work if we're out
> > of the budget. While at it, using a per device workqueue to avoid on
> > demand memory allocation for cvq.
> >
> > Fixes: 5262912ef3cfc ("vdpa/mlx5: Add support for control VQ and MAC setting")
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > ---
> >  drivers/vdpa/mlx5/net/mlx5_vnet.c | 28 +++++++++++++++-------------
> >  1 file changed, 15 insertions(+), 13 deletions(-)
> >
> > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > index d0f91078600e..d5a6fb3f9c41 100644
> > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > @@ -163,6 +163,7 @@ struct mlx5_vdpa_net {
> >       u32 cur_num_vqs;
> >       struct notifier_block nb;
> >       struct vdpa_callback config_cb;
> > +     struct mlx5_vdpa_wq_ent cvq_ent;
> >  };
> >
> >  static void free_resources(struct mlx5_vdpa_net *ndev);
> > @@ -1600,6 +1601,8 @@ static virtio_net_ctrl_ack handle_ctrl_mq(struct mlx5_vdpa_dev *mvdev, u8 cmd)
> >       return status;
> >  }
> >
> > +#define MLX5_CVQ_BUDGET 16
> > +
>
> This is not needed as given a single thread workqueue, a cond_resched()
> can do the job in the worker context instead of requeue of work.
>
> Hillf

I'm not sure I get this, but there's a loop in the work fn:

while(true) {
}

Where there could be no chance for the cond_resched() to run?

Thanks

>
> >  static void mlx5_cvq_kick_handler(struct work_struct *work)
> >  {
> >       virtio_net_ctrl_ack status = VIRTIO_NET_ERR;
> > @@ -1609,17 +1612,17 @@ static void mlx5_cvq_kick_handler(struct work_struct *work)
> >       struct mlx5_control_vq *cvq;
> >       struct mlx5_vdpa_net *ndev;
> >       size_t read, write;
> > -     int err;
> > +     int err, n = 0;
> >
> >       wqent = container_of(work, struct mlx5_vdpa_wq_ent, work);
> >       mvdev = wqent->mvdev;
> >       ndev = to_mlx5_vdpa_ndev(mvdev);
> >       cvq = &mvdev->cvq;
> >       if (!(ndev->mvdev.actual_features & BIT_ULL(VIRTIO_NET_F_CTRL_VQ)))
> > -             goto out;
> > +             return;
> >
> >       if (!cvq->ready)
> > -             goto out;
> > +             return;
> >
> >       while (true) {
> >               err = vringh_getdesc_iotlb(&cvq->vring, &cvq->riov, &cvq->wiov, &cvq->head,
> > @@ -1653,9 +1656,13 @@ static void mlx5_cvq_kick_handler(struct work_struct *work)
> >
> >               if (vringh_need_notify_iotlb(&cvq->vring))
> >                       vringh_notify(&cvq->vring);
> > +
> > +             n++;
> > +             if (n > MLX5_CVQ_BUDGET) {
> > +                     queue_work(mvdev->wq, &wqent->work);
> > +                     break;
> > +             }
> >       }
> > -out:
> > -     kfree(wqent);
> >  }
> >
> >  static void mlx5_vdpa_kick_vq(struct vdpa_device *vdev, u16 idx)
> > @@ -1663,7 +1670,6 @@ static void mlx5_vdpa_kick_vq(struct vdpa_device *vdev, u16 idx)
> >       struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
> >       struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
> >       struct mlx5_vdpa_virtqueue *mvq;
> > -     struct mlx5_vdpa_wq_ent *wqent;
> >
> >       if (!is_index_valid(mvdev, idx))
> >               return;
> > @@ -1672,13 +1678,7 @@ static void mlx5_vdpa_kick_vq(struct vdpa_device *vdev, u16 idx)
> >               if (!mvdev->cvq.ready)
> >                       return;
> >
> > -             wqent = kzalloc(sizeof(*wqent), GFP_ATOMIC);
> > -             if (!wqent)
> > -                     return;
> > -
> > -             wqent->mvdev = mvdev;
> > -             INIT_WORK(&wqent->work, mlx5_cvq_kick_handler);
> > -             queue_work(mvdev->wq, &wqent->work);
> > +             queue_work(mvdev->wq, &ndev->cvq_ent.work);
> >               return;
> >       }
> >
> > @@ -2668,6 +2668,8 @@ static int mlx5_vdpa_dev_add(struct vdpa_mgmt_dev *v_mdev, const char *name,
> >       if (err)
> >               goto err_mr;
> >
> > +     ndev->cvq_ent.mvdev = mvdev;
> > +     INIT_WORK(&ndev->cvq_ent.work, mlx5_cvq_kick_handler);
> >       mvdev->wq = create_singlethread_workqueue("mlx5_vdpa_wq");
> >       if (!mvdev->wq) {
> >               err = -ENOMEM;
> > --
> > 2.18.1
>