linux-kernel - Re: [PATCH 2/3] vdpa_sim_blk: limit the number of request handled per batch

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Message-ID: <CACGkMEsk-O=dVwKoEHRY-nL3XEQktPWiBot2NfOUYNdkoL-s=Q@mail.gmail.com>
Date:   Thu, 23 Jun 2022 11:50:22 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     virtualization <virtualization@...ts.linux-foundation.org>,
        Eugenio Pérez <eperezma@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] vdpa_sim_blk: limit the number of request handled per batch

On Wed, Jun 22, 2022 at 12:09 AM Stefano Garzarella <sgarzare@...hat.com> wrote:
>
> Limit the number of requests (4 per queue as for vdpa_sim_net) handled
> in a batch to prevent the worker from using the CPU for too long.
>
> Suggested-by: Eugenio Pérez <eperezma@...hat.com>
> Signed-off-by: Stefano Garzarella <sgarzare@...hat.com>
> ---
>  drivers/vdpa/vdpa_sim/vdpa_sim_blk.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/vdpa/vdpa_sim/vdpa_sim_blk.c b/drivers/vdpa/vdpa_sim/vdpa_sim_blk.c
> index a83a5c76f620..ac86478845b6 100644
> --- a/drivers/vdpa/vdpa_sim/vdpa_sim_blk.c
> +++ b/drivers/vdpa/vdpa_sim/vdpa_sim_blk.c
> @@ -197,6 +197,7 @@ static bool vdpasim_blk_handle_req(struct vdpasim *vdpasim,
>  static void vdpasim_blk_work(struct work_struct *work)
>  {
>         struct vdpasim *vdpasim = container_of(work, struct vdpasim, work);
> +       bool reschedule = false;
>         int i;
>
>         spin_lock(&vdpasim->lock);
> @@ -206,11 +207,15 @@ static void vdpasim_blk_work(struct work_struct *work)
>
>         for (i = 0; i < VDPASIM_BLK_VQ_NUM; i++) {
>                 struct vdpasim_virtqueue *vq = &vdpasim->vqs[i];
> +               bool vq_work = true;
> +               int reqs = 0;
>
>                 if (!vq->ready)
>                         continue;
>
> -               while (vdpasim_blk_handle_req(vdpasim, vq)) {
> +               while (vq_work) {
> +                       vq_work = vdpasim_blk_handle_req(vdpasim, vq);
> +

Is it better to check and exit the loop early here?

Thanks

>                         /* Make sure used is visible before rasing the interrupt. */
>                         smp_wmb();
>
> @@ -218,10 +223,18 @@ static void vdpasim_blk_work(struct work_struct *work)
>                         if (vringh_need_notify_iotlb(&vq->vring) > 0)
>                                 vringh_notify(&vq->vring);
>                         local_bh_enable();
> +
> +                       if (++reqs > 4) {
> +                               vq_work = false;
> +                               reschedule = true;
> +                       }
>                 }
>         }
>  out:
>         spin_unlock(&vdpasim->lock);
> +
> +       if (reschedule)
> +               schedule_work(&vdpasim->work);
>  }
>
>  static void vdpasim_blk_get_config(struct vdpasim *vdpasim, void *config)
> --
> 2.36.1
>