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

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Message-ID: <CAGxU2F5uv2gMPp6Mfc_yxytMpbPwPdcfJ=NhazoJQkR3WNjUAQ@mail.gmail.com>
Date:   Tue, 28 Jun 2022 09:55:27 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Jason Wang <jasowang@...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 Tue, Jun 28, 2022 at 6:01 AM Jason Wang <jasowang@...hat.com> wrote:
>
> On Thu, Jun 23, 2022 at 4:58 PM Stefano Garzarella <sgarzare@...hat.com> wrote:
> >
> > On Thu, Jun 23, 2022 at 11:50:22AM +0800, Jason Wang wrote:
> > >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?
> >
> > Maybe, but I'm not sure.
> >
> > In vdpa_sim_net we call vringh_complete_iotlb() and send notification
> > also in the error path,
>
> Looks not?
>
>                 read = vringh_iov_pull_iotlb(&cvq->vring, &cvq->in_iov, &ctrl,
>                                              sizeof(ctrl));
>                 if (read != sizeof(ctrl))
>                         break;
>
> We break the loop.

I was looking at vdpasim_net_work(), but I was confused since it
handles 2 queues.

I'll break the loop as it was before.

Thanks,
Stefano