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

[Stefano Garzarella]

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@xxxxxxxxxx> 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@xxxxxxxxxx>
> > Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> > ---
> >  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, so I thought was better to send notification 
also when vdpasim_blk_handle_req() return false, since we will update 
the used.idx.

However, I don't think it's a common path, so if you think it's better 
to exit the loop early, I can do it.

Thanks,
Stefano