Re: [PATCH v4 03/11] virtio-vdpa: Support interrupt affinity spreading mechanism

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Mar 24, 2023 at 02:27:52PM +0800, Jason Wang wrote:
> On Thu, Mar 23, 2023 at 1:31 PM Xie Yongji <xieyongji@bytedance.com> wrote:
> >
> > To support interrupt affinity spreading mechanism,
> > this makes use of group_cpus_evenly() to create
> > an irq callback affinity mask for each virtqueue
> > of vdpa device. Then we will unify set_vq_affinity
> > callback to pass the affinity to the vdpa device driver.
> >
> > Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> 
> Thinking hard of all the logics, I think I've found something interesting.
> 
> Commit ad71473d9c437 ("virtio_blk: use virtio IRQ affinity") tries to
> pass irq_affinity to transport specific find_vqs().  This seems a
> layer violation since driver has no knowledge of
> 
> 1) whether or not the callback is based on an IRQ
> 2) whether or not the device is a PCI or not (the details are hided by
> the transport driver)
> 3) how many vectors could be used by a device
> 
> This means the driver can't actually pass a real affinity masks so the
> commit passes a zero irq affinity structure as a hint in fact, so the
> PCI layer can build a default affinity based that groups cpus evenly
> based on the number of MSI-X vectors (the core logic is the
> group_cpus_evenly). I think we should fix this by replacing the
> irq_affinity structure with
> 
> 1) a boolean like auto_cb_spreading
> 
> or
> 
> 2) queue to cpu mapping
> 
> So each transport can do its own logic based on that. Then virtio-vDPA
> can pass that policy to VDUSE where we only need a group_cpus_evenly()
> and avoid duplicating irq_create_affinity_masks()?
> 
> Thanks

I don't really understand what you propose. Care to post a patch?
Also does it have to block this patchset or can it be done on top?

> > ---
> >  drivers/virtio/virtio_vdpa.c | 68 ++++++++++++++++++++++++++++++++++++
> >  1 file changed, 68 insertions(+)
> >
> > diff --git a/drivers/virtio/virtio_vdpa.c b/drivers/virtio/virtio_vdpa.c
> > index f72696b4c1c2..f3826f42b704 100644
> > --- a/drivers/virtio/virtio_vdpa.c
> > +++ b/drivers/virtio/virtio_vdpa.c
> > @@ -13,6 +13,7 @@
> >  #include <linux/kernel.h>
> >  #include <linux/slab.h>
> >  #include <linux/uuid.h>
> > +#include <linux/group_cpus.h>
> >  #include <linux/virtio.h>
> >  #include <linux/vdpa.h>
> >  #include <linux/virtio_config.h>
> > @@ -272,6 +273,66 @@ static void virtio_vdpa_del_vqs(struct virtio_device *vdev)
> >                 virtio_vdpa_del_vq(vq);
> >  }
> >
> > +static void default_calc_sets(struct irq_affinity *affd, unsigned int affvecs)
> > +{
> > +       affd->nr_sets = 1;
> > +       affd->set_size[0] = affvecs;
> > +}
> > +
> > +static struct cpumask *
> > +create_affinity_masks(unsigned int nvecs, struct irq_affinity *affd)
> > +{
> > +       unsigned int affvecs = 0, curvec, usedvecs, i;
> > +       struct cpumask *masks = NULL;
> > +
> > +       if (nvecs > affd->pre_vectors + affd->post_vectors)
> > +               affvecs = nvecs - affd->pre_vectors - affd->post_vectors;
> > +
> > +       if (!affd->calc_sets)
> > +               affd->calc_sets = default_calc_sets;
> > +
> > +       affd->calc_sets(affd, affvecs);
> > +
> > +       if (!affvecs)
> > +               return NULL;
> > +
> > +       masks = kcalloc(nvecs, sizeof(*masks), GFP_KERNEL);
> > +       if (!masks)
> > +               return NULL;
> > +
> > +       /* Fill out vectors at the beginning that don't need affinity */
> > +       for (curvec = 0; curvec < affd->pre_vectors; curvec++)
> > +               cpumask_setall(&masks[curvec]);
> > +
> > +       for (i = 0, usedvecs = 0; i < affd->nr_sets; i++) {
> > +               unsigned int this_vecs = affd->set_size[i];
> > +               int j;
> > +               struct cpumask *result = group_cpus_evenly(this_vecs);
> > +
> > +               if (!result) {
> > +                       kfree(masks);
> > +                       return NULL;
> > +               }
> > +
> > +               for (j = 0; j < this_vecs; j++)
> > +                       cpumask_copy(&masks[curvec + j], &result[j]);
> > +               kfree(result);
> > +
> > +               curvec += this_vecs;
> > +               usedvecs += this_vecs;
> > +       }
> > +
> > +       /* Fill out vectors at the end that don't need affinity */
> > +       if (usedvecs >= affvecs)
> > +               curvec = affd->pre_vectors + affvecs;
> > +       else
> > +               curvec = affd->pre_vectors + usedvecs;
> > +       for (; curvec < nvecs; curvec++)
> > +               cpumask_setall(&masks[curvec]);
> > +
> > +       return masks;
> > +}
> > +
> >  static int virtio_vdpa_find_vqs(struct virtio_device *vdev, unsigned int nvqs,
> >                                 struct virtqueue *vqs[],
> >                                 vq_callback_t *callbacks[],
> > @@ -282,9 +343,15 @@ static int virtio_vdpa_find_vqs(struct virtio_device *vdev, unsigned int nvqs,
> >         struct virtio_vdpa_device *vd_dev = to_virtio_vdpa_device(vdev);
> >         struct vdpa_device *vdpa = vd_get_vdpa(vdev);
> >         const struct vdpa_config_ops *ops = vdpa->config;
> > +       struct irq_affinity default_affd = { 0 };
> > +       struct cpumask *masks;
> >         struct vdpa_callback cb;
> >         int i, err, queue_idx = 0;
> >
> > +       masks = create_affinity_masks(nvqs, desc ? desc : &default_affd);
> > +       if (!masks)
> > +               return -ENOMEM;
> > +
> >         for (i = 0; i < nvqs; ++i) {
> >                 if (!names[i]) {
> >                         vqs[i] = NULL;
> > @@ -298,6 +365,7 @@ static int virtio_vdpa_find_vqs(struct virtio_device *vdev, unsigned int nvqs,
> >                         err = PTR_ERR(vqs[i]);
> >                         goto err_setup_vq;
> >                 }
> > +               ops->set_vq_affinity(vdpa, i, &masks[i]);
> >         }
> >
> >         cb.callback = virtio_vdpa_config_cb;
> > --
> > 2.20.1
> >