Re:Re: [PATCH] make blk_mq_map_queues more friendly for cpu topology

[luferry <luferry@163.com>]

At 2019-03-26 15:39:54, "Christoph Hellwig" <hch@lst.de> wrote:
>Why isn't this using the automatic PCI-level affinity assignment to
>start with?

When enable virtio-blk with multi queues but with only 2 msix-vector.
vp_dev->per_vq_vectors will be false, vp_get_vq_affintity will return NULL directly
so blk_mq_virtio_map_queues will fallback to blk_mq_map_queues.


448 const struct cpumask *vp_get_vq_affinity(struct virtio_device *vdev, int index)
449 {
450     struct virtio_pci_device *vp_dev = to_vp_device(vdev);
451
452     if (!vp_dev->per_vq_vectors ||
453         vp_dev->vqs[index]->msix_vector == VIRTIO_MSI_NO_VECTOR)
454         return NULL;
455
456     return pci_irq_get_affinity(vp_dev->pci_dev,
457                     vp_dev->vqs[index]->msix_vector);
458 }

 32 int blk_mq_virtio_map_queues(struct blk_mq_queue_map *qmap,
 33         struct virtio_device *vdev, int first_vec)
 34 {
 35     const struct cpumask *mask;
 36     unsigned int queue, cpu;
 37
 38     if (!vdev->config->get_vq_affinity)
 39         goto fallback;
 40
 41     for (queue = 0; queue < qmap->nr_queues; queue++) {
 42         mask = vdev->config->get_vq_affinity(vdev, first_vec + queue); //vp_get_vq_affinity return NULL
 43         if (!mask)
 44             goto fallback;
 45
 46         for_each_cpu(cpu, mask)
 47             qmap->mq_map[cpu] = qmap->queue_offset + queue;
 48     }
 49
 50     return 0;
 51 fallback:
 52     return blk_mq_map_queues(qmap);
 53 }

here is previous discussion
https://patchwork.kernel.org/patch/10865461/