Re: [PATCH v2 2/2] vhost: Perform memory section dirty scans once per iteration

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

On Wed, Feb 14, 2024 at 03:50:19AM -0800, Si-Wei Liu wrote:
> On setups with one or more virtio-net devices with vhost on,
> dirty tracking iteration increases cost the bigger the number
> amount of queues are set up e.g. on idle guests migration the
> following is observed with virtio-net with vhost=on:
> 
> 48 queues -> 78.11%  [.] vhost_dev_sync_region.isra.13
> 8 queues -> 40.50%   [.] vhost_dev_sync_region.isra.13
> 1 queue -> 6.89%     [.] vhost_dev_sync_region.isra.13
> 2 devices, 1 queue -> 18.60%  [.] vhost_dev_sync_region.isra.14

Given the drastic slowdown I am prepared to treat this as
a bugfix if a version addressing all comments and rebased
is sent early during the freeze.

> With high memory rates the symptom is lack of convergence as soon
> as it has a vhost device with a sufficiently high number of queues,
> the sufficient number of vhost devices.
> 
> On every migration iteration (every 100msecs) it will redundantly
> query the *shared log* the number of queues configured with vhost
> that exist in the guest. For the virtqueue data, this is necessary,
> but not for the memory sections which are the same. So
> essentially we end up scanning the dirty log too often.
> 
> To fix that, select a vhost device responsible for scanning the
> log with regards to memory sections dirty tracking. It is selected
> when we enable the logger (during migration) and cleared when we
> disable the logger. If the vhost logger device goes away for some
> reason, the logger will be re-selected from the rest of vhost
> devices.
> 
> Co-developed-by: Joao Martins <joao.m.martins@oracle.com>
> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
> Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
> ---
>  hw/virtio/vhost.c         | 75 +++++++++++++++++++++++++++++++++++++++++++----
>  include/hw/virtio/vhost.h |  1 +
>  2 files changed, 70 insertions(+), 6 deletions(-)
> 
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index ef6d9b5..997d560 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -45,6 +45,9 @@
>  
>  static struct vhost_log *vhost_log[VHOST_BACKEND_TYPE_MAX];
>  static struct vhost_log *vhost_log_shm[VHOST_BACKEND_TYPE_MAX];
> +static struct vhost_dev *vhost_mem_logger[VHOST_BACKEND_TYPE_MAX];
> +static QLIST_HEAD(, vhost_dev) vhost_mlog_devices =
> +    QLIST_HEAD_INITIALIZER(vhost_mlog_devices);
>  
>  /* Memslots used by backends that support private memslots (without an fd). */
>  static unsigned int used_memslots;
> @@ -149,6 +152,53 @@ bool vhost_dev_has_iommu(struct vhost_dev *dev)
>      }
>  }
>  
> +static bool vhost_log_dev_enabled(struct vhost_dev *dev)
> +{
> +    assert(dev->vhost_ops);
> +    assert(dev->vhost_ops->backend_type > VHOST_BACKEND_TYPE_NONE);
> +    assert(dev->vhost_ops->backend_type < VHOST_BACKEND_TYPE_MAX);
> +
> +    return dev == vhost_mem_logger[dev->vhost_ops->backend_type];
> +}
> +
> +static void vhost_mlog_set_dev(struct vhost_dev *hdev, bool enable)
> +{
> +    struct vhost_dev *logdev = NULL;
> +    VhostBackendType backend_type;
> +    bool reelect = false;
> +
> +    assert(hdev->vhost_ops);
> +    assert(hdev->vhost_ops->backend_type > VHOST_BACKEND_TYPE_NONE);
> +    assert(hdev->vhost_ops->backend_type < VHOST_BACKEND_TYPE_MAX);
> +
> +    backend_type = hdev->vhost_ops->backend_type;
> +
> +    if (enable && !QLIST_IS_INSERTED(hdev, logdev_entry)) {
> +        reelect = !vhost_mem_logger[backend_type];
> +        QLIST_INSERT_HEAD(&vhost_mlog_devices, hdev, logdev_entry);
> +    } else if (!enable && QLIST_IS_INSERTED(hdev, logdev_entry)) {
> +        reelect = vhost_mem_logger[backend_type] == hdev;
> +        QLIST_REMOVE(hdev, logdev_entry);
> +    }
> +
> +    if (!reelect)
> +        return;
> +
> +    QLIST_FOREACH(hdev, &vhost_mlog_devices, logdev_entry) {
> +        if (!hdev->vhost_ops ||
> +            hdev->vhost_ops->backend_type == VHOST_BACKEND_TYPE_NONE ||
> +            hdev->vhost_ops->backend_type >= VHOST_BACKEND_TYPE_MAX)
> +            continue;
> +
> +        if (hdev->vhost_ops->backend_type == backend_type) {
> +            logdev = hdev;
> +            break;
> +        }
> +    }
> +
> +    vhost_mem_logger[backend_type] = logdev;
> +}
> +
>  static int vhost_sync_dirty_bitmap(struct vhost_dev *dev,
>                                     MemoryRegionSection *section,
>                                     hwaddr first,
> @@ -166,12 +216,14 @@ static int vhost_sync_dirty_bitmap(struct vhost_dev *dev,
>      start_addr = MAX(first, start_addr);
>      end_addr = MIN(last, end_addr);
>  
> -    for (i = 0; i < dev->mem->nregions; ++i) {
> -        struct vhost_memory_region *reg = dev->mem->regions + i;
> -        vhost_dev_sync_region(dev, section, start_addr, end_addr,
> -                              reg->guest_phys_addr,
> -                              range_get_last(reg->guest_phys_addr,
> -                                             reg->memory_size));
> +    if (vhost_log_dev_enabled(dev)) {
> +        for (i = 0; i < dev->mem->nregions; ++i) {
> +            struct vhost_memory_region *reg = dev->mem->regions + i;
> +            vhost_dev_sync_region(dev, section, start_addr, end_addr,
> +                                  reg->guest_phys_addr,
> +                                  range_get_last(reg->guest_phys_addr,
> +                                                 reg->memory_size));
> +        }
>      }
>      for (i = 0; i < dev->nvqs; ++i) {
>          struct vhost_virtqueue *vq = dev->vqs + i;
> @@ -382,6 +434,7 @@ static void vhost_log_put(struct vhost_dev *dev, bool sync)
>          g_free(log);
>      }
>  
> +    vhost_mlog_set_dev(dev, false);
>      dev->log = NULL;
>      dev->log_size = 0;
>  }
> @@ -997,6 +1050,15 @@ static int vhost_dev_set_log(struct vhost_dev *dev, bool enable_log)
>              goto err_vq;
>          }
>      }
> +
> +    /*
> +     * At log start we select our vhost_device logger that will scan the
> +     * memory sections and skip for the others. This is possible because
> +     * the log is shared amongst all vhost devices for a given type of
> +     * backend.
> +     */
> +    vhost_mlog_set_dev(dev, enable_log);
> +
>      return 0;
>  err_vq:
>      for (; i >= 0; --i) {
> @@ -2072,6 +2134,7 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev, bool vrings)
>              VHOST_OPS_DEBUG(r, "vhost_set_log_base failed");
>              goto fail_log;
>          }
> +        vhost_mlog_set_dev(hdev, true);
>      }
>      if (vrings) {
>          r = vhost_dev_set_vring_enable(hdev, true);
> diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
> index 0247778..d75faf4 100644
> --- a/include/hw/virtio/vhost.h
> +++ b/include/hw/virtio/vhost.h
> @@ -129,6 +129,7 @@ struct vhost_dev {
>      void *opaque;
>      struct vhost_log *log;
>      QLIST_ENTRY(vhost_dev) entry;
> +    QLIST_ENTRY(vhost_dev) logdev_entry;
>      QLIST_HEAD(, vhost_iommu) iommu_list;
>      IOMMUNotifier n;
>      const VhostDevConfigOps *config_ops;
> -- 
> 1.8.3.1