Re: [PATCH v3 2/2] vhost: Perform memory section dirty scans once per iteration

[Eugenio Perez Martin]

On Thu, Mar 14, 2024 at 9:38 AM Si-Wei Liu <si-wei.liu@oracle.com> 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
>
> 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.
>
> After making mem-section logger a singleton instance, constant cost
> of 7%-9% (like the 1 queue report) will be seen, no matter how many
> queues or how many vhost devices are configured:
>
> 48 queues -> 8.71%    [.] vhost_dev_sync_region.isra.13
> 2 devices, 8 queues -> 7.97%   [.] vhost_dev_sync_region.isra.14
>
> 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>
> ---
> v2 -> v3:
>   - add after-fix benchmark to commit log
>   - rename vhost_log_dev_enabled to vhost_dev_should_log
>   - remove unneeded comparisons for backend_type
>   - use QLIST array instead of single flat list to store vhost
>     logger devices
>   - simplify logger election logic
>
> ---
>  hw/virtio/vhost.c         | 63 
> ++++++++++++++++++++++++++++++++++++++++++-----
>  include/hw/virtio/vhost.h |  1 +
>  2 files changed, 58 insertions(+), 6 deletions(-)
>
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index efe2f74..d91858b 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -45,6 +45,7 @@
>
>  static struct vhost_log *vhost_log[VHOST_BACKEND_TYPE_MAX];
>  static struct vhost_log *vhost_log_shm[VHOST_BACKEND_TYPE_MAX];
> +static QLIST_HEAD(, vhost_dev) vhost_log_devs[VHOST_BACKEND_TYPE_MAX];
>
>  /* Memslots used by backends that support private memslots (without an fd). 
> */
>  static unsigned int used_memslots;
> @@ -149,6 +150,43 @@ bool vhost_dev_has_iommu(struct vhost_dev *dev)
>      }
>  }
>
> +static inline bool vhost_dev_should_log(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 == QLIST_FIRST(&vhost_log_devs[dev->vhost_ops->backend_type]);
> +}
> +
> +static inline void vhost_dev_elect_mem_logger(struct vhost_dev *hdev, bool 
> add)
> +{
> +    VhostBackendType backend_type;
> +
> +    assert(hdev->vhost_ops);
> +
> +    backend_type = hdev->vhost_ops->backend_type;
> +    assert(backend_type > VHOST_BACKEND_TYPE_NONE);
> +    assert(backend_type < VHOST_BACKEND_TYPE_MAX);
> +
> +    if (add && !QLIST_IS_INSERTED(hdev, logdev_entry)) {
> +        if (QLIST_EMPTY(&vhost_log_devs[backend_type])) {
> +            QLIST_INSERT_HEAD(&vhost_log_devs[backend_type],
> +                              hdev, logdev_entry);
> +        } else {
> +            /*
> +             * The first vhost_device in the list is selected as the shared
> +             * logger to scan memory sections. Put new entry next to the head
> +             * to avoid inadvertent change to the underlying logger device.
> +             */

Why is changing the logger device a problem? All the code paths are
either changing the QLIST or logging, isn't it?

> +            QLIST_INSERT_AFTER(QLIST_FIRST(&vhost_log_devs[backend_type]),
> +                               hdev, logdev_entry);
> +        }
> +    } else if (!add && QLIST_IS_INSERTED(hdev, logdev_entry)) {
> +        QLIST_REMOVE(hdev, logdev_entry);
> +    }
> +}
> +
>  static int vhost_sync_dirty_bitmap(struct vhost_dev *dev,
>                                     MemoryRegionSection *section,
>                                     hwaddr first,
> @@ -166,12 +204,14 @@ 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_dev_should_log(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;
> @@ -383,6 +423,7 @@ static void vhost_log_put(struct vhost_dev *dev, bool 
> sync)
>          g_free(log);
>      }
>
> +    vhost_dev_elect_mem_logger(dev, false);
>      dev->log = NULL;
>      dev->log_size = 0;
>  }
> @@ -998,6 +1039,15 @@ 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_dev_elect_mem_logger(dev, enable_log);
> +
>      return 0;
>  err_vq:
>      for (; i >= 0; --i) {
> @@ -2073,6 +2123,7 @@ 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_dev_elect_mem_logger(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
>