On Sat, Mar 16, 2024 at 2:45 AM Si-Wei Liu <si-wei....@oracle.com> wrote: > > > > On 3/14/2024 9:03 PM, Jason Wang wrote: > > On Fri, Mar 15, 2024 at 5:39 AM Si-Wei Liu <si-wei....@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.mart...@oracle.com> > >> Signed-off-by: Joao Martins <joao.m.mart...@oracle.com> > >> Signed-off-by: Si-Wei Liu <si-wei....@oracle.com> > >> > >> --- > >> v3 -> v4: > >> - add comment to clarify effect on cache locality and > >> performance > >> > >> 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 | 67 > >> ++++++++++++++++++++++++++++++++++++++++++----- > >> include/hw/virtio/vhost.h | 1 + > >> 2 files changed, 62 insertions(+), 6 deletions(-) > >> > >> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c > >> index 612f4db..58522f1 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 +150,47 @@ 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]); > > A dumb question, why not simple check > > > > dev->log == vhost_log_shm[dev->vhost_ops->backend_type] > Because we are not sure if the logger comes from vhost_log_shm[] or > vhost_log[]. Don't want to complicate the check here by calling into > vhost_dev_log_is_shared() everytime when the .log_sync() is called.
It has very low overhead, isn't it? static bool vhost_dev_log_is_shared(struct vhost_dev *dev) { return dev->vhost_ops->vhost_requires_shm_log && dev->vhost_ops->vhost_requires_shm_log(dev); } And it helps to simplify the logic. Thanks > > -Siwei > > ? > > > > Thanks > > >