On Tue, Mar 19, 2024 at 6:16 AM Si-Wei Liu <si-wei....@oracle.com> wrote: > > > > On 3/17/2024 8:22 PM, Jason Wang wrote: > > 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? > Whether this has low overhead will have to depend on the specific > backend's implementation for .vhost_requires_shm_log(), which the common > vhost layer should not assume upon or rely on the current implementation. > > > > > 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); > > }
For example, if I understand the code correctly, the log type won't be changed during runtime, so we can endup with a boolean to record that instead of a query ops? > > > > And it helps to simplify the logic. > Generally yes, but when it comes to hot path operations the performance > consideration could override this principle. I think there's no harm to > check against logger device cached in vhost layer itself, and the > current patch does not create a lot of complexity or performance side > effect (actually I think the conditional should be very straightforward > to turn into just a couple of assembly compare and branch instructions > rather than indirection through another jmp call). Thanks > > -Siwei > > > > > Thanks > > > >> -Siwei > >>> ? > >>> > >>> Thanks > >>> >