Re: [PATCH v4 2/2] vhost: Perform memory section dirty scans once per iteration

[Jason Wang]

On Tue, Mar 19, 2024 at 6:16 AM Si-Wei Liu <si-wei.liu@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.liu@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.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>
> >>>>
> >>>> ---
> >>>> 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
> >>>
>