On Thu, Mar 21, 2024 at 5:03 AM Si-Wei Liu <si-wei....@oracle.com> wrote:
>
>
>
> On 3/19/2024 8:27 PM, Jason Wang wrote:
> > 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?
> Right now the log type won't change during runtime, but I am not sure if
> this may prohibit future revisit to allow change at the runtime,
We can be bothered when we have such a request then.
> then
> there'll be complex code involvled to maintain the state.
>
> Other than this, I think it's insufficient to just check the shm log
> v.s. normal log. The logger device requires to identify a leading logger
> device that gets elected in vhost_dev_elect_mem_logger(), as all the
> dev->log points to the same logger that is refenerce counted, that we
> have to add extra field and complex logic to maintain the election
> status.
One thing I don't understand here (and in the changelog) is why do we
need an election here?
> I thought that Eugenio's previous suggestion tried to simplify
> the logic in vhost_dev_elect_mem_logger(), as the QLIST_FIRST macro that
> gets expanded to use the lh_first field for the QLIST would simply
> satisfy the basic need. Why extra logic to make the check ever more
> complex, is there any benefit by adding more fields to the vhost_dev?
I don't get here, the idea is to just pick one shared log which should
be much more simpler than what is proposed here.
Thanks
>
>
> Thanks,
> -Siwei
>
> >
> >>> 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
> >>>>>
>
