On Sat, Mar 23, 2024 at 5:14 AM Si-Wei Liu <si-wei....@oracle.com> wrote: > > > > On 3/21/2024 10:08 PM, Jason Wang wrote: > > On Fri, Mar 22, 2024 at 5:43 AM Si-Wei Liu <si-wei....@oracle.com> wrote: > >> > >> > >> On 3/20/2024 8:56 PM, Jason Wang wrote: > >>> 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? > >> vhost_sync_dirty_bitmap() not just scans the guest memory sections but > >> the specific one for virtqueues (used rings) also. To save more CPU > >> cycles to the best extend, the guest memory must be scanned only once in > >> each log iteration, though the logging for used rings would still have > >> to use the specific vhost instance, so all vhost_device instance still > >> keeps the dev->log pointer to the shared log as-is. Generally the shared > >> memory logger can be picked from an arbitrary vhost_device instance, but > >> to keep the code simple, performant and predictable > > This is the point, I don't see why election is simpler than picking an > > arbitrary shared log in this case. > Maybe I missed your point, but I am confused and fail to understand why > electing a fixed vhost_dev is not as simple? Regardless of the > specifics, I think the point is one _fixed_ vhost_dev has to be picked > amongst all the instances per backend type in charge of logging guest > memory, no matter if it's at the start on the list or not.
See below. > > > > >> , logger selection is > >> made on the control path at the vhost add/remove time rather than be > >> determined at the dirty log collection runtime, the latter of which is > >> in the hotpath. > >> > >>>> 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. > >> The code you showed earlier won't work as all vhost_device instance > >> points to the same dev->log device... > > This part I don't understand. > > vhost_log_get() has the following: > > log = share ? vhost_log_shm[backend_type] : vhost_log[backend_type]; > > if (!log || log->size != size) { > log = vhost_log_alloc(size, share); > if (share) { > vhost_log_shm[backend_type] = log; > } else { > vhost_log[backend_type] = log; > } > } else { > ++log->refcnt; > } > > So for a specific backend type, vhost_log_get() would return the same > logger device (stored at either vhost_log_shm[] or vhost_log[]) to all > vhost_dev instances, and the check you suggested earlier: > > dev->log == vhost_log_shm[dev->vhost_ops->backend_type] > > will always hold true if the vhost_dev instance (representing the > specific virtqueue) is active. Right, so the point is see if we can find something simpler to avoid the QLIST as it involves vhost_dev which seems unnecessary. Does something like a counter work? vhost_sync_dirty_bitmap() { rounds ++; vhost_dev_sync_region(rounds); } vhost_dev_sync_region(rounds) { if (dev->log->rounds == rounds) return; else dev->log->rounds; } (pseudo codes, just used to demonstrate the idea). Thanks > > Regards, > -Siwei > > > > Thanks > > > >> Regards, > >> -Siwei > >>> 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 > >>>>>>>>> >