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

[Si-Wei Liu <si-wei.liu@oracle.com>]

On 3/21/2024 10:08 PM, Jason Wang wrote:
> On Fri, Mar 22, 2024 at 5:43 AM Si-Wei Liu <si-wei.liu@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.liu@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.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?
>>>> 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.

>
>> , 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.

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
>>>>>>>>>