Re: [PATCH v2] monitor: Rate-limit MEMORY_DEVICE_SIZE_CHANGE qapi events per device

[Markus Armbruster <armbru@redhat.com>]

"Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:

> * David Hildenbrand (david@redhat.com) wrote:
>> We want to rate-limit MEMORY_DEVICE_SIZE_CHANGE events per device,
>> otherwise we can lose some events for devices. As we might not always have
>> a device id, add the qom-path to the event and use that to rate-limit
>> per device.
>> 
>> This was noticed by starting a VM with two virtio-mem devices that each
>> have a requested size > 0. The Linux guest will initialize both devices
>> in parallel, resulting in losing MEMORY_DEVICE_SIZE_CHANGE events for
>> one of the devices.
>> 
>> Fixes: 722a3c783ef4 ("virtio-pci: Send qapi events when the virtio-mem size changes")
>> Suggested-by: Markus Armbruster <armbru@redhat.com>
>> Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
>> Cc: Markus Armbruster <armbru@redhat.com>
>> Cc: Michael S. Tsirkin <mst@redhat.com>
>> Cc: Eric Blake <eblake@redhat.com>
>> Cc: Igor Mammedov <imammedo@redhat.com>
>> Cc: Michal Privoznik <mprivozn@redhat.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>> 
>> Follow up of:
>>     https://lkml.kernel.org/r/20210921102434.24273-1-david@redhat.com
>> 
>> v1 -> v2:
>> - Add the qom-path and use that identifier to rate-limit per device
>> - Rephrase subject/description
>> 
>> ---
>>  hw/virtio/virtio-mem-pci.c | 3 ++-
>>  monitor/monitor.c          | 9 +++++++++
>>  qapi/machine.json          | 5 ++++-
>>  3 files changed, 15 insertions(+), 2 deletions(-)
>> 
>> diff --git a/hw/virtio/virtio-mem-pci.c b/hw/virtio/virtio-mem-pci.c
>> index fa5395cd88..dd5085497f 100644
>> --- a/hw/virtio/virtio-mem-pci.c
>> +++ b/hw/virtio/virtio-mem-pci.c
>> @@ -87,6 +87,7 @@ static void virtio_mem_pci_size_change_notify(Notifier *notifier, void *data)
>>      VirtIOMEMPCI *pci_mem = container_of(notifier, VirtIOMEMPCI,
>>                                           size_change_notifier);
>>      DeviceState *dev = DEVICE(pci_mem);
>> +    const char * qom_path = object_get_canonical_path(OBJECT(dev));
>>      const uint64_t * const size_p = data;
>>      const char *id = NULL;
>>  
>> @@ -94,7 +95,7 @@ static void virtio_mem_pci_size_change_notify(Notifier *notifier, void *data)
>>          id = g_strdup(dev->id);
>>      }
>>  
>> -    qapi_event_send_memory_device_size_change(!!id, id, *size_p);
>> +    qapi_event_send_memory_device_size_change(!!id, id, *size_p, qom_path);
>>  }
>>  
>>  static void virtio_mem_pci_class_init(ObjectClass *klass, void *data)
>> diff --git a/monitor/monitor.c b/monitor/monitor.c
>> index 46a171bca6..21c7a68758 100644
>> --- a/monitor/monitor.c
>> +++ b/monitor/monitor.c
>> @@ -474,6 +474,10 @@ static unsigned int qapi_event_throttle_hash(const void *key)
>>          hash += g_str_hash(qdict_get_str(evstate->data, "node-name"));
>>      }
>>  
>> +    if (evstate->event == QAPI_EVENT_MEMORY_DEVICE_SIZE_CHANGE) {
>> +        hash += g_str_hash(qdict_get_str(evstate->data, "qom-path"));
>> +    }
>
> It makes me wonder if all these entries could turn into:
>   str = qdict_get_try_str(qdict, "qom-path");
>   if (str) {
>       hash += g_str_hash(str);
>   }
>
> and then stop worrying about checking each eventtype there?

Prone to accidental capture when we add to event data later on.  I feel
it's better to be explicit.