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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* 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?

Dave

> +
>      return hash;
>  }
>  
> @@ -496,6 +500,11 @@ static gboolean qapi_event_throttle_equal(const void *a, const void *b)
>                         qdict_get_str(evb->data, "node-name"));
>      }
>  
> +    if (eva->event == QAPI_EVENT_MEMORY_DEVICE_SIZE_CHANGE) {
> +        return !strcmp(qdict_get_str(eva->data, "qom-path"),
> +                       qdict_get_str(evb->data, "qom-path"));
> +    }
> +
>      return TRUE;
>  }
>  
> diff --git a/qapi/machine.json b/qapi/machine.json
> index 157712f006..2487c92f18 100644
> --- a/qapi/machine.json
> +++ b/qapi/machine.json
> @@ -1245,8 +1245,11 @@
>  # action).
>  #
>  # @id: device's ID
> +#
>  # @size: the new size of memory that the device provides
>  #
> +# @qom-path: path to the device object in the QOM tree (since 6.2)
> +#
>  # Note: this event is rate-limited.
>  #
>  # Since: 5.1
> @@ -1259,7 +1262,7 @@
>  #
>  ##
>  { 'event': 'MEMORY_DEVICE_SIZE_CHANGE',
> -  'data': { '*id': 'str', 'size': 'size' } }
> +  'data': { '*id': 'str', 'size': 'size', 'qom-path' : 'str'} }
>  
>  
>  ##
> -- 
> 2.31.1
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK