Re: [Xen-devel] [PATCH 2/2] xen-bus: Avoid rewriting identical values to xenstore

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> Sent: 21 August 2019 10:20
> To: qemu-devel@xxxxxxxxxx
> Cc: Anthony Perard <anthony.perard@xxxxxxxxxx>; Stefano Stabellini 
> <sstabellini@xxxxxxxxxx>; Paul
> Durrant <Paul.Durrant@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: [PATCH 2/2] xen-bus: Avoid rewriting identical values to xenstore
> 
> When QEMU receive a xenstore watch event suggesting that the "state" or
> "online" status of the frontend or the backend changed, it record this
> in its own state but it also re-write the value back into xenstore even
> so there were no changed. This trigger an unnecessary xenstore watch
> event which QEMU will process again (and maybe the frontend as well).
> 
> Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> ---
>  hw/xen/xen-bus.c | 37 ++++++++++++++++++++++++-------------
>  1 file changed, 24 insertions(+), 13 deletions(-)
> 
> diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
> index 982eca4533..c83f07424a 100644
> --- a/hw/xen/xen-bus.c
> +++ b/hw/xen/xen-bus.c
> @@ -481,20 +481,27 @@ static int xen_device_backend_scanf(XenDevice *xendev, 
> const char *key,
>      return rc;
>  }
> 
> -void xen_device_backend_set_state(XenDevice *xendev,
> -                                  enum xenbus_state state)
> +static bool xen_device_backend_record_state(XenDevice *xendev,
> +                                            enum xenbus_state state)
>  {
>      const char *type = object_get_typename(OBJECT(xendev));
> 
>      if (xendev->backend_state == state) {
> -        return;
> +        return false;
>      }
> 
>      trace_xen_device_backend_state(type, xendev->name,
>                                     xs_strstate(state));
> 
>      xendev->backend_state = state;
> -    xen_device_backend_printf(xendev, "state", "%u", state);
> +    return true;
> +}
> +
> +void xen_device_backend_set_state(XenDevice *xendev,
> +                                  enum xenbus_state state)
> +{
> +    if (xen_device_backend_record_state(xendev, state))
> +        xen_device_backend_printf(xendev, "state", "%u", state);
>  }
> 
>  enum xenbus_state xen_device_backend_get_state(XenDevice *xendev)
> @@ -502,7 +509,8 @@ enum xenbus_state xen_device_backend_get_state(XenDevice 
> *xendev)
>      return xendev->backend_state;
>  }
> 
> -static void xen_device_backend_set_online(XenDevice *xendev, bool online)
> +static void xen_device_backend_set_online(XenDevice *xendev, bool online,
> +                                          bool export)
>  {
>      const char *type = object_get_typename(OBJECT(xendev));
> 
> @@ -513,7 +521,8 @@ static void xen_device_backend_set_online(XenDevice 
> *xendev, bool online)
>      trace_xen_device_backend_online(type, xendev->name, online);
> 
>      xendev->backend_online = online;
> -    xen_device_backend_printf(xendev, "online", "%u", online);
> +    if (export)
> +        xen_device_backend_printf(xendev, "online", "%u", online);
>  }
>

Perhaps the behaviour of backend_set_state() and backend_set_online() could be 
the same? I.e. they both take an 'export' (or perhaps 'publish'?) parameter and 
only write xenstore if that is true. (I realise that would involve modifying 
xen-block to pass 'true' as the extra export/publish param, but I think it 
would be neater overall).
 
>  static void xen_device_backend_changed(void *opaque)
> @@ -529,13 +538,13 @@ static void xen_device_backend_changed(void *opaque)
>          state = XenbusStateUnknown;
>      }
> 
> -    xen_device_backend_set_state(xendev, state);
> +    xen_device_backend_record_state(xendev, state);
> 
>      if (xen_device_backend_scanf(xendev, "online", "%u", &online) != 1) {
>          online = 0;
>      }
> 
> -    xen_device_backend_set_online(xendev, !!online);
> +    xen_device_backend_set_online(xendev, !!online, false);
> 

You could then pass 'false' here in both cases.

>      /*
>       * If the toolstack (or unplug request callback) has set the backend
> @@ -683,7 +692,8 @@ int xen_device_frontend_scanf(XenDevice *xendev, const 
> char *key,
>  }
> 
>  static void xen_device_frontend_set_state(XenDevice *xendev,
> -                                          enum xenbus_state state)
> +                                          enum xenbus_state state,
> +                                          bool export)
>  {
>      const char *type = object_get_typename(OBJECT(xendev));
> 
> @@ -695,7 +705,8 @@ static void xen_device_frontend_set_state(XenDevice 
> *xendev,
>                                      xs_strstate(state));
> 
>      xendev->frontend_state = state;
> -    xen_device_frontend_printf(xendev, "state", "%u", state);
> +    if (export)
> +        xen_device_frontend_printf(xendev, "state", "%u", state);
>  }
> 
>  static void xen_device_frontend_changed(void *opaque)
> @@ -711,7 +722,7 @@ static void xen_device_frontend_changed(void *opaque)
>          state = XenbusStateUnknown;
>      }
> 
> -    xen_device_frontend_set_state(xendev, state);
> +    xen_device_frontend_set_state(xendev, state, false);
> 
>      if (state == XenbusStateInitialising &&
>          xendev->backend_state == XenbusStateClosed &&
> @@ -1146,7 +1157,7 @@ static void xen_device_realize(DeviceState *dev, Error 
> **errp)
>                                xendev->frontend_id);
>      xen_device_backend_printf(xendev, "hotplug-status", "connected");
> 
> -    xen_device_backend_set_online(xendev, true);
> +    xen_device_backend_set_online(xendev, true, true);
>      xen_device_backend_set_state(xendev, XenbusStateInitWait);
> 
>      xen_device_frontend_printf(xendev, "backend", "%s",
> @@ -1154,7 +1165,7 @@ static void xen_device_realize(DeviceState *dev, Error 
> **errp)
>      xen_device_frontend_printf(xendev, "backend-id", "%u",
>                                 xenbus->backend_id);
> 
> -    xen_device_frontend_set_state(xendev, XenbusStateInitialising);
> +    xen_device_frontend_set_state(xendev, XenbusStateInitialising, true);
> 

And similarly pass 'true' here for all three cases.

What do you think?

  Paul

>      xendev->exit.notify = xen_device_exit;
>      qemu_add_exit_notifier(&xendev->exit);
> --
> Anthony PERARD


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