Re: [PATCH] system/qdev-monitor: move drain_call_rcu call under if (!dev) in qmp_device_add()

[Igor Mammedov <imammedo@redhat.com>]

On Fri,  3 Nov 2023 13:56:02 +0300
Dmitrii Gavrilov <ds-gavr@yandex-team.ru> wrote:

Seems related to cpu hotpug issues,
CCing Boris for awareness.

> Original goal of addition of drain_call_rcu to qmp_device_add was to cover
> the failure case of qdev_device_add. It seems call of drain_call_rcu was
> misplaced in 7bed89958bfbf40df what led to waiting for pending RCU callbacks
> under happy path too. What led to overall performance degradation of
> qmp_device_add.
> 
> In this patch call of drain_call_rcu moved under handling of failure of
> qdev_device_add.
> 
> Signed-off-by: Dmitrii Gavrilov <ds-gavr@yandex-team.ru>
> ---
>  system/qdev-monitor.c | 23 +++++++++++------------
>  1 file changed, 11 insertions(+), 12 deletions(-)
> 
> diff --git a/system/qdev-monitor.c b/system/qdev-monitor.c
> index 1b8005a..dc7b02d 100644
> --- a/system/qdev-monitor.c
> +++ b/system/qdev-monitor.c
> @@ -856,19 +856,18 @@ void qmp_device_add(QDict *qdict, QObject **ret_data, Error **errp)
>          return;
>      }
>      dev = qdev_device_add(opts, errp);
> -
> -    /*
> -     * Drain all pending RCU callbacks. This is done because
> -     * some bus related operations can delay a device removal
> -     * (in this case this can happen if device is added and then
> -     * removed due to a configuration error)
> -     * to a RCU callback, but user might expect that this interface
> -     * will finish its job completely once qmp command returns result
> -     * to the user
> -     */
> -    drain_call_rcu();
> -
>      if (!dev) {
> +        /*
> +         * Drain all pending RCU callbacks. This is done because
> +         * some bus related operations can delay a device removal
> +         * (in this case this can happen if device is added and then
> +         * removed due to a configuration error)
> +         * to a RCU callback, but user might expect that this interface
> +         * will finish its job completely once qmp command returns result
> +         * to the user
> +         */
> +        drain_call_rcu();
> +
>          qemu_opts_del(opts);
>          return;
>      }