Re: [Qemu-devel] [PATCH v8 16/23] monitor: send event when command queue full

[Eric Blake <eblake@redhat.com>]

On 03/09/2018 02:59 AM, Peter Xu wrote:
> Set maximum QMP command queue length to 8.  If queue full, instead of

If the queue is full

> queue the command, we directly return a "command-dropped" event, telling

s/queue/queuing/

> client that specific command is dropped.

s/client that/the client that a/

> 
> Note that this flow control mechanism is only valid if OOB is enabled.
> If it's not, the effective queue length will always be 1, which strictly
> follows original behavior of QMP command handling (which never drop

s/drop/drops/

> messages).
> 
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>   monitor.c | 18 +++++++++++++++++-
>   1 file changed, 17 insertions(+), 1 deletion(-)

> 
> diff --git a/monitor.c b/monitor.c
> index 5104e5db07..4d57a8d341 100644
> --- a/monitor.c
> +++ b/monitor.c
> @@ -4007,6 +4007,8 @@ static void monitor_qmp_bh_dispatcher(void *data)
>       }
>   }
>   
> +#define  QMP_REQ_QUEUE_LEN_MAX  (8)

Parens are redundant on a single int, but it doesn't hurt to leave them 
in a macro definition.

> @@ -4049,6 +4054,18 @@ static void handle_qmp_command(JSONMessageParser *parser, GQueue *tokens)
>       if (!qmp_oob_enabled(mon)) {
>           monitor_suspend(mon);
>           req_obj->need_resume = true;
> +    } else {
> +        /* Drop the request if queue is full. */
> +        if (mon->qmp.qmp_requests->length >= QMP_REQ_QUEUE_LEN_MAX) {
> +            qemu_mutex_unlock(&mon->qmp.qmp_queue_lock);
> +            qapi_event_send_command_dropped(id,
> +                                            COMMAND_DROP_REASON_QUEUE_FULL,
> +                                            NULL);

Most qapi_event_send_* callers pass &error_abort (migration/ram.c being 
the only exception I could find); why not this one?  I'm changing it for 
consistency; we can revert back to NULL if testing finds a problem with it.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org