Re: [PATCH 2/2] util/qemu-sockets: make keep-alive enabled by default

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Jul 08, 2020 at 10:15:39PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> Keep-alive won't hurt, let's try to enable it even if not requested by
> user.

Keep-alive intentionally breaks TCP connections earlier than normal
in face of transient networking problems.

The question is more about which type of pain is more desirable. A
stall in the network connection (for a potentially very long time),
or an intentionally broken socket.

I'm not at all convinced it is a good idea to intentionally break
/all/ QEMU sockets in the face of transient problems, even if the
problems last for 2 hours or more. 

I could see keep-alives being ok on some QEMU socket. For example
VNC/SPICE clients, as there is no downside to proactively culling
them as they can trivially reconnect. Migration too is quite
reasonable to use keep alives, as you generally want migration to
run to completion in a short amount of time, and aborting migration
needs to be safe no matter what.

Breaking chardevs or block devices or network devices that use
QEMU sockets though will be disruptive. The only solution once
those backends have a dead socket is going to be to kill QEMU
and cold-boot the VM again.


> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  util/qemu-sockets.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c
> index b961963472..f6851376f5 100644
> --- a/util/qemu-sockets.c
> +++ b/util/qemu-sockets.c
> @@ -438,7 +438,8 @@ static struct addrinfo *inet_parse_connect_saddr(InetSocketAddress *saddr,
>   *
>   * Handle keep_alive settings. If user specified settings explicitly, fail if
>   * can't set the settings. If user just enabled keep-alive, not specifying the
> - * settings, try to set defaults but ignore failures.
> + * settings, try to set defaults but ignore failures. If keep-alive option is
> + * not specified, try to set it but ignore failures.
>   */
>  static int inet_set_keepalive(int sock, bool has_keep_alive,
>                                KeepAliveField *keep_alive, Error **errp)
> @@ -447,8 +448,8 @@ static int inet_set_keepalive(int sock, bool has_keep_alive,
>      int val;
>      bool has_settings = has_keep_alive &&  keep_alive->type == QTYPE_QDICT;
>  
> -    if (!has_keep_alive || (keep_alive->type == QTYPE_QBOOL &&
> -                            !keep_alive->u.enabled))
> +    if (has_keep_alive &&
> +        keep_alive->type == QTYPE_QBOOL && !keep_alive->u.enabled)
>      {
>          return 0;
>      }
> @@ -456,8 +457,12 @@ static int inet_set_keepalive(int sock, bool has_keep_alive,
>      val = 1;
>      ret = qemu_setsockopt(sock, SOL_SOCKET, SO_KEEPALIVE, &val, sizeof(val));
>      if (ret < 0) {
> -        error_setg_errno(errp, errno, "Unable to set KEEPALIVE");
> -        return -1;
> +        if (has_keep_alive) {
> +            error_setg_errno(errp, errno, "Unable to set KEEPALIVE");
> +            return -1;
> +        } else {
> +            return 0;
> +        }
>      }
>  
>      val = has_settings ? keep_alive->u.settings.idle : 30;
> -- 
> 2.21.0
> 

Regards,
Daniel
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