Re: [PATCH v3 1/2] qemu-nbd: Use SOMAXCONN for socket listen() backlog

["Richard W.M. Jones" <rjones@redhat.com>]

On Tue, Feb 09, 2021 at 09:27:58AM -0600, Eric Blake wrote:
> Our default of a backlog of 1 connection is rather puny; it gets in
> the way when we are explicitly allowing multiple clients (such as
> qemu-nbd -e N [--shared], or nbd-server-start with its default
> "max-connections":0 for unlimited), but is even a problem when we
> stick to qemu-nbd's default of only 1 active client but use -t
> [--persistent] where a second client can start using the server once
> the first finishes.  While the effects are less noticeable on TCP
> sockets (since the client can poll() to learn when the server is ready
> again), it is definitely observable on Unix sockets, where on Unix, a
> client will fail with EAGAIN and no recourse but to sleep an arbitrary
> amount of time before retrying if the server backlog is already full.
> 
> Since QMP nbd-server-start is always persistent, it now always
> requests a backlog of SOMAXCONN; meanwhile, qemu-nbd will request
> SOMAXCONN if persistent, otherwise its backlog should be based on the
> expected number of clients.
> 
> See https://bugzilla.redhat.com/1925045 for a demonstration of where
> our low backlog prevents libnbd from connecting as many parallel
> clients as it wants.
> 
> Reported-by: Richard W.M. Jones <rjones@redhat.com>
> Signed-off-by: Eric Blake <eblake@redhat.com>
> CC: qemu-stable@nongnu.org
> ---
>  blockdev-nbd.c |  7 ++++++-
>  qemu-nbd.c     | 10 +++++++++-
>  2 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/blockdev-nbd.c b/blockdev-nbd.c
> index d8443d235b73..b264620b98d8 100644
> --- a/blockdev-nbd.c
> +++ b/blockdev-nbd.c
> @@ -134,7 +134,12 @@ void nbd_server_start(SocketAddress *addr, const char *tls_creds,
>      qio_net_listener_set_name(nbd_server->listener,
>                                "nbd-listener");
> 
> -    if (qio_net_listener_open_sync(nbd_server->listener, addr, 1, errp) < 0) {
> +    /*
> +     * Because this server is persistent, a backlog of SOMAXCONN is
> +     * better than trying to size it to max_connections.
> +     */
> +    if (qio_net_listener_open_sync(nbd_server->listener, addr, SOMAXCONN,
> +                                   errp) < 0) {
>          goto error;
>      }
> 
> diff --git a/qemu-nbd.c b/qemu-nbd.c
> index 608c63e82a25..1a340ea4858d 100644
> --- a/qemu-nbd.c
> +++ b/qemu-nbd.c
> @@ -964,8 +964,16 @@ int main(int argc, char **argv)
> 
>      server = qio_net_listener_new();
>      if (socket_activation == 0) {
> +        int backlog;
> +
> +        if (persistent) {
> +            backlog = SOMAXCONN;
> +        } else {
> +            backlog = MIN(shared, SOMAXCONN);
> +        }
>          saddr = nbd_build_socket_address(sockpath, bindto, port);
> -        if (qio_net_listener_open_sync(server, saddr, 1, &local_err) < 0) {
> +        if (qio_net_listener_open_sync(server, saddr, backlog,
> +                                       &local_err) < 0) {
>              object_unref(OBJECT(server));
>              error_report_err(local_err);
>              exit(EXIT_FAILURE);

Works fine here, so:

Tested-by: Richard W.M. Jones <rjones@redhat.com>

Rich.

-- 
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