Re: [RFC v4 PATCH 30/49] multi-process: send heartbeat messages to remote

[Stefan Hajnoczi]

On Thu, Oct 24, 2019 at 05:09:11AM -0400, Jagannathan Raman wrote:
> +static void broadcast_msg(MPQemuMsg *msg, bool need_reply)
> +{
> +    PCIProxyDev *entry;
> +    unsigned int pid;
> +    int wait;
> +
> +    QLIST_FOREACH(entry, &proxy_dev_list.devices, next) {
> +        if (need_reply) {
> +            wait = eventfd(0, EFD_NONBLOCK);
> +            msg->num_fds = 1;
> +            msg->fds[0] = wait;
> +        }
> +
> +        mpqemu_msg_send(entry->mpqemu_link, msg, entry->mpqemu_link->com);
> +        if (need_reply) {
> +            pid = (uint32_t)wait_for_remote(wait);

Sometimes QEMU really needs to wait for the remote process before it can
make progress.  I think this is not one of those cases though.

Since QEMU is event-driven it's problematic to invoke blocking system
calls.  The remote process might not respond for a significant amount of
time.  Other QEMU threads will be held up waiting for the QEMU global
mutex in the meantime (because we hold it!).

Please implement heartbeat/ping asynchronously.  The wait eventfd should
be read by an event loop fd handler instead.  That way QEMU can continue
with running the VM while waiting for the remote process.

This will also improve guest performance because there will be less
jitter (random latency because the event loop is held up waiting for
remote processes for short periods of time).

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