RE: [PATCH v2 2/2] qtest/migration-tests.c: use "-incoming defer" for postcopy tests

[Wang, Wei W]

On Tuesday, May 30, 2023 10:41 PM, Peter Xu wrote:
> On Tue, May 30, 2023 at 05:02:59PM +0800, Wei Wang wrote:
> > The Postcopy preempt capability requires to be set before incoming
> > starts, so change the postcopy tests to start with deferred incoming
> > and call migrate-incoming after the cap has been set.
> >
> > Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> 
> Hmm.. so we used to do socket_start_incoming_migration_internal() before
> setting the right num for the preempt test, then I'm curious why it wasn't
> failing before this patch when trying to connect with the preempt channel..
> 
> Wei, do you know?

I think there are two reasons:
#1 "backlog" specifies the number of pending connections. As long as the server
accepts the connections faster than the clients side connecting, connection
will succeed. For the preempt test, it uses only 2 channels, so very likely to 
not have
pending connections. (This is easier to trigger for the multifd case, e.g. use 
a much
larger number like 16).
#2 per my tests (on kernel 6.2), the number of pending connections allowed is 
actually
"backlog + 1", which is 2 in this case. Adding more pending connections will 
then be
dropped. I'm not sure if " backlog + 1" is true for older versions of kernel, 
though.