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

[Peter Xu <peterx@redhat.com>]

On Wed, May 31, 2023 at 10:41:37AM +0000, Wang, Wei W wrote:
> 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.

Interesting to know, thanks.

If there'll be a new version, please consider adding some of those into the
commit message.

Reviewed-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu