Re: [PATCH 2/2] tests/qtest: make more migration pre-copy scenarios run non-live

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

On Thu, Apr 20, 2023 at 02:59:00PM +0200, Juan Quintela wrote:
> Daniel P. Berrangé <berrange@redhat.com> wrote:
> > There are 27 pre-copy live migration scenarios being tested. In all of
> > these we force non-convergance and run for one iteration, then let it
> > converge and wait for completion during the second (or following)
> > iterations. At 3 mbps bandwidth limit the first iteration takes a very
> > long time (~30 seconds).
> >
> > While it is important to test the migration passes and convergance
> > logic, it is overkill to do this for all 27 pre-copy scenarios. The
> > TLS migration scenarios in particular are merely exercising different
> > code paths during connection establishment.
> >
> > To optimize time taken, switch most of the test scenarios to run
> > non-live (ie guest CPUs paused) with no bandwidth limits. This gives
> > a massive speed up for most of the test scenarios.
> >
> > For test coverage the following scenarios are unchanged
> >
> >  * Precopy with UNIX sockets
> >  * Precopy with UNIX sockets and dirty ring tracking
> >  * Precopy with XBZRLE
> >  * Precopy with multifd
> 
> Just for completeness: the other test that is still slow is
> /migration/vcpu_dirty_limit.
> 
> > -    migrate_ensure_non_converge(from);
> > +    if (args->live) {
> > +        migrate_ensure_non_converge(from);
> > +    } else {
> > +        migrate_ensure_converge(from);
> > +    }
> 
> Looks ... weird?
> But the only way that I can think of improving it is to pass args to
> migrate_ensure_*() and that is a different kind of weird.

I'm going to change this a little anyway. Currently for the non-live
case, I start the migration and then  stop the CPUs. I want to reverse
that order, as we should have CPUs paused before even starting the
migration to ensure we don't have any re-dirtied pages at all.

> 
> >      } else {
> > -        if (args->iterations) {
> > -            while (args->iterations--) {
> > +        if (args->live) {
> > +            if (args->iterations) {
> > +                while (args->iterations--) {
> > +                    wait_for_migration_pass(from);
> > +                }
> > +            } else {
> >                  wait_for_migration_pass(from);
> >              }
> > +
> > +            migrate_ensure_converge(from);
> 
> I think we should change iterations to be 1 when we create args, but
> otherwise, treat 0 as 1 and change it to something in the lines of:
> 
>         if (args->live) {
>             while (args->iterations-- >= 0) {
>                 wait_for_migration_pass(from);
>             }
>             migrate_ensure_converge(from);
> 
> What do you think?

I think in retrospect 'iterations' was overkill as we only use the
values 0 (implicitly 1) or 2. IOW we could just just a 'bool multipass'
to distinguish the two cases.

> > -        qtest_qmp_eventwait(to, "RESUME");
> > +        if (!args->live) {
> > +            qtest_qmp_discard_response(to, "{ 'execute' : 'cont'}");
> > +        }
> > +        if (!got_resume) {
> > +            qtest_qmp_eventwait(to, "RESUME");
> > +        }
> >  
> >          wait_for_serial("dest_serial");
> >      }
> 
> I was looking at the "culprit" of Lukas problem, and it is not directly
> obvious.  I see that when we expect one event, we just drop any event
> that we are not interested in.  I don't know if that is the proper
> behaviour or if that is what affecting this test.

I've not successfully reproduced it yet, nor figured out a real
scenario where it could plausibly happen. i'm looking to add more
debug to help us out.

With regards,
Daniel
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