Re: [Xen-devel] [xen-unstable test] 128240: regressions - FAIL

[Dario Faggioli <dfaggioli@xxxxxxxx>]

On Mon, 2018-10-01 at 18:07 +0200, Juergen Gross wrote:
> On 01/10/2018 17:48, George Dunlap wrote:
> > On 10/01/2018 04:40 PM, Andrew Cooper wrote:
> > > On 01/10/18 16:35, Wei Liu wrote:
> > > > > Wait, the migration code reads the scheduler parameters --
> > > > > even if these
> > > > > have not been explicitly set by the admin -- and sends them
> > > > > along with
> > > > > the migration stream?  And if the remote scheduler is
> > > > > different, the
> > > > > migration fails?
> > > > > 
> > > > > That's not so good. :-)
> > > > 
> > > > But one can argue that the guest is specific configured that
> > > > way so it's
> > > > parameters should be preserved. We normally analyse things on a
> > > > case by
> > > > case basis.
> > > 
> > > If there isn't an obvious fix, then the switch of default
> > > scheduler
> > > needs reverting until there is a fix present.  This is currently
> > > blocking master.
> > 
> > Agreed.  I'd argue for ignoring failures to set scheduler
> > parameters on
> > migrate, on the grounds that this will be less risk to the project
> > as a
> > whole than reverting credit2 again.  But either way we should do
> > something quickly.
> 
> We should ignore a mismatch of the scheduler. Failures when setting
> parameters for a matching scheduler should not be ignored IMO.
> 
Indeed! Especially considering that this isn't really related on what
the default scheduler is (despite it being making Credit2 default that
triggers the issue).

In fact, what if:
- user uses Credit1 (default and supported) on host A
- user uses Credit2 (supported) on host B
- user migrates VM
- BOOOM!

So, unless it is intended --and, I'd say, also documented somewhere--
that migrating between hosts which use different schedulers is to be
avoided, this is already a bug, whatever the default scheduler is...

George, let me know if you're working on a fix already, or if I should
do that myself.

Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Software Engineer @ SUSE https://www.suse.com/

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