Re: [Xen-devel] [PATCH] libxl: Restore scheduling parameters after migrate in best-effort fashion

[Dario Faggioli <dfaggioli@xxxxxxxx>]

On Wed, 2018-10-03 at 12:53 +0200, Juergen Gross wrote:
> On 02/10/2018 17:49, George Dunlap wrote:
> > Commit 3b4adba ("tools/libxl: include scheduler parameters in the
> > output of xl list -l") added scheduling parameters to the set of
> > information collected by libxl_retrieve_domain_configuration(), in
> > order to report that information in `xl list -l`.
> > 
> > Unfortunately, libxl_retrieve_domain_configuration() is also called
> > by
> > the migration / save code, and the results passed to the restore /
> > receive code.  This meant scheduler parameters were inadvertently
> > added to the migration stream, without proper consideration for how
> > to
> > handle corner cases.  The result was that if migrating from a host
> > running one scheduler to a host running a different scheduler, the
> > migration would fail with an error like the following:
> > 
> > libxl: error: libxl_sched.c:232:sched_credit_domain_set: Domain
> > 1:Getting domain sched credit: Invalid argument
> > libxl: error: libxl_create.c:1275:domcreate_rebuild_done: Domain
> > 1:cannot (re-)build domain: -3
> > 
> > Luckily there's a fairly straightforward way to set parameters in a
> > "best-effort" fashion.  libxl provides a single struct containing
> > the
> > parameters of all schedulers, as well as a parameter specifying
> > which
> > scheduler.  Parameters not used by a given scheduler are ignored.
> > Additionally, the struct contains a parameter to specify the
> > scheduler.  If you specify a specific scheduler,
> > libxl_domain_sched_params_set() will fail if there's a different
> > scheduler.  However, if you pass LIBXL_SCHEDULER_UNKNOWN, it will
> > use
> > the value of the current scheduler for that domain.
> > 
> > In domcreate_stream_done(), before calling libxl__build_post(), set
> > the scheduler to LIBXL_SCHEDULER_UNKNOWN.  This will propagate
> > scheduler parameters from the previous instantiation on a best-
> > effort
> > basis.
> > 
> > Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> 
> Acked-by: Juergen Gross <jgross@xxxxxxxx>
> 
Wei, Ian, ping?

We've reverted Credit2 as default scheduler, not because of a Credit2
own issue, but because of the bug fixed by this patch.

I think it would be both fair and appropriate to re-instate it (i.e.,
Credit2 as default), as soon as possible.

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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