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Re: [Xen-devel] [PATCH] libxl: Restore scheduling parameters after migrate in best-effort fashion
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- From: George Dunlap <george.dunlap@xxxxxxxxxx>
- Date: Wed, 3 Oct 2018 11:52:40 +0100
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxx>
- Delivery-date: Wed, 03 Oct 2018 10:52:49 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 10/02/2018 04:49 PM, 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>
I've tested this with save/restore now, and it works fine (including
changing the weight of the VM from default and having the updated weight
show up in the other scheduler on restore).
-George
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