[Xen-changelog] [xen staging] libxl: Restore scheduling parameters after migrate in best-effort fashion

[p***@xen.org]

commit 162fc8295f31240dc3670190a91e9bbc03b0d7be
Author: George Dunlap <***@citrix.com>
AuthorDate: Wed Oct 10 12:36:25 2018 +0100
Commit: George Dunlap <***@citrix.com>
CommitDate: Wed Oct 10 12:36:25 2018 +0100

libxl: Restore scheduling parameters after migrate in best-effort fashion

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 <***@citrix.com>
Acked-by: Ian Jackson <***@citrix.com>
Acked-by: Wei Liu <***@citrix.com>
---
tools/libxl/libxl_create.c | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
index 320dbed3c6..4bb750e951 100644
--- a/tools/libxl/libxl_create.c
+++ b/tools/libxl/libxl_create.c
@@ -1218,6 +1218,15 @@ static void domcreate_stream_done(libxl__egc *egc,
ret = ERROR_INVAL;
goto out;
}
+
+ /*
+ * The scheduler on the sending domain may be different than the
+ * scheduler running here. Setting the scheduler to UNKNOWN will
+ * cause the code to take to take whatever parameters are
+ * available in that scheduler, while discarding the rest.
+ */
+ info->sched_params.sched = LIBXL_SCHEDULER_UNKNOWN;
+
ret = libxl__build_post(gc, domid, info, state, vments, localents);
if (ret)
goto out;
--
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