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[Xen-devel] [PATCH] 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

Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
CC: Ian Jackson <ian.jackson@xxxxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Jan Beulich <jbeulich@xxxxxxxx>
CC: Dario Faggioli <dfaggioli@xxxxxxxx>
CC: Juergen Gross <jgross@xxxxxxxx>
 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 dcfde7787e..caf79d4620 100644
--- a/tools/libxl/libxl_create.c
+++ b/tools/libxl/libxl_create.c
@@ -1237,6 +1237,15 @@ 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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