[PATCH RFC 3/3] qtest/migration: Use switchover-hold to speedup

[Peter Xu <peterx@redhat.com>]

Use the switchover-hold flag rather than tuning bw+downtime to guide test
convergence.

This can achieve similar goal of previous patch "tests/qtest: massively
speed up migration-test" but without magic offset to write or monitoring.

The initial solution can reduce migration-test time from 8min to 1min40s,
this patch can further reduce it from 1m40s to 1m1s per my local test.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 tests/qtest/migration-test.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/tests/qtest/migration-test.c b/tests/qtest/migration-test.c
index b0c355bbd9..62bdd67fd9 100644
--- a/tests/qtest/migration-test.c
+++ b/tests/qtest/migration-test.c
@@ -433,16 +433,15 @@ static void migrate_set_parameter_bool(QTestState *who, const char *parameter,
 
 static void migrate_ensure_non_converge(QTestState *who)
 {
-    /* Can't converge with 1ms downtime + 3 mbs bandwidth limit */
-    migrate_set_parameter_int(who, "max-bandwidth", 3 * 1000 * 1000);
-    migrate_set_parameter_int(who, "downtime-limit", 1);
+    /* Hold off switchover for precopy only */
+    migrate_set_parameter_bool(who, "switchover-hold", true);
 }
 
 static void migrate_ensure_converge(QTestState *who)
 {
-    /* Should converge with 30s downtime + 1 gbs bandwidth limit */
-    migrate_set_parameter_int(who, "max-bandwidth", 1 * 1000 * 1000 * 1000);
-    migrate_set_parameter_int(who, "downtime-limit", 30 * 1000);
+    /* No limitation on bandwidth so converge faster */
+    migrate_set_parameter_int(who, "max-bandwidth", 0);
+    migrate_set_parameter_bool(who, "switchover-hold", false);
 }
 
 static void migrate_pause(QTestState *who)
@@ -492,6 +491,13 @@ static void migrate_postcopy_start(QTestState *from, QTestState *to)
     }
 
     qtest_qmp_eventwait(to, "RESUME");
+
+    /*
+     * Now allow precopy switchover (which will allow completion).  This
+     * needs to be done after migrate-start-postcopy to make sure we switch
+     * to postcopy first.
+     */
+    migrate_ensure_converge(from);
 }
 
 typedef struct {
@@ -1164,6 +1170,8 @@ static int migrate_postcopy_prepare(QTestState **from_ptr,
     }
 
     migrate_ensure_non_converge(from);
+    /* Still use unlimited precopy speed to finish 1st iteration fast */
+    migrate_set_parameter_int(from, "max-bandwidth", 0);
 
     /* Wait for the first serial output from the source */
     wait_for_serial("src_serial");
-- 
2.40.1