[PATCH 2/2] qtest/migration: Use switchover-hold to speedup

[Peter Xu <peterx@redhat.com>]

This solution is heavily based on Daniel's original approach here, but
hopefully a cleaner way to impl:

https://lore.kernel.org/r/20230601161347.1803440-11-berrange@redhat.com

The difference is we use the switchover-hold flag rather than tuning
bw+downtime to guide test convergence, comparing to use the magic offset.

One thing needs caution is the postcopy test - we used to only make it
non-converge for precopy.  Now we need the paired migrate_ensure_converge()
to make sure postcopy can still complete or otherwise the flag will stops
the final switchover from postcopy->finish.

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

With this flag, we can safely always run migration tests with full
speed (bw=0).

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

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 tests/qtest/migration-test.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/tests/qtest/migration-test.c b/tests/qtest/migration-test.c
index b0c355bbd9..3a583238e1 100644
--- a/tests/qtest/migration-test.c
+++ b/tests/qtest/migration-test.c
@@ -433,16 +433,13 @@ 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);
+    migrate_set_parameter_bool(who, "switchover-hold", false);
 }
 
 static void migrate_pause(QTestState *who)
@@ -492,6 +489,13 @@ 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 {
@@ -736,6 +740,14 @@ static int test_migrate_start(QTestState **from, QTestState **to,
         unlink(shmem_path);
     }
 
+    /*
+     * By default, use full speed for precopy in qtests as that will reduce
+     * the time of testing. The default bandwidth (128MB/s) may be too slow
+     * in this case.  Specific test can overwrite this value after the
+     * function returns but before starting migration.
+     */
+    migrate_set_parameter_int(*from, "max-bandwidth", 0);
+
     return 0;
 }
 
-- 
2.40.1