Usually, incoming migration coroutine yields to the main loop
while its IO-channel is waiting for data to receive. But there is a case
when RAM migration and data receive have the same speed: VM with huge
zeroed RAM. In this case, IO-channel won't read and thus the main loop
is stuck and for instance, it doesn't respond to QMP commands.

For this case, yield periodically, but not too often, so as not to
affect the speed of migration.

Signed-off-by: Yury Kotov <yury-kotov@yandex-team.ru>
---
 migration/ram.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/migration/ram.c b/migration/ram.c
index 5078f94490..9694ee7a0b 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -4227,7 +4227,7 @@ static void colo_flush_ram_cache(void)
  */
 static int ram_load_precopy(QEMUFile *f)
 {
-    int flags = 0, ret = 0, invalid_flags = 0, len = 0;
+    int flags = 0, ret = 0, invalid_flags = 0, len = 0, i = 0;
     /* ADVISE is earlier, it shows the source has the postcopy capability on */
     bool postcopy_advised = postcopy_is_advised();
     if (!migrate_use_compression()) {
@@ -4239,6 +4239,17 @@ static int ram_load_precopy(QEMUFile *f)
         void *host = NULL;
         uint8_t ch;
 
+        /*
+         * Yield periodically to let main loop run, but an iteration of
+         * the main loop is expensive, so do it each some iterations
+         */
+        if ((i & 32767) == 0 && qemu_in_coroutine()) {
+            aio_co_schedule(qemu_get_current_aio_context(),
+                            qemu_coroutine_self());
+            qemu_coroutine_yield();
+        }
+        i++;
+
         addr = qemu_get_be64(f);
         flags = addr & ~TARGET_PAGE_MASK;
         addr &= TARGET_PAGE_MASK;
-- 
2.24.0

Yury Kotov <yury-kotov@yandex-team.ru> wrote:
> Usually, incoming migration coroutine yields to the main loop
> while its IO-channel is waiting for data to receive. But there is a case
> when RAM migration and data receive have the same speed: VM with huge
> zeroed RAM. In this case, IO-channel won't read and thus the main loop
> is stuck and for instance, it doesn't respond to QMP commands.
>
> For this case, yield periodically, but not too often, so as not to
> affect the speed of migration.
>
> Signed-off-by: Yury Kotov <yury-kotov@yandex-team.ru>

Reviewed-by: Juan Quintela <quintela@redhat.com>

Juan Quintela <quintela@redhat.com> wrote:
> Yury Kotov <yury-kotov@yandex-team.ru> wrote:
>> Usually, incoming migration coroutine yields to the main loop
>> while its IO-channel is waiting for data to receive. But there is a case
>> when RAM migration and data receive have the same speed: VM with huge
>> zeroed RAM. In this case, IO-channel won't read and thus the main loop
>> is stuck and for instance, it doesn't respond to QMP commands.
>>
>> For this case, yield periodically, but not too often, so as not to
>> affect the speed of migration.
>>
>> Signed-off-by: Yury Kotov <yury-kotov@yandex-team.ru>
>
> Reviewed-by: Juan Quintela <quintela@redhat.com>

Nack.

The idea is good.  But it fails migration-test to fail from time to time
(50% of the time on my laptop).

Will investigate why this is failing.

Later, Juan.