[Qemu-devel] [PATCH] migration: always initial ram_counters for a new migration

[Ivan Ren <renyime@gmail.com>]

From: Ivan Ren <ivanren@tencent.com>

This patch fix a multifd migration bug in migration speed calculation, this
problem can be reproduced as follows:
1. start a vm and give a heavy memory write stress to prevent the vm be
   successfully migrated to destination
2. begin a migration with multifd
3. migrate for a long time [actually, this can be measured by transferred bytes]
4. migrate cancel
5. begin a new migration with multifd, the migration will directly run into
   migration_completion phase

Reason as follows:

Migration update bandwidth and s->threshold_size in function
migration_update_counters after BUFFER_DELAY time:

    current_bytes = migration_total_bytes(s);
    transferred = current_bytes - s->iteration_initial_bytes;
    time_spent = current_time - s->iteration_start_time;
    bandwidth = (double)transferred / time_spent;
    s->threshold_size = bandwidth * s->parameters.downtime_limit;

In multifd migration, migration_total_bytes function return
qemu_ftell(s->to_dst_file) + ram_counters.multifd_bytes.
s->iteration_initial_bytes will be initialized to 0 at every new migration,
but ram_counters is a global variable, and history migration data will be
accumulated. So if the ram_counters.multifd_bytes is big enough, it may lead
pending_size >= s->threshold_size become false in migration_iteration_run
after the first migration_update_counters.

Signed-off-by: Ivan Ren <ivanren@tencent.com>
---
 migration/migration.c | 15 ++++++++++++++-
 migration/savevm.c    |  1 +
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/migration/migration.c b/migration/migration.c
index 8a607fe1e2..d35a6ae6f9 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1908,6 +1908,11 @@ static bool migrate_prepare(MigrationState *s, bool blk, bool blk_inc,
     }
 
     migrate_init(s);
+    /*
+     * set ram_counters memory to zero for a
+     * new migration
+     */
+    memset(&ram_counters, 0, sizeof(ram_counters));
 
     return true;
 }
@@ -3187,6 +3192,10 @@ static void *migration_thread(void *opaque)
 
     object_ref(OBJECT(s));
     s->iteration_start_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
+    /*
+     * Update s->iteration_initial_bytes to match s->iteration_start_time.
+     */
+    s->iteration_initial_bytes = migration_total_bytes(s);
 
     qemu_savevm_state_header(s->to_dst_file);
 
@@ -3252,7 +3261,11 @@ static void *migration_thread(void *opaque)
              * breaking transferred_bytes and bandwidth calculation
              */
             s->iteration_start_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
-            s->iteration_initial_bytes = 0;
+            /*
+             * Update s->iteration_initial_bytes to current size to
+             * avoid historical data lead wrong bandwidth.
+             */
+            s->iteration_initial_bytes = migration_total_bytes(s);
         }
 
         current_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
diff --git a/migration/savevm.c b/migration/savevm.c
index 79ed44d475..480c511b19 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -1424,6 +1424,7 @@ static int qemu_savevm_state(QEMUFile *f, Error **errp)
     }
 
     migrate_init(ms);
+    memset(&ram_counters, 0, sizeof(ram_counters));
     ms->to_dst_file = f;
 
     qemu_mutex_unlock_iothread();
-- 
2.17.2 (Apple Git-113)