[PATCH 3/3] COLO: Optimize memory back-up process

[Hailiang Zhang]

This patch will reduce the downtime of VM for the initial process,
Privously, we copied all these memory in preparing stage of COLO
while we need to stop VM, which is a time-consuming process.
Here we optimize it by a trick, back-up every page while in migration
process while COLO is enabled, though it affects the speed of the
migration, but it obviously reduce the downtime of back-up all SVM'S
memory in COLO preparing stage.

Signed-off-by: Hailiang Zhang <address@hidden>
---
 migration/colo.c |  3 +++
 migration/ram.c  | 35 +++++++++++++++++++++++++++--------
 migration/ram.h  |  1 +
 3 files changed, 31 insertions(+), 8 deletions(-)

diff --git a/migration/colo.c b/migration/colo.c
index d30c6bc4ad..febf010571 100644
--- a/migration/colo.c
+++ b/migration/colo.c
@@ -26,6 +26,7 @@
 #include "qemu/main-loop.h"
 #include "qemu/rcu.h"
 #include "migration/failover.h"
+#include "migration/ram.h"
 #ifdef CONFIG_REPLICATION
 #include "replication.h"
 #endif
@@ -906,6 +907,8 @@ void *colo_process_incoming_thread(void *opaque)
      */
     qemu_file_set_blocking(mis->from_src_file, true);
 
+    colo_incoming_start_dirty_log();
+
     bioc = qio_channel_buffer_new(COLO_BUFFER_BASE_SIZE);
     fb = qemu_fopen_channel_input(QIO_CHANNEL(bioc));
     object_unref(OBJECT(bioc));
diff --git a/migration/ram.c b/migration/ram.c
index ed23ed1c7c..24a8aa3527 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -2986,7 +2986,6 @@ int colo_init_ram_cache(void)
                 }
                 return -errno;
             }
-            memcpy(block->colo_cache, block->host, block->used_length);
         }
     }
 
@@ -3005,12 +3004,16 @@ int colo_init_ram_cache(void)
             bitmap_set(block->bmap, 0, pages);
         }
     }
+
+    return 0;
+}
+
+void colo_incoming_start_dirty_log(void)
+{
     ram_state = g_new0(RAMState, 1);
     ram_state->migration_dirty_pages = 0;
     qemu_mutex_init(&ram_state->bitmap_mutex);
     memory_global_dirty_log_start();
-
-    return 0;
 }
 
 /* It is need to hold the global lock to call this helper */
@@ -3348,7 +3351,7 @@ static int ram_load_precopy(QEMUFile *f)
 
     while (!ret && !(flags & RAM_SAVE_FLAG_EOS)) {
         ram_addr_t addr, total_ram_bytes;
-        void *host = NULL;
+        void *host = NULL, *host_bak = NULL;
         uint8_t ch;
 
         /*
@@ -3378,13 +3381,26 @@ static int ram_load_precopy(QEMUFile *f)
         if (flags & (RAM_SAVE_FLAG_ZERO | RAM_SAVE_FLAG_PAGE |
                      RAM_SAVE_FLAG_COMPRESS_PAGE | RAM_SAVE_FLAG_XBZRLE)) {
             RAMBlock *block = ram_block_from_stream(f, flags);
-
             /*
-             * After going into COLO, we should load the Page into colo_cache.
+             * After going into COLO, we should load the Page into colo_cache
+             * NOTE: We need to keep a copy of SVM's ram in colo_cache.
+             * Privously, we copied all these memory in preparing stage of COLO
+             * while we need to stop VM, which is a time-consuming process.
+             * Here we optimize it by a trick, back-up every page while in
+             * migration process while COLO is enabled, though it affects the
+             * speed of the migration, but it obviously reduce the downtime of
+             * back-up all SVM'S memory in COLO preparing stage.
              */
-            if (migration_incoming_in_colo_state()) {
+            if (migration_incoming_colo_enabled()) {
                 host = colo_cache_from_block_offset(block, addr);
-            } else {
+                /*
+                 * After going into COLO, load the Page into colo_cache.
+                 */
+                if (!migration_incoming_in_colo_state()) {
+                    host_bak = host;
+                }
+            }
+            if (!migration_incoming_in_colo_state()) {
                 host = host_from_ram_block_offset(block, addr);
             }
             if (!host) {
@@ -3506,6 +3522,9 @@ static int ram_load_precopy(QEMUFile *f)
         if (!ret) {
             ret = qemu_file_get_error(f);
         }
+        if (!ret && host_bak && host) {
+            memcpy(host_bak, host, TARGET_PAGE_SIZE);
+        }
     }
 
     ret |= wait_for_decompress_done();
diff --git a/migration/ram.h b/migration/ram.h
index a553d40751..5ceaff7cb4 100644
--- a/migration/ram.h
+++ b/migration/ram.h
@@ -66,5 +66,6 @@ int ram_dirty_bitmap_reload(MigrationState *s, RAMBlock *rb);
 /* ram cache */
 int colo_init_ram_cache(void);
 void colo_release_ram_cache(void);
+void colo_incoming_start_dirty_log(void);
 
 #endif
-- 
2.21.0