[PATCH v4 0/4] Eliminate multifd flush

[PATCH v4 0/4] Eliminate multifd flush

[Juan Quintela]

Hi

In this v4:
- Rebased on top of migration-20230209 PULL request
- Integrate two patches in that pull request
- Rebase
- Address Eric reviews.

Please review.

In this v3:
- update to latest upstream.
- fix checkpatch errors.

Please, review.

In this v2:
- update to latest upstream
- change 0, 1, 2 values to defines
- Add documentation for SAVE_VM_FLAGS
- Add missing qemu_fflush(), it made random hangs for migration test
  (only for tls, no clue why).

Please, review.

[v1]
Upstream multifd code synchronize all threads after each RAM section.  This is suboptimal.
Change it to only flush after we go trough all ram.

Preserve all semantics for old machine types.

Juan Quintela (4):
  multifd: Create property multifd-sync-after-each-section
  multifd: Protect multifd_send_sync_main() calls
  multifd: Only sync once each full round of memory
  ram: Document migration ram flags

 qapi/migration.json   | 10 +++++++-
 migration/migration.h |  1 +
 hw/core/machine.c     |  1 +
 migration/migration.c | 13 ++++++++--
 migration/ram.c       | 56 +++++++++++++++++++++++++++++++++++--------
 5 files changed, 68 insertions(+), 13 deletions(-)

-- 
2.39.1

[PATCH v4 1/4] multifd: Create property multifd-sync-after-each-section

[Juan Quintela]

We used to synchronize all channels at the end of each RAM section
sent.  That is not needed, so preparing to only synchronize once every
full round in latests patches.

Notice that we initialize the property as true.  We will change the
default when we introduce the new mechanism.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

---

Rename each-iteration to after-each-section

Signed-off-by: Juan Quintela <quintela@redhat.com>
---
 qapi/migration.json   | 10 +++++++++-
 migration/migration.h |  1 +
 hw/core/machine.c     |  1 +
 migration/migration.c | 15 +++++++++++++--
 4 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/qapi/migration.json b/qapi/migration.json
index c84fa10e86..2907241b9c 100644
--- a/qapi/migration.json
+++ b/qapi/migration.json
@@ -478,6 +478,13 @@
 #                    should not affect the correctness of postcopy migration.
 #                    (since 7.1)
 #
+# @multifd-sync-after-each-section: Synchronize channels after each
+#                                   section is sent.  We used to do
+#                                   that in the past, but it is
+#                                   suboptimal.
+#                                   Default value is true until all code is in.
+#                                   (since 8.0)
+#
 # Features:
 # @unstable: Members @x-colo and @x-ignore-shared are experimental.
 #
@@ -492,7 +499,8 @@
            'dirty-bitmaps', 'postcopy-blocktime', 'late-block-activate',
            { 'name': 'x-ignore-shared', 'features': [ 'unstable' ] },
            'validate-uuid', 'background-snapshot',
-           'zero-copy-send', 'postcopy-preempt'] }
+           'zero-copy-send', 'postcopy-preempt',
+           'multifd-sync-after-each-section'] }
 
 ##
 # @MigrationCapabilityStatus:
diff --git a/migration/migration.h b/migration/migration.h
index 2da2f8a164..cf84520196 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -424,6 +424,7 @@ int migrate_multifd_channels(void);
 MultiFDCompression migrate_multifd_compression(void);
 int migrate_multifd_zlib_level(void);
 int migrate_multifd_zstd_level(void);
+bool migrate_multifd_sync_after_each_section(void);
 
 #ifdef CONFIG_LINUX
 bool migrate_use_zero_copy_send(void);
diff --git a/hw/core/machine.c b/hw/core/machine.c
index f73fc4c45c..dc86849402 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -54,6 +54,7 @@ const size_t hw_compat_7_1_len = G_N_ELEMENTS(hw_compat_7_1);
 GlobalProperty hw_compat_7_0[] = {
     { "arm-gicv3-common", "force-8-bit-prio", "on" },
     { "nvme-ns", "eui64-default", "on"},
+    { "migration", "multifd-sync-after-each-section", "on"},
 };
 const size_t hw_compat_7_0_len = G_N_ELEMENTS(hw_compat_7_0);
 
diff --git a/migration/migration.c b/migration/migration.c
index a5c22e327d..b2844d374f 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -167,7 +167,8 @@ INITIALIZE_MIGRATE_CAPS_SET(check_caps_background_snapshot,
     MIGRATION_CAPABILITY_XBZRLE,
     MIGRATION_CAPABILITY_X_COLO,
     MIGRATION_CAPABILITY_VALIDATE_UUID,
-    MIGRATION_CAPABILITY_ZERO_COPY_SEND);
+    MIGRATION_CAPABILITY_ZERO_COPY_SEND,
+    MIGRATION_CAPABILITY_MULTIFD_SYNC_AFTER_EACH_SECTION);
 
 /* When we add fault tolerance, we could have several
    migrations at once.  For now we don't need to add
@@ -2705,6 +2706,15 @@ bool migrate_use_multifd(void)
     return s->enabled_capabilities[MIGRATION_CAPABILITY_MULTIFD];
 }
 
+bool migrate_multifd_sync_after_each_section(void)
+{
+    MigrationState *s = migrate_get_current();
+
+    return true;
+    // We will change this when code gets in.
+    return s->enabled_capabilities[MIGRATION_CAPABILITY_MULTIFD_SYNC_AFTER_EACH_SECTION];
+}
+
 bool migrate_pause_before_switchover(void)
 {
     MigrationState *s;
@@ -4539,7 +4549,8 @@ static Property migration_properties[] = {
     DEFINE_PROP_MIG_CAP("x-zero-copy-send",
             MIGRATION_CAPABILITY_ZERO_COPY_SEND),
 #endif
-
+    DEFINE_PROP_MIG_CAP("multifd-sync-after-each-section",
+                        MIGRATION_CAPABILITY_MULTIFD_SYNC_AFTER_EACH_SECTION),
     DEFINE_PROP_END_OF_LIST(),
 };
 
-- 
2.39.1

Re: [PATCH v4 1/4] multifd: Create property multifd-sync-after-each-section

[Markus Armbruster]

Juan Quintela <quintela@redhat.com> writes:

> We used to synchronize all channels at the end of each RAM section
> sent.  That is not needed, so preparing to only synchronize once every
> full round in latests patches.
>
> Notice that we initialize the property as true.  We will change the
> default when we introduce the new mechanism.
>
> Signed-off-by: Juan Quintela <quintela@redhat.com>
> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Signed-off-by: Juan Quintela <quintela@redhat.com>
>
> ---
>
> Rename each-iteration to after-each-section
>
> Signed-off-by: Juan Quintela <quintela@redhat.com>
> ---

[...]

> diff --git a/migration/migration.c b/migration/migration.c
> index a5c22e327d..b2844d374f 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -167,7 +167,8 @@ INITIALIZE_MIGRATE_CAPS_SET(check_caps_background_snapshot,
>      MIGRATION_CAPABILITY_XBZRLE,
>      MIGRATION_CAPABILITY_X_COLO,
>      MIGRATION_CAPABILITY_VALIDATE_UUID,
> -    MIGRATION_CAPABILITY_ZERO_COPY_SEND);
> +    MIGRATION_CAPABILITY_ZERO_COPY_SEND,
> +    MIGRATION_CAPABILITY_MULTIFD_SYNC_AFTER_EACH_SECTION);
>  
>  /* When we add fault tolerance, we could have several
>     migrations at once.  For now we don't need to add
> @@ -2705,6 +2706,15 @@ bool migrate_use_multifd(void)
>      return s->enabled_capabilities[MIGRATION_CAPABILITY_MULTIFD];
>  }
>  
> +bool migrate_multifd_sync_after_each_section(void)
> +{
> +    MigrationState *s = migrate_get_current();
> +
> +    return true;
> +    // We will change this when code gets in.

It seems the time for that would be right now, doesn't it?  ;)

> +    return s->enabled_capabilities[MIGRATION_CAPABILITY_MULTIFD_SYNC_AFTER_EACH_SECTION];
> +}
> +
>  bool migrate_pause_before_switchover(void)
>  {
>      MigrationState *s;
> @@ -4539,7 +4549,8 @@ static Property migration_properties[] = {
>      DEFINE_PROP_MIG_CAP("x-zero-copy-send",
>              MIGRATION_CAPABILITY_ZERO_COPY_SEND),
>  #endif
> -
> +    DEFINE_PROP_MIG_CAP("multifd-sync-after-each-section",
> +                        MIGRATION_CAPABILITY_MULTIFD_SYNC_AFTER_EACH_SECTION),
>      DEFINE_PROP_END_OF_LIST(),
>  };

[PATCH v4 2/4] multifd: Protect multifd_send_sync_main() calls

[Juan Quintela]

We only need to do that on the ram_save_iterate() call on sending and
on destination when we get a RAM_SAVE_FLAG_EOS.

In setup() and complete() we need to synch in both new and old cases,
so don't add a check there.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

---

Remove the wrappers that we take out on patch 5.
---
 migration/ram.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index 8d114afd4b..899e2cd8af 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -3406,9 +3406,11 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
 out:
     if (ret >= 0
         && migration_is_setup_or_active(migrate_get_current()->state)) {
-        ret = multifd_send_sync_main(rs->pss[RAM_CHANNEL_PRECOPY].pss_channel);
-        if (ret < 0) {
-            return ret;
+        if (migrate_multifd_sync_after_each_section()) {
+            ret = multifd_send_sync_main(rs->pss[RAM_CHANNEL_PRECOPY].pss_channel);
+            if (ret < 0) {
+                return ret;
+            }
         }
 
         qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
@@ -4168,7 +4170,9 @@ int ram_load_postcopy(QEMUFile *f, int channel)
 
         case RAM_SAVE_FLAG_EOS:
             /* normal exit */
-            multifd_recv_sync_main();
+            if (migrate_multifd_sync_after_each_section()) {
+                multifd_recv_sync_main();
+            }
             break;
         default:
             error_report("Unknown combination of migration flags: 0x%x"
@@ -4439,7 +4443,9 @@ static int ram_load_precopy(QEMUFile *f)
             break;
         case RAM_SAVE_FLAG_EOS:
             /* normal exit */
-            multifd_recv_sync_main();
+            if (migrate_multifd_sync_after_each_section()) {
+                multifd_recv_sync_main();
+            }
             break;
         default:
             if (flags & RAM_SAVE_FLAG_HOOK) {
-- 
2.39.1

[PATCH v4 3/4] multifd: Only sync once each full round of memory

[Juan Quintela]

We need to add a new flag to mean to sync at that point.
Notice that we still synchronize at the end of setup and at the end of
complete stages.

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

---

Add missing qemu_fflush(), now it passes all tests always.
---
 qapi/migration.json   |  2 +-
 migration/migration.c |  2 --
 migration/ram.c       | 28 +++++++++++++++++++++++++++-
 3 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/qapi/migration.json b/qapi/migration.json
index 2907241b9c..5d0efa4590 100644
--- a/qapi/migration.json
+++ b/qapi/migration.json
@@ -482,7 +482,7 @@
 #                                   section is sent.  We used to do
 #                                   that in the past, but it is
 #                                   suboptimal.
-#                                   Default value is true until all code is in.
+#                                   Default value is false.
 #                                   (since 8.0)
 #
 # Features:
diff --git a/migration/migration.c b/migration/migration.c
index b2844d374f..9eb061319d 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2710,8 +2710,6 @@ bool migrate_multifd_sync_after_each_section(void)
 {
     MigrationState *s = migrate_get_current();
 
-    return true;
-    // We will change this when code gets in.
     return s->enabled_capabilities[MIGRATION_CAPABILITY_MULTIFD_SYNC_AFTER_EACH_SECTION];
 }
 
diff --git a/migration/ram.c b/migration/ram.c
index 899e2cd8af..32fab7b5ee 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -82,6 +82,7 @@
 #define RAM_SAVE_FLAG_XBZRLE   0x40
 /* 0x80 is reserved in migration.h start with 0x100 next */
 #define RAM_SAVE_FLAG_COMPRESS_PAGE    0x100
+#define RAM_SAVE_FLAG_MULTIFD_SYNC     0x200
 
 int (*xbzrle_encode_buffer_func)(uint8_t *, uint8_t *, int,
      uint8_t *, int) = xbzrle_encode_buffer;
@@ -1593,6 +1594,7 @@ retry:
  * associated with the search process.
  *
  * Returns:
+ *         <0: An error happened
  *         PAGE_ALL_CLEAN: no dirty page found, give up
  *         PAGE_TRY_AGAIN: no dirty page found, retry for next block
  *         PAGE_DIRTY_FOUND: dirty page found
@@ -1620,6 +1622,15 @@ static int find_dirty_block(RAMState *rs, PageSearchStatus *pss)
         pss->page = 0;
         pss->block = QLIST_NEXT_RCU(pss->block, next);
         if (!pss->block) {
+            if (!migrate_multifd_sync_after_each_section()) {
+                QEMUFile *f = rs->pss[RAM_CHANNEL_PRECOPY].pss_channel;
+                int ret = multifd_send_sync_main(f);
+                if (ret < 0) {
+                    return ret;
+                }
+                qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_SYNC);
+                qemu_fflush(f);
+            }
             /*
              * If memory migration starts over, we will meet a dirtied page
              * which may still exists in compression threads's ring, so we
@@ -2612,6 +2623,9 @@ static int ram_find_and_save_block(RAMState *rs)
                     break;
                 } else if (res == PAGE_TRY_AGAIN) {
                     continue;
+                } else if (res < 0) {
+                    pages = res;
+                    break;
                 }
             }
         }
@@ -3298,6 +3312,10 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
         return ret;
     }
 
+    if (!migrate_multifd_sync_after_each_section()) {
+        qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_SYNC);
+    }
+
     qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
     qemu_fflush(f);
 
@@ -3483,6 +3501,9 @@ static int ram_save_complete(QEMUFile *f, void *opaque)
         return ret;
     }
 
+    if (!migrate_multifd_sync_after_each_section()) {
+        qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_SYNC);
+    }
     qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
     qemu_fflush(f);
 
@@ -4167,7 +4188,9 @@ int ram_load_postcopy(QEMUFile *f, int channel)
             }
             decompress_data_with_multi_threads(f, page_buffer, len);
             break;
-
+        case RAM_SAVE_FLAG_MULTIFD_SYNC:
+            multifd_recv_sync_main();
+            break;
         case RAM_SAVE_FLAG_EOS:
             /* normal exit */
             if (migrate_multifd_sync_after_each_section()) {
@@ -4441,6 +4464,9 @@ static int ram_load_precopy(QEMUFile *f)
                 break;
             }
             break;
+        case RAM_SAVE_FLAG_MULTIFD_SYNC:
+            multifd_recv_sync_main();
+            break;
         case RAM_SAVE_FLAG_EOS:
             /* normal exit */
             if (migrate_multifd_sync_after_each_section()) {
-- 
2.39.1

Re: [PATCH v4 3/4] multifd: Only sync once each full round of memory

[Markus Armbruster]

Juan Quintela <quintela@redhat.com> writes:

> We need to add a new flag to mean to sync at that point.
> Notice that we still synchronize at the end of setup and at the end of
> complete stages.
>
> Signed-off-by: Juan Quintela <quintela@redhat.com>
>
> ---
>
> Add missing qemu_fflush(), now it passes all tests always.
> ---

[...]

> diff --git a/migration/migration.c b/migration/migration.c
> index b2844d374f..9eb061319d 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -2710,8 +2710,6 @@ bool migrate_multifd_sync_after_each_section(void)
>  {
>      MigrationState *s = migrate_get_current();
>  
> -    return true;
> -    // We will change this when code gets in.
>      return s->enabled_capabilities[MIGRATION_CAPABILITY_MULTIFD_SYNC_AFTER_EACH_SECTION];
>  }

Ah, nevermind.

[...]

[PATCH v4 4/4] ram: Document migration ram flags

[Juan Quintela]

0x80 is RAM_SAVE_FLAG_HOOK, it is in qemu-file now.
Bigger usable flag is 0x200, noticing that.
We can reuse RAM_SAVe_FLAG_FULL.

Signed-off-by: Juan Quintela <quintela@redhat.com>
---
 migration/ram.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index 32fab7b5ee..3648cfc357 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -67,22 +67,26 @@
 /***********************************************************/
 /* ram save/restore */
 
-/* RAM_SAVE_FLAG_ZERO used to be named RAM_SAVE_FLAG_COMPRESS, it
+/*
+ * RAM_SAVE_FLAG_ZERO used to be named RAM_SAVE_FLAG_COMPRESS, it
  * worked for pages that where filled with the same char.  We switched
  * it to only search for the zero value.  And to avoid confusion with
  * RAM_SSAVE_FLAG_COMPRESS_PAGE just rename it.
  */
-
-#define RAM_SAVE_FLAG_FULL     0x01 /* Obsolete, not used anymore */
+/*
+ * RAM_SAVE_FLAG_FULL was obsoleted in 2009, it can be reused now
+ */
+#define RAM_SAVE_FLAG_FULL     0x01
 #define RAM_SAVE_FLAG_ZERO     0x02
 #define RAM_SAVE_FLAG_MEM_SIZE 0x04
 #define RAM_SAVE_FLAG_PAGE     0x08
 #define RAM_SAVE_FLAG_EOS      0x10
 #define RAM_SAVE_FLAG_CONTINUE 0x20
 #define RAM_SAVE_FLAG_XBZRLE   0x40
-/* 0x80 is reserved in migration.h start with 0x100 next */
+/* 0x80 is reserved in qemu-file.h for RAM_SAVE_FLAG_HOOK */
 #define RAM_SAVE_FLAG_COMPRESS_PAGE    0x100
 #define RAM_SAVE_FLAG_MULTIFD_SYNC     0x200
+/* We can't use any flag that is bigger than 0x200 */
 
 int (*xbzrle_encode_buffer_func)(uint8_t *, uint8_t *, int,
      uint8_t *, int) = xbzrle_encode_buffer;
-- 
2.39.1

Re: [PATCH v4 4/4] ram: Document migration ram flags

[Eric Blake]

On Fri, Feb 10, 2023 at 12:37:30AM +0100, Juan Quintela wrote:
> 0x80 is RAM_SAVE_FLAG_HOOK, it is in qemu-file now.
> Bigger usable flag is 0x200, noticing that.
> We can reuse RAM_SAVe_FLAG_FULL.

SAVE

> 
> Signed-off-by: Juan Quintela <quintela@redhat.com>
> ---
>  migration/ram.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 32fab7b5ee..3648cfc357 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -67,22 +67,26 @@
>  /***********************************************************/
>  /* ram save/restore */
>  
> -/* RAM_SAVE_FLAG_ZERO used to be named RAM_SAVE_FLAG_COMPRESS, it
> +/*
> + * RAM_SAVE_FLAG_ZERO used to be named RAM_SAVE_FLAG_COMPRESS, it
>   * worked for pages that where filled with the same char.  We switched

As long as you're in the area,

s/where/were/

>   * it to only search for the zero value.  And to avoid confusion with
>   * RAM_SSAVE_FLAG_COMPRESS_PAGE just rename it.

s/SSAVE/SAVE/

>   */
> -
> -#define RAM_SAVE_FLAG_FULL     0x01 /* Obsolete, not used anymore */
> +/*
> + * RAM_SAVE_FLAG_FULL was obsoleted in 2009, it can be reused now
> + */
> +#define RAM_SAVE_FLAG_FULL     0x01
>  #define RAM_SAVE_FLAG_ZERO     0x02
>  #define RAM_SAVE_FLAG_MEM_SIZE 0x04
>  #define RAM_SAVE_FLAG_PAGE     0x08
>  #define RAM_SAVE_FLAG_EOS      0x10
>  #define RAM_SAVE_FLAG_CONTINUE 0x20
>  #define RAM_SAVE_FLAG_XBZRLE   0x40
> -/* 0x80 is reserved in migration.h start with 0x100 next */
> +/* 0x80 is reserved in qemu-file.h for RAM_SAVE_FLAG_HOOK */
>  #define RAM_SAVE_FLAG_COMPRESS_PAGE    0x100
>  #define RAM_SAVE_FLAG_MULTIFD_SYNC     0x200
> +/* We can't use any flag that is bigger than 0x200 */

Spelling fixes are trivial; feel free to add:

Reviewed-by: Eric Blake <eblake@redhat.com>


-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org