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Subject: Re: [PATCH v9 5/8] migration/ram.c: add a notifier chain for precopy


On 11/27/2018 03:38 PM, Peter Xu wrote:
On Thu, Nov 15, 2018 at 06:08:01PM +0800, Wei Wang wrote:
+typedef enum PrecopyNotifyReason {
+    PRECOPY_NOTIFY_ERR = 0,
+    PRECOPY_NOTIFY_START_ITERATION = 1,
+    PRECOPY_NOTIFY_BEFORE_SYNC_BITMAP = 2,
+    PRECOPY_NOTIFY_AFTER_SYNC_BITMAP = 3,
+    PRECOPY_NOTIFY_MAX = 4,
It would be nice to add some comments for each of the notify reason.
E.g., from the name PRECOPY_NOTIFY_START_ITERATION seems more like a
hook at the start of each iteration but according to [1] it should be
at the start of migration rather than each iteration (or when
migration restarts, though I'm not sure whether we really have this
yet).

OK. I think It would be better if the name itself could be straightforward.
Probably we could change PRECOPY_NOTIFY_START_ITERATION to
PRECOPY_NOTIFY_START_MIGRATION.


+} PrecopyNotifyReason;
+
+void precopy_infrastructure_init(void);
+void precopy_add_notifier(Notifier *n);
+void precopy_remove_notifier(Notifier *n);
+
  void ram_mig_init(void);
  void qemu_guest_free_page_hint(void *addr, size_t len);
diff --git a/migration/ram.c b/migration/ram.c
index 229b791..65b1223 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -292,6 +292,8 @@ struct RAMState {
      bool ram_bulk_stage;
      /* How many times we have dirty too many pages */
      int dirty_rate_high_cnt;
+    /* ram save states used for notifiers */
+    int ram_save_state;
This can be removed?

Yes, thanks.


      /* these variables are used for bitmap sync */
      /* last time we did a full bitmap_sync */
      int64_t time_last_bitmap_sync;
@@ -328,6 +330,28 @@ typedef struct RAMState RAMState;
static RAMState *ram_state; +static NotifierList precopy_notifier_list;
+
+void precopy_infrastructure_init(void)
+{
+    notifier_list_init(&precopy_notifier_list);
+}
+
+void precopy_add_notifier(Notifier *n)
+{
+    notifier_list_add(&precopy_notifier_list, n);
+}
+
+void precopy_remove_notifier(Notifier *n)
+{
+    notifier_remove(n);
+}
+
+static void precopy_notify(PrecopyNotifyReason reason)
+{
+    notifier_list_notify(&precopy_notifier_list, &reason);
+}
+
  uint64_t ram_bytes_remaining(void)
  {
      return ram_state ? (ram_state->migration_dirty_pages * TARGET_PAGE_SIZE) :
@@ -1642,6 +1666,8 @@ static void migration_bitmap_sync(RAMState *rs)
      int64_t end_time;
      uint64_t bytes_xfer_now;
+ precopy_notify(PRECOPY_NOTIFY_BEFORE_SYNC_BITMAP);
+
      ram_counters.dirty_sync_count++;
if (!rs->time_last_bitmap_sync) {
@@ -1699,6 +1725,8 @@ static void migration_bitmap_sync(RAMState *rs)
      if (migrate_use_events()) {
          qapi_event_send_migration_pass(ram_counters.dirty_sync_count);
      }
+
+    precopy_notify(PRECOPY_NOTIFY_AFTER_SYNC_BITMAP);
  }
/**
@@ -2555,6 +2583,8 @@ static void ram_state_reset(RAMState *rs)
      rs->last_page = 0;
      rs->last_version = ram_list.version;
      rs->ram_bulk_stage = true;
+
+    precopy_notify(PRECOPY_NOTIFY_START_ITERATION);
[1]

  }
#define MAX_WAIT 50 /* ms, half buffered_file limit */
@@ -3324,6 +3354,7 @@ out:
ret = qemu_file_get_error(f);
      if (ret < 0) {
+        precopy_notify(PRECOPY_NOTIFY_ERR);
Could you show me which function is this line in?

Line 3324 here is ram_save_complete(), but I cannot find this exact
place.

Sure, it's in ram_save_iterate():
...
out:
    qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
    qemu_fflush(f);
    ram_counters.transferred += 8;

    ret = qemu_file_get_error(f);
    if (ret < 0) {
+        precopy_notify(PRECOPY_NOTIFY_ERR);
        return ret;
    }

    return done;
}



Another thing to mention about the "reasons" (though I see it more
like "events"): have you thought about adding a PRECOPY_NOTIFY_END?
It might help in some cases:

   - then you don't need to trickily export the migrate_postcopy()
     since you'll notify that before postcopy starts

I'm thinking probably we don't need to export migrate_postcopy even now.
It's more like a sanity check, and not needed because now we have the
notifier registered to the precopy specific callchain, which has ensured that
it is invoked via precopy.

   - you'll have a solid point that you'll 100% guarantee that we'll
     stop the free page hinting and don't need to worry about whether
     there is chance the hinting will be running without an end [2].

Thanks, I think it makes sense. Plan to add PRECOPY_NOTIFY_END in ram_save_complete.



Regarding [2] above: now the series only stops the hinting when
PRECOPY_NOTIFY_BEFORE_SYNC_BITMAP is notified.  Could there be a case
that it's missing?  E.g., what if we cancel/fail a migration during
precopy?  Have you tried it?


I think it has been handled by the above PRECOPY_NOTIFY_ERR

Best,
Wei



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