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Re: [Qemu-block] [PATCH v10 05/12] migration: introduce postcopy-only pe

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [PATCH v10 05/12] migration: introduce postcopy-only pending
Date: Tue, 13 Mar 2018 16:11:44 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

13.03.2018 13:30, Dr. David Alan Gilbert wrote:
* Vladimir Sementsov-Ogievskiy (address@hidden) wrote:
12.03.2018 18:30, Dr. David Alan Gilbert wrote:
* Vladimir Sementsov-Ogievskiy (address@hidden) wrote:
There would be savevm states (dirty-bitmap) which can migrate only in
postcopy stage. The corresponding pending is introduced here.

Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>

  static MigIterateState migration_iteration_run(MigrationState *s)
-    uint64_t pending_size, pend_post, pend_nonpost;
+    uint64_t pending_size, pend_pre, pend_compat, pend_post;
      bool in_postcopy = s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE;
-    qemu_savevm_state_pending(s->to_dst_file, s->threshold_size,
-                              &pend_nonpost, &pend_post);
-    pending_size = pend_nonpost + pend_post;
+    qemu_savevm_state_pending(s->to_dst_file, s->threshold_size, &pend_pre,
+                              &pend_compat, &pend_post);
+    pending_size = pend_pre + pend_compat + pend_post;
      trace_migrate_pending(pending_size, s->threshold_size,
-                          pend_post, pend_nonpost);
+                          pend_pre, pend_compat, pend_post);
      if (pending_size && pending_size >= s->threshold_size) {
          /* Still a significant amount to transfer */
          if (migrate_postcopy() && !in_postcopy &&
-            pend_nonpost <= s->threshold_size &&
-            atomic_read(&s->start_postcopy)) {
+            pend_pre <= s->threshold_size &&
+            (atomic_read(&s->start_postcopy) ||
+             (pend_pre + pend_compat <= s->threshold_size)))
This change does something different from the description;
it causes a postcopy_start even if the user never ran the postcopy-start
command; so sorry, we can't do that; because postcopy for RAM is
something that users can enable but only switch into when they've given
up on it completing normally.

However, I guess that leaves you with a problem; which is what happens
to the system when you've run out of pend_pre+pend_compat but can't
complete because pend_post is non-0; so I don't know the answer to that.

Hmm. Here, we go to postcopy only if "pend_pre + pend_compat <=
s->threshold_size". Pre-patch, in this case we will go to
migration_completion(). So, precopy stage is finishing anyway.

So, we want
in this case to finish ram migration like it was finished by
migration_completion(), and then, run postcopy, which will handle only dirty
bitmaps, yes?
It's a bit tricky; the first important thing is that we can't change the
semantics of the migration without the 'dirty bitmaps'.

So then there's the question of how  a migration with both
postcopy-ram+dirty bitmaps should work;  again I don't think we should
enter the postcopy-ram phase until start-postcopy is issued.

Then there's the 3rd case; dirty-bitmaps but no postcopy-ram; in that
case I worry less about the semantics of how you want to do it.

I have an idea:

in postcopy_start(), in ram_has_postcopy() (and may be some other places?), check atomic_read(&s->start_postcopy) instead of migrate_postcopy_ram()


1. behavior without dirty-bitmaps is not changed, as currently we cant go into postcopy_start and ram_has_postcopy without s->start_postcopy
2. dirty-bitmaps+ram: if user don't set s->start_postcopy, postcopy_start() will operate as if migration capability was not enabled, so ram should complete its migration
3. only dirty-bitmaps: again, postcopy_start() will operate as if migration capability was not enabled, so ram should complete its migration

Hmm2. Looked through migration_completion(), I don't understand, how it
finishes ram migration without postcopy. It calls
qemu_savevm_state_complete_precopy(), which skips states with
has_postcopy=true, which is ram...
Because savevm_state_complete_precopy only skips has_postcopy=true in
the in_postcopy case:

            (in_postcopy && se->ops->has_postcopy &&
             se->ops->has_postcopy(se->opaque)) ||

so when we call it in migration_completion(), if we've not entered
postcopy yet, then that test doesn't trigger.

(Apologies for not spotting this earlier; but I thought this patch was
a nice easy one just adding the postcopy_only_pending - I didn't realise it changed
existing semantics until I spotted that)

oh, yes, I was inattentive :(


Best regards,

Dr. David Alan Gilbert / address@hidden / Manchester, UK

Best regards,

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