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Re: [Qemu-devel] [PATCH] migration: re-active images when migration fail

From: Hailiang Zhang
Subject: Re: [Qemu-devel] [PATCH] migration: re-active images when migration fails to complete
Date: Wed, 11 Jan 2017 13:22:40 +0800
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1

ping .. ?

Any comments ? Or should I send a for formal patch ?

On 2016/12/22 10:56, Hailiang Zhang wrote:
On 2016/12/9 4:02, Dr. David Alan Gilbert wrote:
* Hailiang Zhang (address@hidden) wrote:

On 2016/12/6 23:24, Dr. David Alan Gilbert wrote:
* Kevin Wolf (address@hidden) wrote:
Am 19.11.2016 um 12:43 hat zhanghailiang geschrieben:
commit fe904ea8242cbae2d7e69c052c754b8f5f1ba1d6 fixed a case
which migration aborted QEMU because it didn't regain the control
of images while some errors happened.

Actually, we have another case in that error path to abort QEMU
because of the same reason:
              bdrv_inactivate_all() ----------------> inactivate images
                  socket_writev_buffer() --------> error because destination 
                qemu_fflush() -------------------> set error on migration stream
              qemu_mutex_unlock_iothread() ------> unlock
       qmp_migrate_cancel() ---------------------> user cancelled migration
           migrate_set_state() ------------------> set migrate CANCELLING

Important to note here: qmp_migrate_cancel() is executed by a concurrent
thread, it doesn't depend on any code paths in migration_completion().

       migration_completion() -----------------> go on to fail_invalidate
           if (s->state == MIGRATION_STATUS_ACTIVE) -> Jump this branch
       migration_thread() -----------------------> break migration loop
         vm_start() -----------------------------> restart guest with inactive
We failed to regain the control of images because we only regain it
while the migration state is "active", but here users cancelled the migration
when they found some errors happened (for example, libvirtd daemon is shutdown
in destination unexpectedly).

Signed-off-by: zhanghailiang <address@hidden>
    migration/migration.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/migration/migration.c b/migration/migration.c
index f498ab8..0c1ee6d 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1752,7 +1752,8 @@ fail_invalidate:
        /* If not doing postcopy, vm_start() will be called: let's regain
         * control on images.
-    if (s->state == MIGRATION_STATUS_ACTIVE) {

This if condition tries to check whether we ran the code path that
called bdrv_inactivate_all(), so that we only try to reactivate images
it if we really inactivated them first.

The problem with it is that it ignores a possible concurrent
modification of s->state.

+    if (s->state == MIGRATION_STATUS_ACTIVE ||
+        s->state == MIGRATION_STATUS_CANCELLING) {

This adds another state that we could end up with with a concurrent
modification, so that even in this case we undo the inactivation.

However, it is no longer limited to the cases where we inactivated the
image. It also applies to other code paths (like the postcopy one) where
we didn't inactivate images.

What saves the patch is that bdrv_invalidate_cache() is a no-op for
block devices that aren't inactivated, so calling it more often than
necessary is okay.

But then, if we're going to rely on this, it would be much better to
just remove the if altogether. I can't say whether there are any other
possible values of s->state that we should consider, and by removing the
if we would be guaranteed to catch all of them.

If we don't want to rely on it, just keep a local bool that remembers
whether we inactivated images and check that here.

            Error *local_err = NULL;


So in summary, this is a horrible patch because it checks the wrong
thing, and for I can't really say if it covers everything it needs to
cover, but arguably it happens to correctly fix the outcome of a
previously failing case.

Normally I would reject such a patch and require a clean solution, but
then we're on the day of -rc3, so if you can't send v2 right away, we
might not have the time for it.

Tough call...

Hmm, this case is messy; I created this function having split it out
of the main loop a couple of years back but it did get more messy
with more s->state checks; as far as I can tell it's always
done the transition to COMPLETED at the end well after the locked
section, so there's always been that chance that cancellation sneaks
in just before or just after the locked section.

Some of the bad cases that can happen:
      a) A cancel sneaks in after the ACTIVE check but before or after
        the locked section;  should we reactivate the disks? Well that
        depends on whether the destination actually got the full migration
        stream - we don't know!
           If the destination actually starts running we must not reactivate
           the disk on the source even if the CPU is stopped.

Yes, we didn't have a mechanism to know exactly whether or not the VM in
destination is well received.

      b) If the bdrv_inactive_all fails for one device, but the others
         are fine, we go down the fail: label and don't reactivate, so
         the source dies even though it might have been mostly OK.

We can move the _lock to before the check of s->state at the top,
which would stop the cancel sneaking in early.
In the case where postcopy was never enabled (!migrate_postcopy_ram())
we can move the COMPLETED transition into the lock as well; so I think
then we kind of become safe.
In the case where postcopy was enabled I think we can do the COMPLETED
transition before waiting for the return path to close - I think but
I need to think more about that.
And there seem to be some dodgy cases where we call the invalidate
there after a late postcopy failure; that's bad, we shouldn't reactivate
the source disks after going into postcopy.

So, in summary; this function is a mess - it needs a much bigger
fix than this patch.

So what's the conclusion ?
Will you send a patch to fix it ? Or let's fix it step by step ?
I think Kevin's suggestion which just remove the *if* check is OK.

I don't see any of the simple solutions are easy;  so I plan
to look at it properly, but am not sure when;  if you have time
to do it then feel free.

Hmm, we still have gaps between bdrv_inactivate_all() and migration thread
totally exit, which migration cancelling can slip in.
We do caught that case while we finished migration_completion() but
before the begin of cleanup work (It has global lock to be protected).
The related codes is:

                  migration_completion(s, current_active_state,
                                       &old_vm_running, &start_time);
------------------------------------------------------> gap begin
          if (qemu_file_get_error(s->to_dst_file)) {
              migrate_set_state(&s->state, current_active_state,
          current_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
          if (current_time >= initial_time + BUFFER_DELAY) {
              uint64_t transferred_bytes = qemu_ftell(s->to_dst_file) -
              uint64_t time_spent = current_time - initial_time;
              double bandwidth = (double)transferred_bytes / time_spent;
              max_size = bandwidth * s->parameters.downtime_limit;

              s->mbps = (((double) transferred_bytes * 8.0) /
                      ((double) time_spent / 1000.0)) / 1000.0 / 1000.0;

              trace_migrate_transferred(transferred_bytes, time_spent,
                                        bandwidth, max_size);
              /* if we haven't sent anything, we don't want to recalculate
                 10000 is a small enough number for our purposes */
              if (s->dirty_bytes_rate && transferred_bytes > 10000) {
                  s->expected_downtime = s->dirty_bytes_rate / bandwidth;

              initial_time = current_time;
              initial_bytes = qemu_ftell(s->to_dst_file);
          if (qemu_file_rate_limit(s->to_dst_file)) {
              /* usleep expects microseconds */
              g_usleep((initial_time + BUFFER_DELAY - current_time)*1000);

      /* If we enabled cpu throttling for auto-converge, turn it off. */
      end_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
--------------------------------------------------------------> gap end

So maybe we should call bdrv_invalidate_cache_all() in qmp_migrate_cancel()
directly ? which i think will cover all the cases. (Including another potential
case, which QEMU has finished the migration process, but libvirtd decides to
cancel the migration before shutdown it.)

The things here I'm not sure is, is it OK to call bdrv_invalidate_cache_all()
without bdrv_inactivate_all() has been called ? If not, we need to record
if we have inactive all blocks before call bdrv_invalidate_cache_all()。

Any ideas ?






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


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


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