[PATCH 0/2] migration: failover: continue to wait card unplug on error

[PATCH 0/2] migration: failover: continue to wait card unplug on error

[Laurent Vivier]

We need to wait the end of the unplug phase to be able
to plug back the card.

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1976852

Laurent Vivier (2):
  migration: move wait-unplug loop to its own function
  migration: failover: continue to wait card unplug on error

 migration/migration.c | 65 ++++++++++++++++++++++++-------------------
 1 file changed, 37 insertions(+), 28 deletions(-)

-- 
2.31.1

[PATCH 1/2] migration: move wait-unplug loop to its own function

[Laurent Vivier]

The loop is used in migration_thread() and bg_migration_thread(),
so we can move it to its own function and call it from these both places.

Moreover, in migration_thread() we have a wrong state transition from
SETUP to ACTIVE while state could be WAIT_UNPLUG. This is correctly
managed in bg_migration_thread() so use this code instead.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 migration/migration.c | 54 +++++++++++++++++++++----------------------
 1 file changed, 26 insertions(+), 28 deletions(-)

diff --git a/migration/migration.c b/migration/migration.c
index 4228635d1880..3e92c405a2b6 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -3664,6 +3664,28 @@ bool migration_rate_limit(void)
     return urgent;
 }
 
+/*
+ * if failover devices are present, wait they are completely
+ * unplugged
+ */
+
+static void qemu_savevm_wait_unplug(MigrationState *s, int old_state,
+                                    int new_state)
+{
+    if (qemu_savevm_state_guest_unplug_pending()) {
+        migrate_set_state(&s->state, old_state, MIGRATION_STATUS_WAIT_UNPLUG);
+
+        while (s->state == MIGRATION_STATUS_WAIT_UNPLUG &&
+               qemu_savevm_state_guest_unplug_pending()) {
+            qemu_sem_timedwait(&s->wait_unplug_sem, 250);
+        }
+
+        migrate_set_state(&s->state, MIGRATION_STATUS_WAIT_UNPLUG, new_state);
+    } else {
+        migrate_set_state(&s->state, old_state, new_state);
+    }
+}
+
 /*
  * Master migration thread on the source VM.
  * It drives the migration and pumps the data down the outgoing channel.
@@ -3710,22 +3732,10 @@ static void *migration_thread(void *opaque)
 
     qemu_savevm_state_setup(s->to_dst_file);
 
-    if (qemu_savevm_state_guest_unplug_pending()) {
-        migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
-                          MIGRATION_STATUS_WAIT_UNPLUG);
-
-        while (s->state == MIGRATION_STATUS_WAIT_UNPLUG &&
-               qemu_savevm_state_guest_unplug_pending()) {
-            qemu_sem_timedwait(&s->wait_unplug_sem, 250);
-        }
-
-        migrate_set_state(&s->state, MIGRATION_STATUS_WAIT_UNPLUG,
-                MIGRATION_STATUS_ACTIVE);
-    }
+    qemu_savevm_wait_unplug(s, MIGRATION_STATUS_SETUP,
+                               MIGRATION_STATUS_ACTIVE);
 
     s->setup_time = qemu_clock_get_ms(QEMU_CLOCK_HOST) - setup_start;
-    migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
-                      MIGRATION_STATUS_ACTIVE);
 
     trace_migration_thread_setup_complete();
 
@@ -3833,21 +3843,9 @@ static void *bg_migration_thread(void *opaque)
     qemu_savevm_state_header(s->to_dst_file);
     qemu_savevm_state_setup(s->to_dst_file);
 
-    if (qemu_savevm_state_guest_unplug_pending()) {
-        migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
-                          MIGRATION_STATUS_WAIT_UNPLUG);
-
-        while (s->state == MIGRATION_STATUS_WAIT_UNPLUG &&
-               qemu_savevm_state_guest_unplug_pending()) {
-            qemu_sem_timedwait(&s->wait_unplug_sem, 250);
-        }
+    qemu_savevm_wait_unplug(s, MIGRATION_STATUS_SETUP,
+                               MIGRATION_STATUS_ACTIVE);
 
-        migrate_set_state(&s->state, MIGRATION_STATUS_WAIT_UNPLUG,
-                          MIGRATION_STATUS_ACTIVE);
-    } else {
-        migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
-                MIGRATION_STATUS_ACTIVE);
-    }
     s->setup_time = qemu_clock_get_ms(QEMU_CLOCK_HOST) - setup_start;
 
     trace_migration_thread_setup_complete();
-- 
2.31.1

Re: [PATCH 1/2] migration: move wait-unplug loop to its own function

[Dr. David Alan Gilbert]

* Laurent Vivier (lvivier@redhat.com) wrote:
> The loop is used in migration_thread() and bg_migration_thread(),
> so we can move it to its own function and call it from these both places.
> 
> Moreover, in migration_thread() we have a wrong state transition from
> SETUP to ACTIVE while state could be WAIT_UNPLUG. This is correctly
> managed in bg_migration_thread() so use this code instead.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
>  migration/migration.c | 54 +++++++++++++++++++++----------------------
>  1 file changed, 26 insertions(+), 28 deletions(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index 4228635d1880..3e92c405a2b6 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -3664,6 +3664,28 @@ bool migration_rate_limit(void)
>      return urgent;
>  }
>  
> +/*
> + * if failover devices are present, wait they are completely
> + * unplugged
> + */
> +
> +static void qemu_savevm_wait_unplug(MigrationState *s, int old_state,
> +                                    int new_state)
> +{
> +    if (qemu_savevm_state_guest_unplug_pending()) {
> +        migrate_set_state(&s->state, old_state, MIGRATION_STATUS_WAIT_UNPLUG);
> +
> +        while (s->state == MIGRATION_STATUS_WAIT_UNPLUG &&
> +               qemu_savevm_state_guest_unplug_pending()) {
> +            qemu_sem_timedwait(&s->wait_unplug_sem, 250);
> +        }
> +
> +        migrate_set_state(&s->state, MIGRATION_STATUS_WAIT_UNPLUG, new_state);
> +    } else {
> +        migrate_set_state(&s->state, old_state, new_state);
> +    }
> +}
> +
>  /*
>   * Master migration thread on the source VM.
>   * It drives the migration and pumps the data down the outgoing channel.
> @@ -3710,22 +3732,10 @@ static void *migration_thread(void *opaque)
>  
>      qemu_savevm_state_setup(s->to_dst_file);
>  
> -    if (qemu_savevm_state_guest_unplug_pending()) {
> -        migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
> -                          MIGRATION_STATUS_WAIT_UNPLUG);
> -
> -        while (s->state == MIGRATION_STATUS_WAIT_UNPLUG &&
> -               qemu_savevm_state_guest_unplug_pending()) {
> -            qemu_sem_timedwait(&s->wait_unplug_sem, 250);
> -        }
> -
> -        migrate_set_state(&s->state, MIGRATION_STATUS_WAIT_UNPLUG,
> -                MIGRATION_STATUS_ACTIVE);
> -    }
> +    qemu_savevm_wait_unplug(s, MIGRATION_STATUS_SETUP,
> +                               MIGRATION_STATUS_ACTIVE);
>  
>      s->setup_time = qemu_clock_get_ms(QEMU_CLOCK_HOST) - setup_start;
> -    migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
> -                      MIGRATION_STATUS_ACTIVE);
>  
>      trace_migration_thread_setup_complete();
>  
> @@ -3833,21 +3843,9 @@ static void *bg_migration_thread(void *opaque)
>      qemu_savevm_state_header(s->to_dst_file);
>      qemu_savevm_state_setup(s->to_dst_file);
>  
> -    if (qemu_savevm_state_guest_unplug_pending()) {
> -        migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
> -                          MIGRATION_STATUS_WAIT_UNPLUG);
> -
> -        while (s->state == MIGRATION_STATUS_WAIT_UNPLUG &&
> -               qemu_savevm_state_guest_unplug_pending()) {
> -            qemu_sem_timedwait(&s->wait_unplug_sem, 250);
> -        }
> +    qemu_savevm_wait_unplug(s, MIGRATION_STATUS_SETUP,
> +                               MIGRATION_STATUS_ACTIVE);
>  
> -        migrate_set_state(&s->state, MIGRATION_STATUS_WAIT_UNPLUG,
> -                          MIGRATION_STATUS_ACTIVE);
> -    } else {
> -        migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
> -                MIGRATION_STATUS_ACTIVE);
> -    }
>      s->setup_time = qemu_clock_get_ms(QEMU_CLOCK_HOST) - setup_start;
>  
>      trace_migration_thread_setup_complete();
> -- 
> 2.31.1
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

Re: [PATCH 1/2] migration: move wait-unplug loop to its own function

[Juan Quintela]

Laurent Vivier <lvivier@redhat.com> wrote:
> The loop is used in migration_thread() and bg_migration_thread(),
> so we can move it to its own function and call it from these both places.
>
> Moreover, in migration_thread() we have a wrong state transition from
> SETUP to ACTIVE while state could be WAIT_UNPLUG. This is correctly
> managed in bg_migration_thread() so use this code instead.
>
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

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

If you have to repost:


> +/*
> + * if failover devices are present, wait they are completely
> + * unplugged
> + */
> +
> +static void qemu_savevm_wait_unplug(MigrationState *s, int old_state,
> +                                    int new_state)

old_state and new state are always the same. SETUP -> ACTIVE.  I think
we can hardcode them.


> +{
> +    if (qemu_savevm_state_guest_unplug_pending()) {
> +        migrate_set_state(&s->state, old_state, MIGRATION_STATUS_WAIT_UNPLUG);
> +
> +        while (s->state == MIGRATION_STATUS_WAIT_UNPLUG &&
> +               qemu_savevm_state_guest_unplug_pending()) {
> +            qemu_sem_timedwait(&s->wait_unplug_sem, 250);

I still don't understand why are we using a semaphore when we just want
a timer :-(

Yes, this is independent of this patch.

Re: [PATCH 1/2] migration: move wait-unplug loop to its own function

[Dr. David Alan Gilbert]

* Juan Quintela (quintela@redhat.com) wrote:
> Laurent Vivier <lvivier@redhat.com> wrote:
> > The loop is used in migration_thread() and bg_migration_thread(),
> > so we can move it to its own function and call it from these both places.
> >
> > Moreover, in migration_thread() we have a wrong state transition from
> > SETUP to ACTIVE while state could be WAIT_UNPLUG. This is correctly
> > managed in bg_migration_thread() so use this code instead.
> >
> > Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> 
> Reviewed-by: Juan Quintela <quintela@redhat.com>
> 
> If you have to repost:
> 
> 
> > +/*
> > + * if failover devices are present, wait they are completely
> > + * unplugged
> > + */
> > +
> > +static void qemu_savevm_wait_unplug(MigrationState *s, int old_state,
> > +                                    int new_state)
> 
> old_state and new state are always the same. SETUP -> ACTIVE.  I think
> we can hardcode them.
> 
> 
> > +{
> > +    if (qemu_savevm_state_guest_unplug_pending()) {
> > +        migrate_set_state(&s->state, old_state, MIGRATION_STATUS_WAIT_UNPLUG);
> > +
> > +        while (s->state == MIGRATION_STATUS_WAIT_UNPLUG &&
> > +               qemu_savevm_state_guest_unplug_pending()) {
> > +            qemu_sem_timedwait(&s->wait_unplug_sem, 250);
> 
> I still don't understand why are we using a semaphore when we just want
> a timer :-(
> 
> Yes, this is independent of this patch.

So yes I was going to ask on the 2nd patch; no one anywhere seems to set
that semaphore?

Dave

-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

[PATCH 2/2] migration: failover: continue to wait card unplug on error

[Laurent Vivier]

If the user cancels the migration in the unplug-wait state,
QEMU will try to plug back the card and this fails because the card
is partially unplugged.
To avoid the problem, continue to wait the card unplug, but to
allow the migration to be canceled if the card never finishes to unplug
use a timeout.

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1976852
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 migration/migration.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/migration/migration.c b/migration/migration.c
index 3e92c405a2b6..3b06d43a7f42 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -3679,6 +3679,17 @@ static void qemu_savevm_wait_unplug(MigrationState *s, int old_state,
                qemu_savevm_state_guest_unplug_pending()) {
             qemu_sem_timedwait(&s->wait_unplug_sem, 250);
         }
+        if (s->state != MIGRATION_STATUS_WAIT_UNPLUG) {
+            int timeout = 120; /* 30 seconds */
+            /*
+             * migration has been canceled
+             * but as we have started an unplug we must wait the end
+             * to be able to plug back the card
+             */
+            while (timeout-- && qemu_savevm_state_guest_unplug_pending()) {
+                qemu_sem_timedwait(&s->wait_unplug_sem, 250);
+            }
+        }
 
         migrate_set_state(&s->state, MIGRATION_STATUS_WAIT_UNPLUG, new_state);
     } else {
-- 
2.31.1

Re: [PATCH 2/2] migration: failover: continue to wait card unplug on error

[Juan Quintela]

Laurent Vivier <lvivier@redhat.com> wrote:
> If the user cancels the migration in the unplug-wait state,
> QEMU will try to plug back the card and this fails because the card
> is partially unplugged.
> To avoid the problem, continue to wait the card unplug, but to
> allow the migration to be canceled if the card never finishes to unplug
> use a timeout.
>
> Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1976852
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  migration/migration.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/migration/migration.c b/migration/migration.c
> index 3e92c405a2b6..3b06d43a7f42 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -3679,6 +3679,17 @@ static void qemu_savevm_wait_unplug(MigrationState *s, int old_state,
>                 qemu_savevm_state_guest_unplug_pending()) {
>              qemu_sem_timedwait(&s->wait_unplug_sem, 250);
>          }
> +        if (s->state != MIGRATION_STATUS_WAIT_UNPLUG) {
> +            int timeout = 120; /* 30 seconds */
> +            /*
> +             * migration has been canceled
> +             * but as we have started an unplug we must wait the end
> +             * to be able to plug back the card
> +             */
> +            while (timeout-- && qemu_savevm_state_guest_unplug_pending()) {
> +                qemu_sem_timedwait(&s->wait_unplug_sem, 250);
> +            }
> +        }
>  
>          migrate_set_state(&s->state, MIGRATION_STATUS_WAIT_UNPLUG, new_state);
>      } else {

I agree with the idea.  But if we are getting out due to timeout == 0,
shouldn't we return some error, warning, whatever?

Later, Juan.

Re: [PATCH 2/2] migration: failover: continue to wait card unplug on error

[Laurent Vivier]

On 29/06/2021 19:50, Juan Quintela wrote:
> Laurent Vivier <lvivier@redhat.com> wrote:
>> If the user cancels the migration in the unplug-wait state,
>> QEMU will try to plug back the card and this fails because the card
>> is partially unplugged.
>> To avoid the problem, continue to wait the card unplug, but to
>> allow the migration to be canceled if the card never finishes to unplug
>> use a timeout.
>>
>> Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1976852
>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
>> ---
>>  migration/migration.c | 11 +++++++++++
>>  1 file changed, 11 insertions(+)
>>
>> diff --git a/migration/migration.c b/migration/migration.c
>> index 3e92c405a2b6..3b06d43a7f42 100644
>> --- a/migration/migration.c
>> +++ b/migration/migration.c
>> @@ -3679,6 +3679,17 @@ static void qemu_savevm_wait_unplug(MigrationState *s, int old_state,
>>                 qemu_savevm_state_guest_unplug_pending()) {
>>              qemu_sem_timedwait(&s->wait_unplug_sem, 250);
>>          }
>> +        if (s->state != MIGRATION_STATUS_WAIT_UNPLUG) {
>> +            int timeout = 120; /* 30 seconds */
>> +            /*
>> +             * migration has been canceled
>> +             * but as we have started an unplug we must wait the end
>> +             * to be able to plug back the card
>> +             */
>> +            while (timeout-- && qemu_savevm_state_guest_unplug_pending()) {
>> +                qemu_sem_timedwait(&s->wait_unplug_sem, 250);
>> +            }
>> +        }
>>  
>>          migrate_set_state(&s->state, MIGRATION_STATUS_WAIT_UNPLUG, new_state);
>>      } else {
> I agree with the idea.  But if we are getting out due to timeout == 0,
> shouldn't we return some error, warning, whatever?

In that case, we keep the current behaviour: guest kernel will report an error when it
will try to plug back the card that has not been unplugged. This is a corner case: if it
happens we have something really wrong with the machine. Perhaps we can remove the
timeout, but I don't like to block the user, or increase it to be sure.

Thanks,

Laurent

Re: [PATCH 2/2] migration: failover: continue to wait card unplug on error

[Juan Quintela]

Laurent Vivier <lvivier@redhat.com> wrote:
> On 29/06/2021 19:50, Juan Quintela wrote:
>> Laurent Vivier <lvivier@redhat.com> wrote:
>>> If the user cancels the migration in the unplug-wait state,
>>> QEMU will try to plug back the card and this fails because the card
>>> is partially unplugged.
>>> To avoid the problem, continue to wait the card unplug, but to
>>> allow the migration to be canceled if the card never finishes to unplug
>>> use a timeout.
>>>
>>> Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1976852
>>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
>>> ---
>>>  migration/migration.c | 11 +++++++++++
>>>  1 file changed, 11 insertions(+)
>>>
>>> diff --git a/migration/migration.c b/migration/migration.c
>>> index 3e92c405a2b6..3b06d43a7f42 100644
>>> --- a/migration/migration.c
>>> +++ b/migration/migration.c
>>> @@ -3679,6 +3679,17 @@ static void qemu_savevm_wait_unplug(MigrationState *s, int old_state,
>>>                 qemu_savevm_state_guest_unplug_pending()) {
>>>              qemu_sem_timedwait(&s->wait_unplug_sem, 250);
>>>          }
>>> +        if (s->state != MIGRATION_STATUS_WAIT_UNPLUG) {
>>> +            int timeout = 120; /* 30 seconds */
>>> +            /*
>>> +             * migration has been canceled
>>> +             * but as we have started an unplug we must wait the end
>>> +             * to be able to plug back the card
>>> +             */
>>> +            while (timeout-- && qemu_savevm_state_guest_unplug_pending()) {
>>> +                qemu_sem_timedwait(&s->wait_unplug_sem, 250);
>>> +            }
>>> +        }
>>>  
>>>          migrate_set_state(&s->state, MIGRATION_STATUS_WAIT_UNPLUG, new_state);
>>>      } else {
>> I agree with the idea.  But if we are getting out due to timeout == 0,
>> shouldn't we return some error, warning, whatever?
>
> In that case, we keep the current behaviour: guest kernel will report
> an error when it
> will try to plug back the card that has not been unplugged. This is a
> corner case: if it
> happens we have something really wrong with the machine. Perhaps we can remove the
> timeout, but I don't like to block the user, or increase it to be sure.

Oh, I whole agree that it is a corner case, and that it shouldn't
happen.

But if it happens, we don't log it anywhere.  That was my complaint.

Later, Juan.

Re: [PATCH 2/2] migration: failover: continue to wait card unplug on error

[Dr. David Alan Gilbert]

* Laurent Vivier (lvivier@redhat.com) wrote:
> If the user cancels the migration in the unplug-wait state,
> QEMU will try to plug back the card and this fails because the card
> is partially unplugged.
> To avoid the problem, continue to wait the card unplug, but to
> allow the migration to be canceled if the card never finishes to unplug
> use a timeout.
> 
> Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1976852
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

I'll take this for now, but as Juan says, we could really do with some
diags when this happens, so when someone comes and tells us that
the hotplug has failed we can see.  Please send something to add it.


Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
>  migration/migration.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index 3e92c405a2b6..3b06d43a7f42 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -3679,6 +3679,17 @@ static void qemu_savevm_wait_unplug(MigrationState *s, int old_state,
>                 qemu_savevm_state_guest_unplug_pending()) {
>              qemu_sem_timedwait(&s->wait_unplug_sem, 250);
>          }
> +        if (s->state != MIGRATION_STATUS_WAIT_UNPLUG) {
> +            int timeout = 120; /* 30 seconds */
> +            /*
> +             * migration has been canceled
> +             * but as we have started an unplug we must wait the end
> +             * to be able to plug back the card
> +             */
> +            while (timeout-- && qemu_savevm_state_guest_unplug_pending()) {
> +                qemu_sem_timedwait(&s->wait_unplug_sem, 250);
> +            }
> +        }
>  
>          migrate_set_state(&s->state, MIGRATION_STATUS_WAIT_UNPLUG, new_state);
>      } else {
> -- 
> 2.31.1
> 
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

Re: [PATCH 0/2] migration: failover: continue to wait card unplug on error

[Dr. David Alan Gilbert]

* Laurent Vivier (lvivier@redhat.com) wrote:
> We need to wait the end of the unplug phase to be able
> to plug back the card.
> 
> Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1976852

Queued

> 
> Laurent Vivier (2):
>   migration: move wait-unplug loop to its own function
>   migration: failover: continue to wait card unplug on error
> 
>  migration/migration.c | 65 ++++++++++++++++++++++++-------------------
>  1 file changed, 37 insertions(+), 28 deletions(-)
> 
> -- 
> 2.31.1
> 
> 
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK