Re: [Qemu-devel] [RFC 11/29] migration: new postcopy-pause state

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Xu (peterx@redhat.com) wrote:
> Introducing a new state "postcopy-paused", which can be used to pause a
> postcopy migration. It is targeted to support network failures during
> postcopy migration. Now when network down for postcopy, the source side
> will not fail the migration. Instead we convert the status into this new
> paused state, and we will try to wait for a rescue in the future.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>

I think this should probably be split into:
   a) A patch that adds a new state and the entries in query_migrate etc
   b) A patch that wires up the semaphore and the use of the state.

> ---
>  migration/migration.c  | 78 +++++++++++++++++++++++++++++++++++++++++++++++---
>  migration/migration.h  |  3 ++
>  migration/trace-events |  1 +
>  qapi-schema.json       |  5 +++-
>  4 files changed, 82 insertions(+), 5 deletions(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index efee87e..0bc70c8 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -470,6 +470,7 @@ static bool migration_is_setup_or_active(int state)
>      switch (state) {
>      case MIGRATION_STATUS_ACTIVE:
>      case MIGRATION_STATUS_POSTCOPY_ACTIVE:
> +    case MIGRATION_STATUS_POSTCOPY_PAUSED:
>      case MIGRATION_STATUS_SETUP:
>          return true;
>  
> @@ -545,6 +546,7 @@ MigrationInfo *qmp_query_migrate(Error **errp)
>      case MIGRATION_STATUS_ACTIVE:
>      case MIGRATION_STATUS_CANCELLING:
>      case MIGRATION_STATUS_POSTCOPY_ACTIVE:
> +    case MIGRATION_STATUS_POSTCOPY_PAUSED:
>           /* TODO add some postcopy stats */
>          info->has_status = true;
>          info->has_total_time = true;
> @@ -991,6 +993,8 @@ static void migrate_fd_cleanup(void *opaque)
>  
>      notifier_list_notify(&migration_state_notifiers, s);
>      block_cleanup_parameters(s);
> +
> +    qemu_sem_destroy(&s->postcopy_pause_sem);
>  }
>  
>  void migrate_fd_error(MigrationState *s, const Error *error)
> @@ -1134,6 +1138,7 @@ MigrationState *migrate_init(void)
>      s->migration_thread_running = false;
>      error_free(s->error);
>      s->error = NULL;
> +    qemu_sem_init(&s->postcopy_pause_sem, 0);
>  
>      migrate_set_state(&s->state, MIGRATION_STATUS_NONE, MIGRATION_STATUS_SETUP);
>  
> @@ -1942,6 +1947,69 @@ static bool postcopy_should_start(MigrationState *s)
>  }
>  
>  /*
> + * We don't return until we are in a safe state to continue current
> + * postcopy migration.  Returns true to continue the migration, or
> + * false to terminate current migration.
> + */
> +static bool postcopy_pause(MigrationState *s)
> +{
> +    assert(s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE);

I never like asserts on the sending side.

> +    migrate_set_state(&s->state, MIGRATION_STATUS_POSTCOPY_ACTIVE,
> +                      MIGRATION_STATUS_POSTCOPY_PAUSED);
> +
> +    /* Current channel is possibly broken. Release it. */
> +    assert(s->to_dst_file);
> +    qemu_file_shutdown(s->to_dst_file);
> +    qemu_fclose(s->to_dst_file);
> +    s->to_dst_file = NULL;

That does scare me a little; I think it's OK, I'm not sure what happens
to the ->from_dst_file fd and the return-path processing.

> +    /*
> +     * We wait until things fixed up. Then someone will setup the
> +     * status back for us.
> +     */
> +    while (s->state == MIGRATION_STATUS_POSTCOPY_PAUSED) {
> +        qemu_sem_wait(&s->postcopy_pause_sem);
> +    }

Something should get written to stderr prior to this, so when we
find a migration apparently stuck we can tell why.

Dave

> +
> +    trace_postcopy_pause_continued();
> +
> +    return true;
> +}
> +
> +/* Return true if we want to stop the migration, otherwise false. */
> +static bool migration_detect_error(MigrationState *s)
> +{
> +    int ret;
> +
> +    /* Try to detect any file errors */
> +    ret = qemu_file_get_error(s->to_dst_file);
> +
> +    if (!ret) {
> +        /* Everything is fine */
> +        return false;
> +    }
> +
> +    if (s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE && ret == -EIO) {
> +        /*
> +         * For postcopy, we allow the network to be down for a
> +         * while. After that, it can be continued by a
> +         * recovery phase.
> +         */
> +        return !postcopy_pause(s);
> +    } else {
> +        /*
> +         * For precopy (or postcopy with error outside IO), we fail
> +         * with no time.
> +         */
> +        migrate_set_state(&s->state, s->state, MIGRATION_STATUS_FAILED);
> +        trace_migration_thread_file_err();
> +
> +        /* Time to stop the migration, now. */
> +        return true;
> +    }
> +}
> +
> +/*
>   * Master migration thread on the source VM.
>   * It drives the migration and pumps the data down the outgoing channel.
>   */
> @@ -2037,12 +2105,14 @@ static void *migration_thread(void *opaque)
>              }
>          }
>  
> -        if (qemu_file_get_error(s->to_dst_file)) {
> -            migrate_set_state(&s->state, current_active_state,
> -                              MIGRATION_STATUS_FAILED);
> -            trace_migration_thread_file_err();
> +        /*
> +         * Try to detect any kind of failures, and see whether we
> +         * should stop the migration now.
> +         */
> +        if (migration_detect_error(s)) {
>              break;
>          }
> +
>          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) -
> diff --git a/migration/migration.h b/migration/migration.h
> index e902bae..24cdaf6 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -151,6 +151,9 @@ struct MigrationState
>      bool send_configuration;
>      /* Whether we send section footer during migration */
>      bool send_section_footer;
> +
> +    /* Needed by postcopy-pause state */
> +    QemuSemaphore postcopy_pause_sem;
>  };
>  
>  void migrate_set_state(int *state, int old_state, int new_state);
> diff --git a/migration/trace-events b/migration/trace-events
> index 08d00fa..2211acc 100644
> --- a/migration/trace-events
> +++ b/migration/trace-events
> @@ -98,6 +98,7 @@ migration_thread_setup_complete(void) ""
>  open_return_path_on_source(void) ""
>  open_return_path_on_source_continue(void) ""
>  postcopy_start(void) ""
> +postcopy_pause_continued(void) ""
>  postcopy_start_set_run(void) ""
>  source_return_path_thread_bad_end(void) ""
>  source_return_path_thread_end(void) ""
> diff --git a/qapi-schema.json b/qapi-schema.json
> index 9c6c3e1..2a36b80 100644
> --- a/qapi-schema.json
> +++ b/qapi-schema.json
> @@ -667,6 +667,8 @@
>  #
>  # @postcopy-active: like active, but now in postcopy mode. (since 2.5)
>  #
> +# @postcopy-paused: during postcopy but paused. (since 2.10)
> +#
>  # @completed: migration is finished.
>  #
>  # @failed: some error occurred during migration process.
> @@ -679,7 +681,8 @@
>  ##
>  { 'enum': 'MigrationStatus',
>    'data': [ 'none', 'setup', 'cancelling', 'cancelled',
> -            'active', 'postcopy-active', 'completed', 'failed', 'colo' ] }
> +            'active', 'postcopy-active', 'postcopy-paused',
> +            'completed', 'failed', 'colo' ] }
>  
>  ##
>  # @MigrationInfo:
> -- 
> 2.7.4
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK