Re: [Qemu-devel] [RFC v2 09/33] migration: implement "postcopy-pause" src logic

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

* Peter Xu (peterx@redhat.com) wrote:
> 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.
> 
> If a recovery is detected, migration_thread() will reset its local
> variables to prepare for that.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  migration/migration.c  | 98 +++++++++++++++++++++++++++++++++++++++++++++++---
>  migration/migration.h  |  3 ++
>  migration/trace-events |  1 +
>  3 files changed, 98 insertions(+), 4 deletions(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index f6130db..8d26ea8 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -993,6 +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)
> @@ -1136,6 +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);
>  
> @@ -1938,6 +1941,80 @@ bool migrate_colo_enabled(void)
>      return s->enabled_capabilities[MIGRATION_CAPABILITY_X_COLO];
>  }
>  
> +typedef enum MigThrError {
> +    /* No error detected */
> +    MIG_THR_ERR_NONE = 0,
> +    /* Detected error, but resumed successfully */
> +    MIG_THR_ERR_RECOVERED = 1,
> +    /* Detected fatal error, need to exit */
> +    MIG_THR_ERR_FATAL = 2,

I don't think it's necessary to assign the values there, but it's OK.

> +} MigThrError;
> +
> +/*
> + * We don't return until we are in a safe state to continue current
> + * postcopy migration.  Returns MIG_THR_ERR_RECOVERED if recovered, or
> + * MIG_THR_ERR_FATAL if unrecovery failure happened.
> + */
> +static MigThrError postcopy_pause(MigrationState *s)
> +{
> +    assert(s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE);
> +    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;
> +
> +    error_report("Detected IO failure for postcopy. "
> +                 "Migration paused.");
> +
> +    /*
> +     * 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);
> +    }
> +
> +    trace_postcopy_pause_continued();
> +
> +    return MIG_THR_ERR_RECOVERED;
> +}
> +
> +static MigThrError 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 MIG_THR_ERR_NONE;
> +    }
> +
> +    if (s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE && ret == -EIO) {

We do need to make sure that whenever we hit a failure in migration
due to a device that we pass that up rather than calling
qemu_file_set_error - e.g. an EIO in a block device or network.

However,

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

> +        /*
> +         * 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 MIG_THR_ERR_FATAL;
> +    }
> +}
> +
>  /*
>   * Master migration thread on the source VM.
>   * It drives the migration and pumps the data down the outgoing channel.
> @@ -1962,6 +2039,7 @@ static void *migration_thread(void *opaque)
>      /* The active state we expect to be in; ACTIVE or POSTCOPY_ACTIVE */
>      enum MigrationStatus current_active_state = MIGRATION_STATUS_ACTIVE;
>      bool enable_colo = migrate_colo_enabled();
> +    MigThrError thr_error;
>  
>      rcu_register_thread();
>  
> @@ -2034,12 +2112,24 @@ 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.
> +         */
> +        thr_error = migration_detect_error(s);
> +        if (thr_error == MIG_THR_ERR_FATAL) {
> +            /* Stop migration */
>              break;
> +        } else if (thr_error == MIG_THR_ERR_RECOVERED) {
> +            /*
> +             * Just recovered from a e.g. network failure, reset all
> +             * the local variables. This is important to avoid
> +             * breaking transferred_bytes and bandwidth calculation
> +             */
> +            initial_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
> +            initial_bytes = 0;
>          }
> +
>          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 70e3094..0c957c9 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -149,6 +149,9 @@ 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 d2910a6..907564b 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) ""
> -- 
> 2.7.4
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK