Re: [PATCH v11 4/5] migration: implementation of background snapshot thread

[Dr. David Alan Gilbert]

* Andrey Gruzdev (andrey.gruzdev@virtuozzo.com) wrote:
> Introducing implementation of 'background' snapshot thread
> which in overall follows the logic of precopy migration
> while internally utilizes completely different mechanism
> to 'freeze' vmstate at the start of snapshot creation.
> 
> This mechanism is based on userfault_fd with wr-protection
> support and is Linux-specific.

I noticed there weren't any bdrv_ calls in here; I guess with a snapshot
you still have the source running so still have it accessing the disk;
do you do anything to try and wire the ram snapshotting up to disk
snapshotting?

> Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
> Acked-by: Peter Xu <peterx@redhat.com>
> ---
>  migration/migration.c | 255 +++++++++++++++++++++++++++++++++++++++++-
>  migration/migration.h |   3 +
>  migration/ram.c       |   2 +
>  migration/savevm.c    |   1 -
>  migration/savevm.h    |   2 +
>  5 files changed, 260 insertions(+), 3 deletions(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index 2c2cb9ef01..0901a15ac5 100644

<snip>

> -    qemu_thread_create(&s->thread, "live_migration", migration_thread, s,
> -                       QEMU_THREAD_JOINABLE);
> +
> +    if (migrate_background_snapshot()) {
> +        qemu_thread_create(&s->thread, "background_snapshot",

Unfortunately that wont work - there's a 14 character limit on
the thread name length; I guess we just shorten that to bg_snapshot

Other than that,



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

> +                bg_migration_thread, s, QEMU_THREAD_JOINABLE);
> +    } else {
> +        qemu_thread_create(&s->thread, "live_migration",
> +                migration_thread, s, QEMU_THREAD_JOINABLE);
> +    }
>      s->migration_thread_running = true;
>  }
>  
> diff --git a/migration/migration.h b/migration/migration.h
> index f40338cfbf..0723955cd7 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -20,6 +20,7 @@
>  #include "qemu/thread.h"
>  #include "qemu/coroutine_int.h"
>  #include "io/channel.h"
> +#include "io/channel-buffer.h"
>  #include "net/announce.h"
>  #include "qom/object.h"
>  
> @@ -147,8 +148,10 @@ struct MigrationState {
>  
>      /*< public >*/
>      QemuThread thread;
> +    QEMUBH *vm_start_bh;
>      QEMUBH *cleanup_bh;
>      QEMUFile *to_dst_file;
> +    QIOChannelBuffer *bioc;
>      /*
>       * Protects to_dst_file pointer.  We need to make sure we won't
>       * yield or hang during the critical section, since this lock will
> diff --git a/migration/ram.c b/migration/ram.c
> index 5707382db1..05fe0c8592 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1471,6 +1471,7 @@ static RAMBlock *poll_fault_page(RAMState *rs, 
> ram_addr_t *offset)
>      page_address = (void *) uffd_msg.arg.pagefault.address;
>      bs = qemu_ram_block_from_host(page_address, false, offset);
>      assert(bs && (bs->flags & RAM_UF_WRITEPROTECT) != 0);
> +
>      return bs;
>  }
>  #endif /* CONFIG_LINUX */
> @@ -1836,6 +1837,7 @@ static void ram_save_host_page_post(RAMState *rs, 
> PageSearchStatus *pss,
>          /* Un-protect memory range. */
>          res = uffd_change_protection(rs->uffdio_fd, page_address, run_length,
>                  false, false);
> +
>          /* We don't want to override existing error from 
> ram_save_host_page(). */
>          if (res < 0 && *res_override >= 0) {
>              *res_override = res;
> diff --git a/migration/savevm.c b/migration/savevm.c
> index 27e842812e..dd4ad0aaaf 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -1354,7 +1354,6 @@ int 
> qemu_savevm_state_complete_precopy_iterable(QEMUFile *f, bool in_postcopy)
>      return 0;
>  }
>  
> -static
>  int qemu_savevm_state_complete_precopy_non_iterable(QEMUFile *f,
>                                                      bool in_postcopy,
>                                                      bool inactivate_disks)
> diff --git a/migration/savevm.h b/migration/savevm.h
> index ba64a7e271..aaee2528ed 100644
> --- a/migration/savevm.h
> +++ b/migration/savevm.h
> @@ -64,5 +64,7 @@ int qemu_loadvm_state(QEMUFile *f);
>  void qemu_loadvm_state_cleanup(void);
>  int qemu_loadvm_state_main(QEMUFile *f, MigrationIncomingState *mis);
>  int qemu_load_device_state(QEMUFile *f);
> +int qemu_savevm_state_complete_precopy_non_iterable(QEMUFile *f,
> +        bool in_postcopy, bool inactivate_disks);
>  
>  #endif
> -- 
> 2.25.1
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK