Re: [Qemu-devel] [PATCH 17/17] migration: flush receive queue

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

* Juan Quintela (quintela@redhat.com) wrote:
> Each time that we sync the bitmap, it is a possiblity that we receive
> a page that is being processed by a different thread.  We fix this
> problem just making sure that we wait for all receiving threads to
> finish its work before we procedeed with the next stage.
> 
> We are low on page flags, so we use a combination that is not valid to
> emit that message:  MULTIFD_PAGE and COMPRESSED.
> 
> I tried to make a migration command for it, but it don't work because
> we sync the bitmap sometimes when we have already sent the beggining
> of the section, so I just added a new page flag.
> 
> Signed-off-by: Juan Quintela <quintela@redhat.com>

Is there something that makes sure that the very last page is marked
with a flush to ensure that the read completes before we start
trying to do anything after ram_load ?
As I read this the flag gets added to the first page of the next round.

Dave

> ---
>  include/migration/migration.h |  1 +
>  migration/ram.c               | 46 +++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 47 insertions(+)
> 
> diff --git a/include/migration/migration.h b/include/migration/migration.h
> index b3e4f31..1bd6bc0 100644
> --- a/include/migration/migration.h
> +++ b/include/migration/migration.h
> @@ -259,6 +259,7 @@ void migrate_multifd_send_threads_create(void);
>  void migrate_multifd_send_threads_join(void);
>  void migrate_multifd_recv_threads_create(void);
>  void migrate_multifd_recv_threads_join(void);
> +void qemu_savevm_send_multifd_flush(QEMUFile *f);
> 
>  void migrate_compress_threads_create(void);
>  void migrate_compress_threads_join(void);
> diff --git a/migration/ram.c b/migration/ram.c
> index 28d099f..3baead8 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -63,6 +63,13 @@ static uint64_t bitmap_sync_count;
>  #define RAM_SAVE_FLAG_COMPRESS_PAGE    0x100
>  #define RAM_SAVE_FLAG_MULTIFD_PAGE     0x200
> 
> +/* We are getting low on pages flags, so we start using combinations 
> +   When we need to flush a page, we sent it as
> +   RAM_SAVE_FLAG_MULTIFD_PAGE | RAM_SAVE_FLAG_COMPRESS_PAGE
> +   We don't allow that combination
> +*/
> +
> +
>  static uint8_t *ZERO_TARGET_PAGE;
> 
>  static inline bool is_zero_range(uint8_t *p, uint64_t size)
> @@ -391,6 +398,9 @@ void migrate_compress_threads_create(void)
> 
>  /* Multiple fd's */
> 
> +/* Indicates if we have synced the bitmap and we need to assure that
> +   target has processeed all previous pages */
> +bool multifd_needs_flush = false;
> 
>  typedef struct {
>      int num;
> @@ -752,6 +762,25 @@ static void multifd_recv_page(uint8_t *address, uint16_t fd_num)
>      qemu_mutex_unlock(&params->mutex);
>  }
> 
> +
> +static int multifd_flush(void)
> +{
> +    int i, thread_count;
> +
> +    if (!migrate_use_multifd()) {
> +        return 0;
> +    }
> +    thread_count = migrate_multifd_threads();
> +    qemu_mutex_lock(&multifd_recv_mutex);
> +    for (i = 0; i < thread_count; i++) {
> +        while(!multifd_recv[i].done) {
> +            qemu_cond_wait(&multifd_recv_cond, &multifd_recv_mutex);
> +        }
> +    }
> +    qemu_mutex_unlock(&multifd_recv_mutex);
> +    return 0;
> +}
> +
>  /**
>   * save_page_header: Write page header to wire
>   *
> @@ -768,6 +797,12 @@ static size_t save_page_header(QEMUFile *f, RAMBlock *block, ram_addr_t offset)
>  {
>      size_t size, len;
> 
> +    if (multifd_needs_flush &&
> +        (offset & RAM_SAVE_FLAG_MULTIFD_PAGE)) {
> +        offset |= RAM_SAVE_FLAG_COMPRESS;
> +        multifd_needs_flush = false;
> +    }
> +
>      qemu_put_be64(f, offset);
>      size = 8;
> 
> @@ -2450,6 +2485,9 @@ static int ram_save_complete(QEMUFile *f, void *opaque)
> 
>      if (!migration_in_postcopy(migrate_get_current())) {
>          migration_bitmap_sync();
> +        if (migrate_use_multifd()) {
> +            multifd_needs_flush = true;
> +        }
>      }
> 
>      ram_control_before_iterate(f, RAM_CONTROL_FINISH);
> @@ -2491,6 +2529,9 @@ static void ram_save_pending(QEMUFile *f, void *opaque, uint64_t max_size,
>          qemu_mutex_lock_iothread();
>          rcu_read_lock();
>          migration_bitmap_sync();
> +        if (migrate_use_multifd()) {
> +            multifd_needs_flush = true;
> +        }
>          rcu_read_unlock();
>          qemu_mutex_unlock_iothread();
>          remaining_size = ram_save_remaining() * TARGET_PAGE_SIZE;
> @@ -2930,6 +2971,11 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
>              break;
>          }
> 
> +        if ((flags & (RAM_SAVE_FLAG_MULTIFD_PAGE|RAM_SAVE_FLAG_COMPRESS))
> +                  == (RAM_SAVE_FLAG_MULTIFD_PAGE|RAM_SAVE_FLAG_COMPRESS)) {
> +            multifd_flush();
> +            flags = flags & ~RAM_SAVE_FLAG_COMPRESS;
> +        }
>          if (flags & (RAM_SAVE_FLAG_COMPRESS | RAM_SAVE_FLAG_PAGE |
>                       RAM_SAVE_FLAG_COMPRESS_PAGE | RAM_SAVE_FLAG_XBZRLE |
>                       RAM_SAVE_FLAG_MULTIFD_PAGE)) {
> -- 
> 2.9.3
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK