Re: [Qemu-devel] [PATCH v9 5/8] migration/ram.c: add a notifier chain for precopy

[Peter Xu <peterx@redhat.com>]

On Thu, Nov 15, 2018 at 06:08:01PM +0800, Wei Wang wrote:
> This patch adds a notifier chain for the memory precopy. This enables various
> precopy optimizations to be invoked at specific places.
> 
> Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> CC: Dr. David Alan Gilbert <dgilbert@redhat.com>
> CC: Juan Quintela <quintela@redhat.com>
> CC: Michael S. Tsirkin <mst@redhat.com>
> CC: Peter Xu <peterx@redhat.com>
> ---
>  include/migration/misc.h | 12 ++++++++++++
>  migration/ram.c          | 31 +++++++++++++++++++++++++++++++
>  vl.c                     |  1 +
>  3 files changed, 44 insertions(+)
> 
> diff --git a/include/migration/misc.h b/include/migration/misc.h
> index 113320e..0bac623 100644
> --- a/include/migration/misc.h
> +++ b/include/migration/misc.h
> @@ -19,6 +19,18 @@
>  
>  /* migration/ram.c */
>  
> +typedef enum PrecopyNotifyReason {
> +    PRECOPY_NOTIFY_ERR = 0,
> +    PRECOPY_NOTIFY_START_ITERATION = 1,
> +    PRECOPY_NOTIFY_BEFORE_SYNC_BITMAP = 2,
> +    PRECOPY_NOTIFY_AFTER_SYNC_BITMAP = 3,
> +    PRECOPY_NOTIFY_MAX = 4,

It would be nice to add some comments for each of the notify reason.
E.g., from the name PRECOPY_NOTIFY_START_ITERATION seems more like a
hook at the start of each iteration but according to [1] it should be
at the start of migration rather than each iteration (or when
migration restarts, though I'm not sure whether we really have this
yet).

> +} PrecopyNotifyReason;
> +
> +void precopy_infrastructure_init(void);
> +void precopy_add_notifier(Notifier *n);
> +void precopy_remove_notifier(Notifier *n);
> +
>  void ram_mig_init(void);
>  void qemu_guest_free_page_hint(void *addr, size_t len);
>  
> diff --git a/migration/ram.c b/migration/ram.c
> index 229b791..65b1223 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -292,6 +292,8 @@ struct RAMState {
>      bool ram_bulk_stage;
>      /* How many times we have dirty too many pages */
>      int dirty_rate_high_cnt;
> +    /* ram save states used for notifiers */
> +    int ram_save_state;

This can be removed?

>      /* these variables are used for bitmap sync */
>      /* last time we did a full bitmap_sync */
>      int64_t time_last_bitmap_sync;
> @@ -328,6 +330,28 @@ typedef struct RAMState RAMState;
>  
>  static RAMState *ram_state;
>  
> +static NotifierList precopy_notifier_list;
> +
> +void precopy_infrastructure_init(void)
> +{
> +    notifier_list_init(&precopy_notifier_list);
> +}
> +
> +void precopy_add_notifier(Notifier *n)
> +{
> +    notifier_list_add(&precopy_notifier_list, n);
> +}
> +
> +void precopy_remove_notifier(Notifier *n)
> +{
> +    notifier_remove(n);
> +}
> +
> +static void precopy_notify(PrecopyNotifyReason reason)
> +{
> +    notifier_list_notify(&precopy_notifier_list, &reason);
> +}
> +
>  uint64_t ram_bytes_remaining(void)
>  {
>      return ram_state ? (ram_state->migration_dirty_pages * TARGET_PAGE_SIZE) :
> @@ -1642,6 +1666,8 @@ static void migration_bitmap_sync(RAMState *rs)
>      int64_t end_time;
>      uint64_t bytes_xfer_now;
>  
> +    precopy_notify(PRECOPY_NOTIFY_BEFORE_SYNC_BITMAP);
> +
>      ram_counters.dirty_sync_count++;
>  
>      if (!rs->time_last_bitmap_sync) {
> @@ -1699,6 +1725,8 @@ static void migration_bitmap_sync(RAMState *rs)
>      if (migrate_use_events()) {
>          qapi_event_send_migration_pass(ram_counters.dirty_sync_count);
>      }
> +
> +    precopy_notify(PRECOPY_NOTIFY_AFTER_SYNC_BITMAP);
>  }
>  
>  /**
> @@ -2555,6 +2583,8 @@ static void ram_state_reset(RAMState *rs)
>      rs->last_page = 0;
>      rs->last_version = ram_list.version;
>      rs->ram_bulk_stage = true;
> +
> +    precopy_notify(PRECOPY_NOTIFY_START_ITERATION);

[1]

>  }
>  
>  #define MAX_WAIT 50 /* ms, half buffered_file limit */
> @@ -3324,6 +3354,7 @@ out:
>  
>      ret = qemu_file_get_error(f);
>      if (ret < 0) {
> +        precopy_notify(PRECOPY_NOTIFY_ERR);

Could you show me which function is this line in?

Line 3324 here is ram_save_complete(), but I cannot find this exact
place.

Another thing to mention about the "reasons" (though I see it more
like "events"): have you thought about adding a PRECOPY_NOTIFY_END?
It might help in some cases:

  - then you don't need to trickily export the migrate_postcopy()
    since you'll notify that before postcopy starts

  - you'll have a solid point that you'll 100% guarantee that we'll
    stop the free page hinting and don't need to worry about whether
    there is chance the hinting will be running without an end [2].

Regarding [2] above: now the series only stops the hinting when
PRECOPY_NOTIFY_BEFORE_SYNC_BITMAP is notified.  Could there be a case
that it's missing?  E.g., what if we cancel/fail a migration during
precopy?  Have you tried it?

Regards,

-- 
Peter Xu