Re: [Qemu-devel] [PATCH v12 16/21] migration: Synchronize multifd threads with main thread

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

* Juan Quintela (quintela@redhat.com) wrote:
> We synchronize all threads each RAM_SAVE_FLAG_EOS.  Bitmap
> synchronizations don't happen inside a  ram section, so we are safe
> about two channels trying to overwrite the same memory.

OK, that's quite neat - so you don't need any extra flags in the stream
to do the sync;  it probably needs a comment in the code somewhere so we
don't forget!


> Signed-off-by: Juan Quintela <quintela@redhat.com>
> ---
>  migration/ram.c        | 118 +++++++++++++++++++++++++++++++++++++----
>  migration/trace-events |   6 +++
>  2 files changed, 113 insertions(+), 11 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index c4c185cc4c..398cb0af3b 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -405,6 +405,8 @@ static void compress_threads_save_setup(void)
>  #define MULTIFD_MAGIC 0x11223344U
>  #define MULTIFD_VERSION 1
>  
> +#define MULTIFD_FLAG_SYNC (1 << 0)
> +
>  typedef struct {
>      uint32_t magic;
>      uint32_t version;
> @@ -471,6 +473,8 @@ typedef struct {
>      uint32_t num_packets;
>      /* pages sent through this channel */
>      uint32_t num_pages;
> +    /* syncs main thread and channels */
> +    QemuSemaphore sem_sync;
>  }  MultiFDSendParams;
>  
>  typedef struct {
> @@ -507,6 +511,8 @@ typedef struct {
>      uint32_t num_packets;
>      /* pages sent through this channel */
>      uint32_t num_pages;
> +    /* syncs main thread and channels */
> +    QemuSemaphore sem_sync;
>  } MultiFDRecvParams;
>  
>  static int multifd_send_initial_packet(MultiFDSendParams *p, Error **errp)
> @@ -682,6 +688,10 @@ struct {
>      int count;
>      /* array of pages to sent */
>      MultiFDPages_t *pages;
> +    /* syncs main thread and channels */
> +    QemuSemaphore sem_sync;
> +    /* global number of generated multifd packets */
> +    uint32_t seq;

It's interesting you use the same comment for 'seq' in
MultiFDSendParams - but I guess that means only this one is the global
version and the others aren't really global number - they're just
local to that thread?

>  } *multifd_send_state;
>  
>  static void multifd_send_terminate_threads(Error *err)
> @@ -727,6 +737,7 @@ int multifd_save_cleanup(Error **errp)
>          p->c = NULL;
>          qemu_mutex_destroy(&p->mutex);
>          qemu_sem_destroy(&p->sem);
> +        qemu_sem_destroy(&p->sem_sync);
>          g_free(p->name);
>          p->name = NULL;
>          multifd_pages_clear(p->pages);
> @@ -735,6 +746,7 @@ int multifd_save_cleanup(Error **errp)
>          g_free(p->packet);
>          p->packet = NULL;
>      }
> +    qemu_sem_destroy(&multifd_send_state->sem_sync);
>      g_free(multifd_send_state->params);
>      multifd_send_state->params = NULL;
>      multifd_pages_clear(multifd_send_state->pages);
> @@ -744,6 +756,33 @@ int multifd_save_cleanup(Error **errp)
>      return ret;
>  }
>  
> +static void multifd_send_sync_main(void)
> +{
> +    int i;
> +
> +    if (!migrate_use_multifd()) {
> +        return;
> +    }
> +    for (i = 0; i < migrate_multifd_channels(); i++) {
> +        MultiFDSendParams *p = &multifd_send_state->params[i];
> +
> +        trace_multifd_send_sync_main_signal(p->id);
> +
> +        qemu_mutex_lock(&p->mutex);
> +        p->flags |= MULTIFD_FLAG_SYNC;
> +        p->pending_job++;
> +        qemu_mutex_unlock(&p->mutex);
> +        qemu_sem_post(&p->sem);
> +    }
> +    for (i = 0; i < migrate_multifd_channels(); i++) {
> +        MultiFDSendParams *p = &multifd_send_state->params[i];
> +
> +        trace_multifd_send_sync_main_wait(p->id);
> +        qemu_sem_wait(&multifd_send_state->sem_sync);
> +    }
> +    trace_multifd_send_sync_main(multifd_send_state->seq);
> +}
> +

OK, so this just makes each of the sending threads ack, so that seems
OK.
But what happens with an error? multifd_send_sync_main exits it's
loop with a 'break' if the writes fail, and that could mean they never
come and post the flag-sync sem.

>  static void *multifd_send_thread(void *opaque)
>  {
>      MultiFDSendParams *p = opaque;
> @@ -778,17 +817,20 @@ static void *multifd_send_thread(void *opaque)
>              /* ToDo: send packet here */
>  
>              qemu_mutex_lock(&p->mutex);
> +            p->flags = 0;
>              p->pending_job--;
>              qemu_mutex_unlock(&p->mutex);
> -            continue;
> +
> +            if (flags & MULTIFD_FLAG_SYNC) {
> +                qemu_sem_post(&multifd_send_state->sem_sync);
> +            }
>          } else if (p->quit) {
>              qemu_mutex_unlock(&p->mutex);
>              break;
> +        } else {
> +            qemu_mutex_unlock(&p->mutex);
> +            /* sometimes there are spurious wakeups */
>          }
> -        qemu_mutex_unlock(&p->mutex);
> -        /* this is impossible */
> -        error_setg(&local_err, "multifd_send_thread: Unknown command");
> -        break;
>      }
>  
>  out:
> @@ -840,12 +882,14 @@ int multifd_save_setup(void)
>      multifd_send_state->params = g_new0(MultiFDSendParams, thread_count);
>      atomic_set(&multifd_send_state->count, 0);
>      multifd_pages_init(&multifd_send_state->pages, page_count);
> +    qemu_sem_init(&multifd_send_state->sem_sync, 0);
>  
>      for (i = 0; i < thread_count; i++) {
>          MultiFDSendParams *p = &multifd_send_state->params[i];
>  
>          qemu_mutex_init(&p->mutex);
>          qemu_sem_init(&p->sem, 0);
> +        qemu_sem_init(&p->sem_sync, 0);
>          p->quit = false;
>          p->pending_job = 0;
>          p->id = i;
> @@ -863,6 +907,10 @@ struct {
>      MultiFDRecvParams *params;
>      /* number of created threads */
>      int count;
> +    /* syncs main thread and channels */
> +    QemuSemaphore sem_sync;
> +    /* global number of generated multifd packets */
> +    uint32_t seq;
>  } *multifd_recv_state;
>  
>  static void multifd_recv_terminate_threads(Error *err)
> @@ -908,6 +956,7 @@ int multifd_load_cleanup(Error **errp)
>          p->c = NULL;
>          qemu_mutex_destroy(&p->mutex);
>          qemu_sem_destroy(&p->sem);
> +        qemu_sem_destroy(&p->sem_sync);
>          g_free(p->name);
>          p->name = NULL;
>          multifd_pages_clear(p->pages);
> @@ -916,6 +965,7 @@ int multifd_load_cleanup(Error **errp)
>          g_free(p->packet);
>          p->packet = NULL;
>      }
> +    qemu_sem_destroy(&multifd_recv_state->sem_sync);
>      g_free(multifd_recv_state->params);
>      multifd_recv_state->params = NULL;
>      g_free(multifd_recv_state);
> @@ -924,6 +974,42 @@ int multifd_load_cleanup(Error **errp)
>      return ret;
>  }
>  
> +static void multifd_recv_sync_main(void)
> +{
> +    int i;
> +
> +    if (!migrate_use_multifd()) {
> +        return;
> +    }
> +    for (i = 0; i < migrate_multifd_channels(); i++) {
> +        MultiFDRecvParams *p = &multifd_recv_state->params[i];
> +
> +        trace_multifd_recv_sync_main_signal(p->id);
> +        qemu_mutex_lock(&p->mutex);
> +        p->pending_job = true;
> +        qemu_mutex_unlock(&p->mutex);
> +    }
> +    for (i = 0; i < migrate_multifd_channels(); i++) {
> +        MultiFDRecvParams *p = &multifd_recv_state->params[i];
> +
> +        trace_multifd_recv_sync_main_wait(p->id);
> +        qemu_sem_wait(&multifd_recv_state->sem_sync);
> +        qemu_mutex_lock(&p->mutex);
> +        if (multifd_recv_state->seq < p->seq) {
> +            multifd_recv_state->seq = p->seq;
> +        }

Can you explain what this is for?
Something like the latest received block?

> +        qemu_mutex_unlock(&p->mutex);
> +    }
> +    for (i = 0; i < migrate_multifd_channels(); i++) {
> +        MultiFDRecvParams *p = &multifd_recv_state->params[i];
> +
> +        trace_multifd_recv_sync_main_signal(p->id);
> +
> +        qemu_sem_post(&p->sem_sync);
> +    }
> +    trace_multifd_recv_sync_main(multifd_recv_state->seq);
> +}
> +
>  static void *multifd_recv_thread(void *opaque)
>  {
>      MultiFDRecvParams *p = opaque;
> @@ -933,9 +1019,8 @@ static void *multifd_recv_thread(void *opaque)
>      trace_multifd_recv_thread_start(p->id);
>  
>      while (true) {
> -        qemu_sem_wait(&p->sem);
>          qemu_mutex_lock(&p->mutex);
> -        if (p->pending_job) {
> +        if (true || p->pending_job) {

A TODO I guess???

>              uint32_t used;
>              uint32_t flags;
>              qemu_mutex_unlock(&p->mutex);
> @@ -956,14 +1041,18 @@ static void *multifd_recv_thread(void *opaque)
>              p->num_packets++;
>              p->num_pages += used;
>              qemu_mutex_unlock(&p->mutex);
> +
> +            if (flags & MULTIFD_FLAG_SYNC) {
> +                qemu_sem_post(&multifd_recv_state->sem_sync);
> +                qemu_sem_wait(&p->sem_sync);
> +            }

Can you explain the receive side logic - I think this is waiting for all
receive threads to 'ack' - but how do we know that they've finished
receiving all data that was sent?

Dave

>          } else if (p->quit) {
>              qemu_mutex_unlock(&p->mutex);
>              break;
> +        } else {
> +            qemu_mutex_unlock(&p->mutex);
> +            /* sometimes there are spurious wakeups */
>          }
> -        qemu_mutex_unlock(&p->mutex);
> -        /* this is impossible */
> -        error_setg(&local_err, "multifd_recv_thread: Unknown command");
> -        break;
>      }
>  
>      if (local_err) {
> @@ -991,12 +1080,14 @@ int multifd_load_setup(void)
>      multifd_recv_state = g_malloc0(sizeof(*multifd_recv_state));
>      multifd_recv_state->params = g_new0(MultiFDRecvParams, thread_count);
>      atomic_set(&multifd_recv_state->count, 0);
> +    qemu_sem_init(&multifd_recv_state->sem_sync, 0);
>  
>      for (i = 0; i < thread_count; i++) {
>          MultiFDRecvParams *p = &multifd_recv_state->params[i];
>  
>          qemu_mutex_init(&p->mutex);
>          qemu_sem_init(&p->sem, 0);
> +        qemu_sem_init(&p->sem_sync, 0);
>          p->quit = false;
>          p->pending_job = false;
>          p->id = i;
> @@ -2695,6 +2786,7 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>      ram_control_before_iterate(f, RAM_CONTROL_SETUP);
>      ram_control_after_iterate(f, RAM_CONTROL_SETUP);
>  
> +    multifd_send_sync_main();
>      qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
>  
>      return 0;
> @@ -2770,6 +2862,7 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
>       */
>      ram_control_after_iterate(f, RAM_CONTROL_ROUND);
>  
> +    multifd_send_sync_main();
>  out:
>      qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
>      ram_counters.transferred += 8;
> @@ -2823,6 +2916,7 @@ static int ram_save_complete(QEMUFile *f, void *opaque)
>  
>      rcu_read_unlock();
>  
> +    multifd_send_sync_main();
>      qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
>  
>      return 0;
> @@ -3253,6 +3347,7 @@ static int ram_load_postcopy(QEMUFile *f)
>              break;
>          case RAM_SAVE_FLAG_EOS:
>              /* normal exit */
> +            multifd_recv_sync_main();
>              break;
>          default:
>              error_report("Unknown combination of migration flags: %#x"
> @@ -3438,6 +3533,7 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
>              break;
>          case RAM_SAVE_FLAG_EOS:
>              /* normal exit */
> +            multifd_recv_sync_main();
>              break;
>          default:
>              if (flags & RAM_SAVE_FLAG_HOOK) {
> diff --git a/migration/trace-events b/migration/trace-events
> index 9eee048287..b0ab8e2d03 100644
> --- a/migration/trace-events
> +++ b/migration/trace-events
> @@ -83,6 +83,12 @@ multifd_recv_thread_start(uint8_t id) "%d"
>  multifd_recv_thread_end(uint8_t id, uint32_t packets, uint32_t pages) "channel %d packets %d pages %d"
>  multifd_send(uint8_t id, uint32_t seq, uint32_t used, uint32_t flags) "channel %d seq number %d pages %d flags 0x%x"
>  multifd_recv(uint8_t id, uint32_t seq, uint32_t used, uint32_t flags) "channel %d seq number %d pages %d flags 0x%x"
> +multifd_send_sync_main(uint32_t seq) "seq %d"
> +multifd_send_sync_main_signal(uint8_t id) "channel %d"
> +multifd_send_sync_main_wait(uint8_t id) "channel %d"
> +multifd_recv_sync_main(uint32_t seq) "seq %d"
> +multifd_recv_sync_main_signal(uint8_t id) "channel %d"
> +multifd_recv_sync_main_wait(uint8_t id) "channel %d"
>  
>  # migration/migration.c
>  await_return_path_close_on_source_close(void) ""
> -- 
> 2.17.0
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK