Re: [Qemu-devel] [PATCH v15 07/12] 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.
> 
> Signed-off-by: Juan Quintela <quintela@redhat.com>

Note the testing didn't like the atomics on the 64bit packet_num.

Dave

> --
> seq needs to be atomic now, will also be accessed from main thread.
> Fix the if (true || ...) leftover
> ---
>  migration/ram.c        | 147 ++++++++++++++++++++++++++++++++---------
>  migration/trace-events |   6 ++
>  2 files changed, 122 insertions(+), 31 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 793f0dc5d3..516f347d24 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -510,6 +510,8 @@ exit:
>  #define MULTIFD_MAGIC 0x11223344U
>  #define MULTIFD_VERSION 1
>  
> +#define MULTIFD_FLAG_SYNC (1 << 0)
> +
>  typedef struct {
>      uint32_t magic;
>      uint32_t version;
> @@ -577,6 +579,8 @@ 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 {
> @@ -614,6 +618,8 @@ 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)
> @@ -801,6 +807,10 @@ 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 */
> +    uint64_t packet_num;
>  } *multifd_send_state;
>  
>  static void multifd_send_terminate_threads(Error *err)
> @@ -848,6 +858,7 @@ 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);
> @@ -856,6 +867,7 @@ 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);
> @@ -865,6 +877,33 @@ 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(atomic_read(&multifd_send_state->packet_num));
> +}
> +
>  static void *multifd_send_thread(void *opaque)
>  {
>      MultiFDSendParams *p = opaque;
> @@ -901,15 +940,17 @@ static void *multifd_send_thread(void *opaque)
>              qemu_mutex_lock(&p->mutex);
>              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:
> @@ -961,12 +1002,14 @@ int multifd_save_setup(void)
>      multifd_send_state->params = g_new0(MultiFDSendParams, thread_count);
>      atomic_set(&multifd_send_state->count, 0);
>      multifd_send_state->pages = multifd_pages_init(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;
> @@ -991,6 +1034,10 @@ struct {
>      MultiFDRecvParams *params;
>      /* number of created threads */
>      int count;
> +    /* syncs main thread and channels */
> +    QemuSemaphore sem_sync;
> +    /* global number of generated multifd packets */
> +    uint64_t packet_num;
>  } *multifd_recv_state;
>  
>  static void multifd_recv_terminate_threads(Error *err)
> @@ -1036,6 +1083,7 @@ 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);
> @@ -1044,6 +1092,7 @@ 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);
> @@ -1052,6 +1101,42 @@ 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 (atomic_read(&multifd_recv_state->packet_num) < p->packet_num) {
> +            atomic_set(&multifd_recv_state->packet_num, p->packet_num);
> +        }
> +        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(atomic_read(&multifd_recv_state->packet_num));
> +}
> +
>  static void *multifd_recv_thread(void *opaque)
>  {
>      MultiFDRecvParams *p = opaque;
> @@ -1061,37 +1146,30 @@ static void *multifd_recv_thread(void *opaque)
>      trace_multifd_recv_thread_start(p->id);
>  
>      while (true) {
> -        qemu_sem_wait(&p->sem);
> +        uint32_t used;
> +        uint32_t flags;
> +
> +        /* ToDo: recv packet here */
> +
>          qemu_mutex_lock(&p->mutex);
> -        if (p->pending_job) {
> -            uint32_t used;
> -            uint32_t flags;
> -            qemu_mutex_unlock(&p->mutex);
> -
> -            /* ToDo: recv packet here */
> -
> -            qemu_mutex_lock(&p->mutex);
> -            ret = multifd_recv_unfill_packet(p, &local_err);
> -            if (ret) {
> -                qemu_mutex_unlock(&p->mutex);
> -                break;
> -            }
> -
> -            used = p->pages->used;
> -            flags = p->flags;
> -            trace_multifd_recv(p->id, p->packet_num, used, flags);
> -            p->pending_job = false;
> -            p->num_packets++;
> -            p->num_pages += used;
> -            qemu_mutex_unlock(&p->mutex);
> -        } else if (p->quit) {
> +        ret = multifd_recv_unfill_packet(p, &local_err);
> +        if (ret) {
>              qemu_mutex_unlock(&p->mutex);
>              break;
>          }
> +
> +        used = p->pages->used;
> +        flags = p->flags;
> +        trace_multifd_recv(p->id, p->packet_num, used, flags);
> +        p->pending_job = false;
> +        p->num_packets++;
> +        p->num_pages += used;
>          qemu_mutex_unlock(&p->mutex);
> -        /* this is impossible */
> -        error_setg(&local_err, "multifd_recv_thread: Unknown command");
> -        break;
> +
> +        if (flags & MULTIFD_FLAG_SYNC) {
> +            qemu_sem_post(&multifd_recv_state->sem_sync);
> +            qemu_sem_wait(&p->sem_sync);
> +        }
>      }
>  
>      if (local_err) {
> @@ -1119,12 +1197,14 @@ 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;
> @@ -2882,6 +2962,7 @@ 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;
> @@ -2962,6 +3043,7 @@ 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;
> @@ -3015,6 +3097,7 @@ 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;
> @@ -3504,6 +3587,7 @@ 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"
> @@ -3692,6 +3776,7 @@ 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 c667d98529..ea39bb2bc5 100644
> --- a/migration/trace-events
> +++ b/migration/trace-events
> @@ -77,9 +77,15 @@ migration_bitmap_sync_start(void) ""
>  migration_bitmap_sync_end(uint64_t dirty_pages) "dirty_pages %" PRIu64
>  migration_throttle(void) ""
>  multifd_recv(uint8_t id, uint64_t packet_num, uint32_t used, uint32_t flags) "channel %d packet number %ld pages %d flags 0x%x"
> +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"
>  multifd_recv_thread_end(uint8_t id, uint32_t packets, uint32_t pages) "channel %d packets %d pages %d"
>  multifd_recv_thread_start(uint8_t id) "%d"
>  multifd_send(uint8_t id, uint64_t packet_num, uint32_t used, uint32_t flags) "channel %d packet_num %ld 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_send_thread_end(uint8_t id, uint32_t packets, uint32_t pages) "channel %d packets %d pages %d"
>  multifd_send_thread_start(uint8_t id) "%d"
>  ram_discard_range(const char *rbname, uint64_t start, size_t len) "%s: start: %" PRIx64 " %zx"
> -- 
> 2.17.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK