Re: [Qemu-devel] [PATCH v6 11/11] migration: create a dedicated thread to release rdma resource

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

* Lidong Chen (jemmy858585@gmail.com) wrote:
> ibv_dereg_mr wait for a long time for big memory size virtual server.
> 
> The test result is:
>   10GB  326ms
>   20GB  699ms
>   30GB  1021ms
>   40GB  1387ms
>   50GB  1712ms
>   60GB  2034ms
>   70GB  2457ms
>   80GB  2807ms
>   90GB  3107ms
>   100GB 3474ms
>   110GB 3735ms
>   120GB 4064ms
>   130GB 4567ms
>   140GB 4886ms
> 
> this will cause the guest os hang for a while when migration finished.
> So create a dedicated thread to release rdma resource.
> 
> Signed-off-by: Lidong Chen <lidongchen@tencent.com>
> ---
>  migration/migration.c |  6 ++++++
>  migration/migration.h |  3 +++
>  migration/rdma.c      | 47 +++++++++++++++++++++++++++++++----------------
>  3 files changed, 40 insertions(+), 16 deletions(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index f7d6e26..25d9009 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -1499,6 +1499,7 @@ void migrate_init(MigrationState *s)
>      s->vm_was_running = false;
>      s->iteration_initial_bytes = 0;
>      s->threshold_size = 0;
> +    s->rdma_cleanup_thread_quit = true;
>  }
>  
>  static GSList *migration_blockers;
> @@ -1660,6 +1661,10 @@ static bool migrate_prepare(MigrationState *s, bool blk, bool blk_inc,
>          return false;
>      }
>  
> +    if (s->rdma_cleanup_thread_quit != true) {
> +        return false;
> +    }

That's not good! We error out without saying anything to the user
about why;  if we error we must always say why, otherwise we'll get
bug reports from people without anyway to debug them.

However, we also don't need to; all we need to do is turn this into
a semaphore or similar, and then wait for it at the start of 
'rdma_start_outgoing_migration' - that way there's no error exit, it
just waits a few seconds and then carries on correctly.
(Maybe we need to wait in incoming as well to cope with
postcopy-recovery).

Dave

>      if (runstate_check(RUN_STATE_INMIGRATE)) {
>          error_setg(errp, "Guest is waiting for an incoming migration");
>          return false;
> @@ -3213,6 +3218,7 @@ static void migration_instance_init(Object *obj)
>  
>      ms->state = MIGRATION_STATUS_NONE;
>      ms->mbps = -1;
> +    ms->rdma_cleanup_thread_quit = true;
>      qemu_sem_init(&ms->pause_sem, 0);
>      qemu_mutex_init(&ms->error_mutex);
>  
> diff --git a/migration/migration.h b/migration/migration.h
> index 64a7b33..60138dd 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -224,6 +224,9 @@ struct MigrationState
>       * do not trigger spurious decompression errors.
>       */
>      bool decompress_error_check;
> +
> +    /* Set this when rdma resource have released */
> +    bool rdma_cleanup_thread_quit;
>  };
>  
>  void migrate_set_state(int *state, int old_state, int new_state);
> diff --git a/migration/rdma.c b/migration/rdma.c
> index e1498f2..3282f35 100644
> --- a/migration/rdma.c
> +++ b/migration/rdma.c
> @@ -2995,35 +2995,50 @@ static void qio_channel_rdma_set_aio_fd_handler(QIOChannel *ioc,
>      }
>  }
>  
> -static int qio_channel_rdma_close(QIOChannel *ioc,
> -                                  Error **errp)
> +static void *qio_channel_rdma_close_thread(void *arg)
>  {
> -    QIOChannelRDMA *rioc = QIO_CHANNEL_RDMA(ioc);
> -    RDMAContext *rdmain, *rdmaout;
> -    trace_qemu_rdma_close();
> -
> -    rdmain = rioc->rdmain;
> -    if (rdmain) {
> -        atomic_rcu_set(&rioc->rdmain, NULL);
> -    }
> +    RDMAContext **rdma = arg;
> +    RDMAContext *rdmain = rdma[0];
> +    RDMAContext *rdmaout = rdma[1];
> +    MigrationState *s = migrate_get_current();
>  
> -    rdmaout = rioc->rdmaout;
> -    if (rdmaout) {
> -        atomic_rcu_set(&rioc->rdmaout, NULL);
> -    }
> +    rcu_register_thread();
>  
>      synchronize_rcu();
> -
>      if (rdmain) {
>          qemu_rdma_cleanup(rdmain);
>      }
> -
>      if (rdmaout) {
>          qemu_rdma_cleanup(rdmaout);
>      }
>  
>      g_free(rdmain);
>      g_free(rdmaout);
> +    g_free(rdma);
> +
> +    rcu_unregister_thread();
> +    s->rdma_cleanup_thread_quit = true;
> +    return NULL;
> +}
> +
> +static int qio_channel_rdma_close(QIOChannel *ioc,
> +                                  Error **errp)
> +{
> +    QemuThread t;
> +    QIOChannelRDMA *rioc = QIO_CHANNEL_RDMA(ioc);
> +    RDMAContext **rdma = g_new0(RDMAContext*, 2);
> +    MigrationState *s = migrate_get_current();
> +
> +    trace_qemu_rdma_close();
> +    if (rioc->rdmain || rioc->rdmaout) {
> +        rdma[0] =  rioc->rdmain;
> +        rdma[1] =  rioc->rdmaout;
> +        atomic_rcu_set(&rioc->rdmain, NULL);
> +        atomic_rcu_set(&rioc->rdmaout, NULL);
> +        s->rdma_cleanup_thread_quit = false;
> +        qemu_thread_create(&t, "rdma cleanup", qio_channel_rdma_close_thread,
> +                           rdma, QEMU_THREAD_DETACHED);
> +    }
>  
>      return 0;
>  }
> -- 
> 1.8.3.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK