Re: [Qemu-devel] [RFC 15/29] migration: allow fault thread to pause

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

* Peter Xu (peterx@redhat.com) wrote:
> Allows the fault thread to stop handling page faults temporarily. When
> network failure happened (and if we expect a recovery afterwards), we
> should not allow the fault thread to continue sending things to source,
> instead, it should halt for a while until the connection is rebuilt.
> 
> When the dest main thread noticed the failure, it kicks the fault thread
> to switch to pause state.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
>  migration/migration.c    |  1 +
>  migration/migration.h    |  1 +
>  migration/postcopy-ram.c | 50 ++++++++++++++++++++++++++++++++++++++++++++----
>  migration/savevm.c       |  3 +++
>  migration/trace-events   |  2 ++
>  5 files changed, 53 insertions(+), 4 deletions(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index 9a0b5b0..9d93836 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -147,6 +147,7 @@ MigrationIncomingState *migration_incoming_get_current(void)
>          qemu_mutex_init(&mis_current.rp_mutex);
>          qemu_event_init(&mis_current.main_thread_load_event, false);
>          qemu_sem_init(&mis_current.postcopy_pause_sem_dst, 0);
> +        qemu_sem_init(&mis_current.postcopy_pause_sem_fault, 0);
>          once = true;
>      }
>      return &mis_current;
> diff --git a/migration/migration.h b/migration/migration.h
> index 047872b..574fedd 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -63,6 +63,7 @@ struct MigrationIncomingState {
>  
>      /* notify PAUSED postcopy incoming migrations to try to continue */
>      QemuSemaphore postcopy_pause_sem_dst;
> +    QemuSemaphore postcopy_pause_sem_fault;
>  };
>  
>  MigrationIncomingState *migration_incoming_get_current(void);
> diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> index 9ce391d..ba53155 100644
> --- a/migration/postcopy-ram.c
> +++ b/migration/postcopy-ram.c
> @@ -418,6 +418,17 @@ static int ram_block_enable_notify(const char *block_name, void *host_addr,
>      return 0;
>  }
>  
> +static bool postcopy_pause_fault_thread(MigrationIncomingState *mis)
> +{
> +    trace_postcopy_pause_fault_thread();
> +
> +    qemu_sem_wait(&mis->postcopy_pause_sem_fault);
> +
> +    trace_postcopy_pause_fault_thread_continued();
> +
> +    return true;
> +}
> +
>  /*
>   * Handle faults detected by the USERFAULT markings
>   */
> @@ -465,6 +476,22 @@ static void *postcopy_ram_fault_thread(void *opaque)
>              }
>          }
>  
> +        if (!mis->to_src_file) {
> +            /*
> +             * Possibly someone tells us that the return path is
> +             * broken already using the event. We should hold until
> +             * the channel is rebuilt.
> +             */
> +            if (postcopy_pause_fault_thread(mis)) {
> +                last_rb = NULL;
> +                /* Continue to read the userfaultfd */
> +            } else {
> +                error_report("%s: paused but don't allow to continue",
> +                             __func__);
> +                break;
> +            }
> +        }
> +
>          ret = read(mis->userfault_fd, &msg, sizeof(msg));
>          if (ret != sizeof(msg)) {
>              if (errno == EAGAIN) {
> @@ -504,18 +531,33 @@ static void *postcopy_ram_fault_thread(void *opaque)
>                                                  qemu_ram_get_idstr(rb),
>                                                  rb_offset);
>  
> +retry:
>          /*
>           * Send the request to the source - we want to request one
>           * of our host page sizes (which is >= TPS)
>           */
>          if (rb != last_rb) {
>              last_rb = rb;
> -            migrate_send_rp_req_pages(mis, qemu_ram_get_idstr(rb),
> -                                     rb_offset, qemu_ram_pagesize(rb));
> +            ret = migrate_send_rp_req_pages(mis, qemu_ram_get_idstr(rb),
> +                                            rb_offset, qemu_ram_pagesize(rb));
>          } else {
>              /* Save some space */
> -            migrate_send_rp_req_pages(mis, NULL,
> -                                     rb_offset, qemu_ram_pagesize(rb));
> +            ret = migrate_send_rp_req_pages(mis, NULL,
> +                                            rb_offset, qemu_ram_pagesize(rb));
> +        }
> +
> +        if (ret) {
> +            /* May be network failure, try to wait for recovery */
> +            if (ret == -EIO && postcopy_pause_fault_thread(mis)) {
> +                /* We got reconnected somehow, try to continue */
> +                last_rb = NULL;
> +                goto retry;
> +            } else {
> +                /* This is a unavoidable fault */
> +                error_report("%s: migrate_send_rp_req_pages() get %d",
> +                             __func__, ret);
> +                break;
> +            }
>          }
>      }
>      trace_postcopy_ram_fault_thread_exit();
> diff --git a/migration/savevm.c b/migration/savevm.c
> index 1f62268..386788d 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -1974,6 +1974,9 @@ static bool postcopy_pause_incoming(MigrationIncomingState *mis)
>      mis->to_src_file = NULL;
>      qemu_mutex_unlock(&mis->rp_mutex);
>  
> +    /* Notify the fault thread for the invalidated file handle */
> +    postcopy_fault_thread_notify(mis);
> +
>      while (mis->state == MIGRATION_STATUS_POSTCOPY_PAUSED) {
>          qemu_sem_wait(&mis->postcopy_pause_sem_dst);
>      }
> diff --git a/migration/trace-events b/migration/trace-events
> index a269eec..dbb4971 100644
> --- a/migration/trace-events
> +++ b/migration/trace-events
> @@ -100,6 +100,8 @@ open_return_path_on_source_continue(void) ""
>  postcopy_start(void) ""
>  postcopy_pause_return_path(void) ""
>  postcopy_pause_return_path_continued(void) ""
> +postcopy_pause_fault_thread(void) ""
> +postcopy_pause_fault_thread_continued(void) ""
>  postcopy_pause_continued(void) ""
>  postcopy_pause_incoming(void) ""
>  postcopy_pause_incoming_continued(void) ""
> -- 
> 2.7.4
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK