Re: [PATCH v4 4/4] migration: Sync requested pages after postcopy recovery

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

* Peter Xu (peterx@redhat.com) wrote:
> We synchronize the requested pages right after a postcopy recovery happens.
> This helps to synchronize the prioritized pages on source so that the faulted
> threads can be served faster.
> 
> Reported-by: Xiaohui Li <xiaohli@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>

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

> ---
>  migration/savevm.c     | 57 ++++++++++++++++++++++++++++++++++++++++++
>  migration/trace-events |  1 +
>  2 files changed, 58 insertions(+)
> 
> diff --git a/migration/savevm.c b/migration/savevm.c
> index 34e4b71052..1dc021ee53 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -2011,6 +2011,49 @@ static int loadvm_postcopy_handle_run(MigrationIncomingState *mis)
>      return LOADVM_QUIT;
>  }
>  
> +/* We must be with page_request_mutex held */
> +static gboolean postcopy_sync_page_req(gpointer key, gpointer value,
> +                                       gpointer data)
> +{
> +    MigrationIncomingState *mis = data;
> +    void *host_addr = (void *) key;
> +    ram_addr_t rb_offset;
> +    RAMBlock *rb;
> +    int ret;
> +
> +    rb = qemu_ram_block_from_host(host_addr, true, &rb_offset);
> +    if (!rb) {
> +        /*
> +         * This should _never_ happen.  However be nice for a migrating VM to
> +         * not crash/assert.  Post an error (note: intended to not use *_once
> +         * because we do want to see all the illegal addresses; and this can
> +         * never be triggered by the guest so we're safe) and move on next.
> +         */
> +        error_report("%s: illegal host addr %p", __func__, host_addr);
> +        /* Try the next entry */
> +        return FALSE;
> +    }
> +
> +    ret = migrate_send_rp_message_req_pages(mis, rb, rb_offset);
> +    if (ret) {
> +        /* Please refer to above comment. */
> +        error_report("%s: send rp message failed for addr %p",
> +                     __func__, host_addr);
> +        return FALSE;
> +    }
> +
> +    trace_postcopy_page_req_sync(host_addr);
> +
> +    return FALSE;
> +}
> +
> +static void migrate_send_rp_req_pages_pending(MigrationIncomingState *mis)
> +{
> +    WITH_QEMU_LOCK_GUARD(&mis->page_request_mutex) {
> +        g_tree_foreach(mis->page_requested, postcopy_sync_page_req, mis);
> +    }
> +}
> +
>  static int loadvm_postcopy_handle_resume(MigrationIncomingState *mis)
>  {
>      if (mis->state != MIGRATION_STATUS_POSTCOPY_RECOVER) {
> @@ -2033,6 +2076,20 @@ static int loadvm_postcopy_handle_resume(MigrationIncomingState *mis)
>      /* Tell source that "we are ready" */
>      migrate_send_rp_resume_ack(mis, MIGRATION_RESUME_ACK_VALUE);
>  
> +    /*
> +     * After a postcopy recovery, the source should have lost the postcopy
> +     * queue, or potentially the requested pages could have been lost during
> +     * the network down phase.  Let's re-sync with the source VM by re-sending
> +     * all the pending pages that we eagerly need, so these threads won't get
> +     * blocked too long due to the recovery.
> +     *
> +     * Without this procedure, the faulted destination VM threads (waiting for
> +     * page requests right before the postcopy is interrupted) can keep hanging
> +     * until the pages are sent by the source during the background copying of
> +     * pages, or another thread faulted on the same address accidentally.
> +     */
> +    migrate_send_rp_req_pages_pending(mis);
> +
>      return 0;
>  }
>  
> diff --git a/migration/trace-events b/migration/trace-events
> index e4d5eb94ca..0fbfd2da60 100644
> --- a/migration/trace-events
> +++ b/migration/trace-events
> @@ -49,6 +49,7 @@ vmstate_save(const char *idstr, const char *vmsd_name) "%s, %s"
>  vmstate_load(const char *idstr, const char *vmsd_name) "%s, %s"
>  postcopy_pause_incoming(void) ""
>  postcopy_pause_incoming_continued(void) ""
> +postcopy_page_req_sync(void *host_addr) "sync page req %p"
>  
>  # vmstate.c
>  vmstate_load_field_error(const char *field, int ret) "field \"%s\" load failed, ret = %d"
> -- 
> 2.26.2
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK