coll/libnbc: Work around for non-uniform data types in ibcast

 * If (legal) non-uniform data type signatures are used in ibcast
   then the chosen algorithm may fail on the request, and worst case
   it could produce wrong answers.
 * Add an MCA parameter that, by default, protects the user from this
   scenario. If the user really wants to use it then they have to
   'opt-in' by setting the following parameter to false:
   - `-mca coll_libnbc_ibcast_skip_dt_decision f`
 * Once the following Issues are resolved then this parameter can
   be removed.
   - open-mpi#2256
   - open-mpi#1763

Signed-off-by: Joshua Hursey <jhursey@us.ibm.com>
(cherry picked from commit 350ef67)
Signed-off-by: Joshua Hursey <jhursey@us.ibm.com>

Author: jjhursey

ompi/mca/coll/libnbc/coll_libnbc.h

@@ -15,6 +15,7 @@
  *                         reserved.
  * Copyright (c) 2014-2015 Research Organization for Information Science
  *                         and Technology (RIST). All rights reserved.
+ * Copyright (c) 2016      IBM Corporation.  All rights reserved.
  * $COPYRIGHT$
  *
  * Additional copyrights may follow
@@ -67,6 +68,8 @@ BEGIN_C_DECLS
 /* number of implemented collective functions */
 #define NBC_NUM_COLL 17
 
+extern bool libnbc_ibcast_skip_dt_decision;
+
 struct ompi_coll_libnbc_component_t {
     mca_coll_base_component_2_0_0_t super;
     opal_free_list_t requests;

ompi/mca/coll/libnbc/coll_libnbc_component.c

@@ -39,6 +39,7 @@ const char *mca_coll_libnbc_component_version_string =
 
 
 static int libnbc_priority = 10;
+bool libnbc_ibcast_skip_dt_decision = true;
 
 
 static int libnbc_open(void);
@@ -131,6 +132,27 @@ libnbc_register(void)
                                            MCA_BASE_VAR_SCOPE_READONLY,
                                            &libnbc_priority);
 
+    /* ibcast decision function can make the wrong decision if a legal
+     * non-uniform data type signature is used. This has resulted in the
+     * collective operation failing, and possibly producing wrong answers.
+     * We are investigating a fix for this problem, but it is taking a while.
+     *   https://github.com/open-mpi/ompi/issues/2256
+     *   https://github.com/open-mpi/ompi/issues/1763
+     * As a result we are adding an MCA parameter to make a conservative
+     * decision to avoid this issue. If the user knows that their application
+     * does not use data types in this way, then they can set this parameter
+     * to get the old behavior. Once the issue is truely fixed, then this
+     * parameter can be removed.
+     */
+    libnbc_ibcast_skip_dt_decision = true;
+    (void) mca_base_component_var_register(&mca_coll_libnbc_component.super.collm_version,
+                                           "ibcast_skip_dt_decision",
+                                           "In ibcast only use size of communicator to choose algorithm, exclude data type signature. Set to 'false' to use data type signature in decision. WARNING: If you set this to 'false' then your application should not use non-uniform data type signatures in calls to ibcast.",
+                                           MCA_BASE_VAR_TYPE_BOOL, NULL, 0, 0,
+                                           OPAL_INFO_LVL_9,
+                                           MCA_BASE_VAR_SCOPE_READONLY,
+                                           &libnbc_ibcast_skip_dt_decision);
+
     return OMPI_SUCCESS;
 }
 

ompi/mca/coll/libnbc/nbc_ibcast.c

@@ -9,6 +9,7 @@
  *                         and Technology (RIST). All rights reserved.
  * Copyright (c) 2015      Los Alamos National Security, LLC. All rights
  *                         reserved.
+ * Copyright (c) 2016      IBM Corporation.  All rights reserved.
  *
  * Author(s): Torsten Hoefler <htor@cs.indiana.edu>
  *
@@ -65,16 +66,26 @@ int ompi_coll_libnbc_ibcast(void *buffer, int count, MPI_Datatype datatype, int
 
   segsize = 16384;
   /* algorithm selection */
-  if (p <= 4) {
-    alg = NBC_BCAST_LINEAR;
-  } else if (size * count < 65536) {
-    alg = NBC_BCAST_BINOMIAL;
-  } else if (size * count < 524288) {
-    alg = NBC_BCAST_CHAIN;
-    segsize = 8192;
-  } else {
-    alg = NBC_BCAST_CHAIN;
-    segsize = 32768;
+  if( libnbc_ibcast_skip_dt_decision ) {
+    if (p <= 4) {
+      alg = NBC_BCAST_LINEAR;
+    }
+    else {
+      alg = NBC_BCAST_BINOMIAL;
+    }
+  }
+  else {
+    if (p <= 4) {
+      alg = NBC_BCAST_LINEAR;
+    } else if (size * count < 65536) {
+      alg = NBC_BCAST_BINOMIAL;
+    } else if (size * count < 524288) {
+      alg = NBC_BCAST_CHAIN;
+      segsize = 8192;
+    } else {
+      alg = NBC_BCAST_CHAIN;
+      segsize = 32768;
+    }
   }
 
 #ifdef NBC_CACHE_SCHEDULE