comm/cid: fix threaded CID allocation

This commit should restore the pre-non-blocking behavior of the CID
allocator when threads are used. There are two primary changes: 1)
do not hold the cid allocator lock past the end of a request callback,
and 2) if a lower id communicator is detected during CID allocation
back off and let the lower id communicator finish before continuing.

Signed-off-by: Nathan Hjelm <[email protected]>

Author: hjelmn

ompi/communicator/comm_cid.c

@@ -181,6 +181,7 @@ static ompi_comm_cid_context_t *mca_comm_cid_context_alloc (ompi_communicator_t
     context->newcomm       = newcomm;
     context->comm          = comm;
     context->bridgecomm    = bridgecomm;
+    context->pml_tag       = 0;
 
     /* Determine which implementation of allreduce we have to use
      * for the current mode. */
@@ -245,8 +246,8 @@ static int ompi_comm_allreduce_getnextcid (ompi_comm_request_t *request);
 static int ompi_comm_checkcid (ompi_comm_request_t *request);
 /* verify that the cid was available globally */
 static int ompi_comm_nextcid_check_flag (ompi_comm_request_t *request);
-/* lock the cid generator */
-static int ompi_comm_cid_lock (ompi_comm_request_t *request);
+
+static volatile int64_t ompi_comm_cid_lowest_id = INT64_MAX;
 
 int ompi_comm_nextcid_nb (ompi_communicator_t *newcomm, ompi_communicator_t *comm,
                           ompi_communicator_t *bridgecomm, const void *arg0, const void *arg1,
@@ -271,7 +272,7 @@ int ompi_comm_nextcid_nb (ompi_communicator_t *newcomm, ompi_communicator_t *com
 
     request->context = &context->super;
 
-    ompi_comm_request_schedule_append (request, ompi_comm_cid_lock, NULL, 0);
+    ompi_comm_request_schedule_append (request, ompi_comm_allreduce_getnextcid, NULL, 0);
     ompi_comm_request_start (request);
 
     *req = &request->super;
@@ -299,30 +300,33 @@ int ompi_comm_nextcid (ompi_communicator_t *newcomm, ompi_communicator_t *comm,
     return rc;
 }
 
-static int ompi_comm_cid_lock (ompi_comm_request_t *request)
-{
-    if (!OPAL_THREAD_TRYLOCK(&ompi_cid_lock)) {
-        return ompi_comm_request_schedule_append (request, ompi_comm_allreduce_getnextcid, NULL, 0);
-    }
-
-    return ompi_comm_request_schedule_append (request, ompi_comm_cid_lock, NULL, 0);
-}
-
 static int ompi_comm_allreduce_getnextcid (ompi_comm_request_t *request)
 {
     ompi_comm_cid_context_t *context = (ompi_comm_cid_context_t *) request->context;
+    int64_t my_id = ((int64_t) ompi_comm_get_cid (context->comm) << 32 | context->pml_tag);
     ompi_request_t *subreq;
     bool flag;
     int ret;
 
+    if (OPAL_THREAD_TRYLOCK(&ompi_cid_lock)) {
+        return ompi_comm_request_schedule_append (request, ompi_comm_allreduce_getnextcid, NULL, 0);
+    }
+
+    if (ompi_comm_cid_lowest_id < my_id) {
+        OPAL_THREAD_UNLOCK(&ompi_cid_lock);
+        return ompi_comm_request_schedule_append (request, ompi_comm_allreduce_getnextcid, NULL, 0);
+    }
+
+    ompi_comm_cid_lowest_id = my_id;
+
     /**
      * This is the real algorithm described in the doc
      */
     flag = false;
     context->nextlocal_cid = mca_pml.pml_max_contextid;
     for (unsigned int i = context->start ; i < mca_pml.pml_max_contextid ; ++i) {
-        flag = opal_pointer_array_test_and_set_item(&ompi_mpi_communicators,
-                                                    i, context->comm);
+        flag = opal_pointer_array_test_and_set_item (&ompi_mpi_communicators, i,
+                                                     context->comm);
         if (true == flag) {
             context->nextlocal_cid = i;
             break;
@@ -332,6 +336,7 @@ static int ompi_comm_allreduce_getnextcid (ompi_comm_request_t *request)
     ret = context->allreduce_fn (&context->nextlocal_cid, &context->nextcid, 1, MPI_MAX,
                                  context, &subreq);
     if (OMPI_SUCCESS != ret) {
+        ompi_comm_cid_lowest_id = INT64_MAX;
         OPAL_THREAD_UNLOCK(&ompi_cid_lock);
         return ret;
     }
@@ -341,10 +346,12 @@ static int ompi_comm_allreduce_getnextcid (ompi_comm_request_t *request)
         if (flag) {
             opal_pointer_array_test_and_set_item(&ompi_mpi_communicators, context->nextlocal_cid, NULL);
         }
-
+ 
+        ompi_comm_cid_lowest_id = INT64_MAX;
         OPAL_THREAD_UNLOCK(&ompi_cid_lock);
         return OMPI_ERR_OUT_OF_RESOURCE;
     }
+    OPAL_THREAD_UNLOCK(&ompi_cid_lock);
 
     /* next we want to verify that the resulting commid is ok */
     return ompi_comm_request_schedule_append (request, ompi_comm_checkcid, &subreq, 1);
@@ -356,6 +363,10 @@ static int ompi_comm_checkcid (ompi_comm_request_t *request)
     ompi_request_t *subreq;
     int ret;
 
+    if (OPAL_THREAD_TRYLOCK(&ompi_cid_lock)) {
+        return ompi_comm_request_schedule_append (request, ompi_comm_checkcid, NULL, 0);
+    }
+
     context->flag = (context->nextcid == context->nextlocal_cid);
 
     if (!context->flag) {
@@ -372,19 +383,26 @@ static int ompi_comm_checkcid (ompi_comm_request_t *request)
         ompi_comm_request_schedule_append (request, ompi_comm_nextcid_check_flag, &subreq, 1);
     }
 
+    OPAL_THREAD_UNLOCK(&ompi_cid_lock);
+
     return ret;
 }
 
 static int ompi_comm_nextcid_check_flag (ompi_comm_request_t *request)
 {
     ompi_comm_cid_context_t *context = (ompi_comm_cid_context_t *) request->context;
 
+    if (OPAL_THREAD_TRYLOCK(&ompi_cid_lock)) {
+        return ompi_comm_request_schedule_append (request, ompi_comm_nextcid_check_flag, NULL, 0);
+    }
+
     if (1 == context->rflag) {
         /* set the according values to the newcomm */
         context->newcomm->c_contextid = context->nextcid;
         opal_pointer_array_set_item (&ompi_mpi_communicators, context->nextcid, context->newcomm);
 
         /* unlock the cid generator */
+        ompi_comm_cid_lowest_id = INT64_MAX;
         OPAL_THREAD_UNLOCK(&ompi_cid_lock);
 
         /* done! */
@@ -399,6 +417,8 @@ static int ompi_comm_nextcid_check_flag (ompi_comm_request_t *request)
 
     ++context->iter;
 
+    OPAL_THREAD_UNLOCK(&ompi_cid_lock);
+
     /* try again */
     return ompi_comm_allreduce_getnextcid (request);
 }