[SYCL][Graph] Avoid unnecessary inter-partition dependencies

Improves management of inter-partition dependencies, so that only required dependencies are added.
As removing these dependencies can results in multiple executions paths, we have added a map to track all events returned from submitted partitions.
All these events are linked to the main event returned to user.
Adds tests.

sycl/doc/design/CommandGraph.md

@@ -192,6 +192,32 @@ illustrated in the following diagrams:
 ![Graph partition illustration step 10b.](images/SYCL-Graph-partitions_step11.jpg)
 ![Graph partition illustration step 11b.](images/SYCL-Graph-partitions_step12.jpg)
 
+### Multiple Roots Execution Flow
+The following diagram shows the partitions of a graph with two roots 
+and a host-task in each branch.
+
+![Multiple roots graph partition illustration.](images/SYCL-Graph-multiple_roots_partitions.jpg)
+
+When executing this graph, the partitions were enqueued one after the other, 
+with each partition waiting for the previous one to complete 
+(see top of the following diagram).
+However, for multi-root graph, this behavior adds unnecessary dependency 
+between partitions, slowing down the execution of the whole graph.
+Now, we keep track of the actual predecessors of each partition and 
+only enforce dependencies between partitions when necessary.
+In our example, the extra dependency is therefore removed and 
+both branches can be executed concurrently.
+But as we can see on this diagram, this new approach can involve 
+multiple execution tails, which leads to difficulties when 
+we want to know when the graph execution has finished.
+To cope with this issue, the events associated to the completion of 
+each partition are linked to the event returned to users. 
+Hence, when the returned event is complete, we can guarantee that 
+all works associated with the graph has been completed.  
+
+![Multiple roots graph partition execution flow.](images/SYCL-Graph-partition_execution_flow.jpg)
+
+
 ## Memory handling: Buffer and Accessor
 
 There is no extra support for graph-specific USM allocations in the current

sycl/source/detail/graph_impl.cpp

@@ -263,6 +263,18 @@ void exec_graph_impl::makePartitions() {
                      Partition->MSchedule.end());
   }
 
+  // Compute partition dependencies
+  for (auto Partition : MPartitions) {
+    for (auto const &Root : Partition->MRoots) {
+      auto RootNode = Root.lock();
+      for (const auto &Dep : RootNode->MPredecessors) {
+        auto NodeDep = Dep.lock();
+        Partition->MPredecessors.push_back(
+            MPartitions[MPartitionNodes[NodeDep]]);
+      }
+    }
+  }
+
   // Reset node groups (if node have to be re-processed - e.g. subgraph)
   for (auto &Node : MGraphImpl->MNodeStorage) {
     Node->MPartitionNum = -1;
@@ -761,7 +773,22 @@ exec_graph_impl::enqueue(const std::shared_ptr<sycl::detail::queue_impl> &Queue,
   });
 
   sycl::detail::EventImplPtr NewEvent;
-  for (auto CurrentPartition : MPartitions) {
+  std::vector<sycl::detail::EventImplPtr> BackupCGDataMEvents;
+  if (MPartitions.size() > 1) {
+    BackupCGDataMEvents = CGData.MEvents;
+  }
+  for (uint32_t currentPartitionsNum = 0;
+       currentPartitionsNum < MPartitions.size(); currentPartitionsNum++) {
+    auto CurrentPartition = MPartitions[currentPartitionsNum];
+    // restore initial MEvents to add only needed additional depenencies
+    if (currentPartitionsNum > 0) {
+      CGData.MEvents = BackupCGDataMEvents;
+    }
+
+    for (auto const &DepPartition : CurrentPartition->MPredecessors) {
+      CGData.MEvents.push_back(MPartitionsExecutionEvents[DepPartition]);
+    }
+
     auto CommandBuffer =
         CurrentPartition->MPiCommandBuffers[Queue->get_device()];
 
@@ -901,12 +928,19 @@ exec_graph_impl::enqueue(const std::shared_ptr<sycl::detail::queue_impl> &Queue,
       NewEvent->setStateIncomplete();
       NewEvent->getPreparedDepsEvents() = ScheduledEvents;
     }
-    CGData.MEvents.push_back(NewEvent);
+    MPartitionsExecutionEvents[CurrentPartition] = NewEvent;
   }
 
   // Keep track of this execution event so we can make sure it's completed in
   // the destructor.
   MExecutionEvents.push_back(NewEvent);
+  // Attach events of previous partitions to ensure that when the returned event
+  // is complete all execution associated with the graph have been completed.
+  for (auto const &Elem : MPartitionsExecutionEvents) {
+    if (Elem.second != NewEvent) {
+      NewEvent->attachEventToComplete(Elem.second);
+    }
+  }
   sycl::event QueueEvent =
       sycl::detail::createSyclObjFromImpl<sycl::event>(NewEvent);
   return QueueEvent;

sycl/source/detail/graph_impl.hpp

@@ -496,6 +496,8 @@ class partition {
   /// Map of devices to command buffers.
   std::unordered_map<sycl::device, sycl::detail::pi::PiExtCommandBuffer>
       MPiCommandBuffers;
+  /// List of predecessors to this partition.
+  std::vector<std::shared_ptr<partition>> MPredecessors;
 
   /// @return True if the partition contains a host task
   bool isHostTask() const {
@@ -1072,6 +1074,9 @@ class exec_graph_impl {
   std::vector<sycl::detail::EventImplPtr> MExecutionEvents;
   /// List of the partitions that compose the exec graph.
   std::vector<std::shared_ptr<partition>> MPartitions;
+  /// Map of the partitions to their execution events
+  std::unordered_map<std::shared_ptr<partition>, sycl::detail::EventImplPtr>
+      MPartitionsExecutionEvents;
 };
 
 } // namespace detail

sycl/test-e2e/Graph/Explicit/host_task2_multiple_roots.cpp

@@ -0,0 +1,9 @@
+// RUN: %{build} -o %t.out
+// RUN: %{run} %t.out
+// Extra run to check for leaks in Level Zero using UR_L0_LEAKS_DEBUG
+// RUN: %if level_zero %{env UR_L0_LEAKS_DEBUG=1 %{run} %t.out 2>&1 | FileCheck %s --implicit-check-not=LEAK %}
+//
+
+#define GRAPH_E2E_EXPLICIT
+
+#include "../Inputs/host_task2_multiple_roots.cpp"

sycl/test-e2e/Graph/Explicit/host_task_multiple_roots.cpp

@@ -0,0 +1,9 @@
+// RUN: %{build} -o %t.out
+// RUN: %{run} %t.out
+// Extra run to check for leaks in Level Zero using UR_L0_LEAKS_DEBUG
+// RUN: %if level_zero %{env UR_L0_LEAKS_DEBUG=1 %{run} %t.out 2>&1 | FileCheck %s --implicit-check-not=LEAK %}
+//
+
+#define GRAPH_E2E_EXPLICIT
+
+#include "../Inputs/host_task_multiple_roots.cpp"

sycl/test-e2e/Graph/Inputs/host_task2_multiple_roots.cpp

@@ -0,0 +1,174 @@
+// This test uses a host_task when adding a command_graph node for graph with
+// multiple roots.
+
+#include "../graph_common.hpp"
+
+int main() {
+  queue Queue{{sycl::ext::intel::property::queue::no_immediate_command_list{}}};
+
+  if (!are_graphs_supported(Queue)) {
+    return 0;
+  }
+
+  using T = int;
+
+  if (!Queue.get_device().has(sycl::aspect::usm_shared_allocations)) {
+    return 0;
+  }
+
+  const T ModValue = T{7};
+  std::vector<T> DataA(Size), DataB(Size), DataC(Size), Res2(Size);
+
+  std::iota(DataA.begin(), DataA.end(), 1);
+  std::iota(DataB.begin(), DataB.end(), 10);
+  std::iota(DataC.begin(), DataC.end(), 1000);
+
+  std::vector<T> Reference(DataC);
+  for (unsigned n = 0; n < Iterations; n++) {
+    for (size_t i = 0; i < Size; i++) {
+      Reference[i] = (((DataA[i] + DataB[i]) * ModValue) + 1) * DataB[i];
+    }
+  }
+
+  exp_ext::command_graph Graph{Queue.get_context(), Queue.get_device()};
+
+  T *PtrA = malloc_device<T>(Size, Queue);
+  T *PtrB = malloc_device<T>(Size, Queue);
+  T *PtrC = malloc_shared<T>(Size, Queue);
+  T *PtrA2 = malloc_device<T>(Size, Queue);
+  T *PtrB2 = malloc_device<T>(Size, Queue);
+  T *PtrC2 = malloc_shared<T>(Size, Queue);
+
+  Queue.copy(DataA.data(), PtrA, Size);
+  Queue.copy(DataB.data(), PtrB, Size);
+  Queue.copy(DataC.data(), PtrC, Size);
+  Queue.copy(DataA.data(), PtrA2, Size);
+  Queue.copy(DataB.data(), PtrB2, Size);
+  Queue.copy(DataC.data(), PtrC2, Size);
+  Queue.wait_and_throw();
+
+  // Vector add to output
+  auto NodeA = add_node(Graph, Queue, [&](handler &CGH) {
+    CGH.parallel_for(range<1>(Size), [=](item<1> id) { PtrC[id] = PtrA[id]; });
+  });
+
+  // Vector add to output
+  auto NodeB = add_node(
+      Graph, Queue,
+      [&](handler &CGH) {
+        depends_on_helper(CGH, NodeA);
+        CGH.parallel_for(range<1>(Size),
+                         [=](item<1> id) { PtrC[id] += PtrB[id]; });
+      },
+      NodeA);
+
+  // Modify the output values in a host_task
+  auto NodeC = add_node(
+      Graph, Queue,
+      [&](handler &CGH) {
+        depends_on_helper(CGH, NodeB);
+        CGH.host_task([=]() {
+          for (size_t i = 0; i < Size; i++) {
+            PtrC[i] *= ModValue;
+          }
+        });
+      },
+      NodeB);
+
+  // Modify temp buffer and write to output buffer
+  auto NodeD = add_node(
+      Graph, Queue,
+      [&](handler &CGH) {
+        depends_on_helper(CGH, NodeC);
+        CGH.parallel_for(range<1>(Size), [=](item<1> id) { PtrC[id] += 1; });
+      },
+      NodeC);
+
+  // Modify temp buffer and write to output buffer
+  add_node(
+      Graph, Queue,
+      [&](handler &CGH) {
+        depends_on_helper(CGH, NodeD);
+        CGH.parallel_for(range<1>(Size),
+                         [=](item<1> id) { PtrC[id] *= PtrB[id]; });
+      },
+      NodeD);
+
+  // Vector add to output
+  auto NodeA2 = add_node(Graph, Queue, [&](handler &CGH) {
+    CGH.parallel_for(range<1>(Size),
+                     [=](item<1> id) { PtrC2[id] = PtrA2[id]; });
+  });
+
+  // Vector add to output
+  auto NodeB2 = add_node(
+      Graph, Queue,
+      [&](handler &CGH) {
+        depends_on_helper(CGH, NodeA2);
+        CGH.parallel_for(range<1>(Size),
+                         [=](item<1> id) { PtrC2[id] += PtrB2[id]; });
+      },
+      NodeA2);
+
+  // Modify the output values in a host_task
+  auto NodeC2 = add_node(
+      Graph, Queue,
+      [&](handler &CGH) {
+        depends_on_helper(CGH, NodeB2);
+        CGH.host_task([=]() {
+          for (size_t i = 0; i < Size; i++) {
+            PtrC2[i] *= ModValue;
+          }
+        });
+      },
+      NodeB2);
+
+  // Modify temp buffer and write to output buffer
+  auto NodeD2 = add_node(
+      Graph, Queue,
+      [&](handler &CGH) {
+        depends_on_helper(CGH, NodeC2);
+        CGH.parallel_for(range<1>(Size), [=](item<1> id) { PtrC2[id] += 1; });
+      },
+      NodeC2);
+
+  // Modify temp buffer and write to output buffer
+  add_node(
+      Graph, Queue,
+      [&](handler &CGH) {
+        depends_on_helper(CGH, NodeD2);
+        CGH.parallel_for(range<1>(Size),
+                         [=](item<1> id) { PtrC2[id] *= PtrB2[id]; });
+      },
+      NodeD2);
+
+  auto GraphExec = Graph.finalize();
+
+  event Event;
+  for (unsigned n = 0; n < Iterations; n++) {
+    Event = Queue.submit([&](handler &CGH) {
+      CGH.depends_on(Event);
+      CGH.ext_oneapi_graph(GraphExec);
+    });
+    Event.wait();
+  }
+  Queue.wait_and_throw();
+
+  Queue.copy(PtrC, DataC.data(), Size);
+  Queue.copy(PtrC2, Res2.data(), Size);
+  Queue.wait_and_throw();
+
+  free(PtrA, Queue);
+  free(PtrB, Queue);
+  free(PtrC, Queue);
+  free(PtrA2, Queue);
+  free(PtrB2, Queue);
+  free(PtrC2, Queue);
+
+  for (size_t i = 0; i < Size; i++) {
+    assert(check_value(i, Reference[i], DataC[i], "DataC"));
+    assert(check_value(i, Reference[i], Res2[i], "Res2"));
+  }
+
+  return 0;
+}