[RPC] Fix the multihop cpu case (#5522)

src/runtime/rpc/rpc_endpoint.cc

@@ -390,20 +390,18 @@ class RPCEndpoint::EventHandler : public dmlc::Stream {
     size_t elem_bytes = (type_hint.bits * type_hint.lanes + 7) / 8;
 
     char* data_ptr;
+    auto* sess = GetServingSession();
 
-    if (ctx.device_type == kDLCPU) {
+    // When session is local, we can directly treat handle
+    // as the cpu pointer without allocating a temp space.
+    if (ctx.device_type == kDLCPU &&
+        sess->IsLocalSession() &&
+        DMLC_IO_NO_ENDIAN_SWAP) {
       data_ptr = reinterpret_cast<char*>(handle) + offset;
-      // endian aware handling
-      if (!DMLC_IO_NO_ENDIAN_SWAP) {
-        char* temp = this->ArenaAlloc<char>(num_bytes);
-        std::memcpy(temp, data_ptr, num_bytes);
-        dmlc::ByteSwap(temp, elem_bytes, num_bytes / elem_bytes);
-        data_ptr = temp;
-      }
     } else {
       try {
         data_ptr = this->ArenaAlloc<char>(num_bytes);
-        GetServingSession()->CopyFromRemote(
+        sess->CopyFromRemote(
             reinterpret_cast<void*>(handle), offset,
             data_ptr, 0,
             num_bytes, ctx, type_hint);
@@ -440,8 +438,11 @@ class RPCEndpoint::EventHandler : public dmlc::Stream {
     this->Read(&type_hint);
 
     size_t elem_bytes = (type_hint.bits * type_hint.lanes + 7) / 8;
+    auto* sess = GetServingSession();
 
-    if (ctx.device_type == kDLCPU) {
+    // When session is local, we can directly treat handle
+    // as the cpu pointer without allocating a temp space.
+    if (ctx.device_type == kDLCPU && sess->IsLocalSession()) {
        char* dptr = reinterpret_cast<char*>(handle) + offset;
        this->ReadArray(dptr, num_bytes);
 
@@ -457,7 +458,7 @@ class RPCEndpoint::EventHandler : public dmlc::Stream {
       }
 
       try {
-        GetServingSession()->CopyToRemote(
+        sess->CopyToRemote(
             temp_data, 0,
             reinterpret_cast<void*>(handle), offset,
             num_bytes, ctx, type_hint);
@@ -1046,6 +1047,10 @@ class RPCClientSession : public RPCSession,
     return this;
   }
 
+  bool IsLocalSession() const final {
+    return false;
+  }
+
  private:
   std::shared_ptr<RPCEndpoint> endpoint_;
 };

src/runtime/rpc/rpc_local_session.h

@@ -68,6 +68,10 @@ class LocalSession : public RPCSession {
 
   DeviceAPI* GetDeviceAPI(TVMContext ctx, bool allow_missing = false) final;
 
+  bool IsLocalSession() const final {
+    return true;
+  }
+
  protected:
   /*!
    * \brief Internal implementation of GetFunction.

src/runtime/rpc/rpc_session.h

@@ -177,6 +177,18 @@ class RPCSession {
    */
   virtual DeviceAPI* GetDeviceAPI(TVMContext ctx, bool allow_missing = false) = 0;
 
+  /*!
+   * \brief Whether the session is a local session and we can directly
+   *        the data handle returned by the session and treat it as pointer
+   *        to the local memory.
+   *
+   * This information is useful for RPC server to directly copy into the
+   * local memory without creating a temporary buffer.
+   *
+   * \return Whether it is a local session.
+   */
+  virtual bool IsLocalSession() const = 0;
+
   /*!
    * \return The session table index of the session.
    */

tests/python/unittest/test_runtime_rpc.py

@@ -167,8 +167,13 @@ def test_rpc_remote_module():
     B = te.compute(A.shape, lambda *i: A(*i) + 1.0, name='B')
     s = te.create_schedule(B.op)
 
-    server = rpc.Server("localhost")
-    client = rpc.connect(server.host, server.port)
+    server0 = rpc.Server("localhost", key="x0")
+    server1 = rpc.Server("localhost", key="x1")
+
+    client = rpc.connect(
+        server0.host, server0.port, key="x0",
+        session_constructor_args=[
+        "rpc.Connect", server1.host, server1.port, "x1"])
 
     def check_remote(remote):
         if not tvm.runtime.enabled("llvm"):