Avoid calling CUDA code on CPU for linear model. (#7154)

src/gbm/gblinear.cc

@@ -1,5 +1,5 @@
 /*!
- * Copyright 2014-2020 by Contributors
+ * Copyright 2014-2021 by Contributors
  * \file gblinear.cc
  * \brief Implementation of Linear booster, with L1/L2 regularization: Elastic Net
  *        the update rule is parallel coordinate descent (shotgun)
@@ -37,6 +37,17 @@ struct GBLinearTrainParam : public XGBoostParameter<GBLinearTrainParam> {
   std::string updater;
   float tolerance;
   size_t max_row_perbatch;
+
+  void CheckGPUSupport() {
+    auto n_gpus = common::AllVisibleGPUs();
+    if (n_gpus == 0 && this->updater == "gpu_coord_descent") {
+      common::AssertGPUSupport();
+      this->UpdateAllowUnknown(Args{{"updater", "coord_descent"}});
+      LOG(WARNING) << "Loading configuration on a CPU only machine.   Changing "
+                      "updater to `coord_descent`.";
+    }
+  }
+
   DMLC_DECLARE_PARAMETER(GBLinearTrainParam) {
     DMLC_DECLARE_FIELD(updater)
         .set_default("shotgun")
@@ -74,12 +85,10 @@ class GBLinear : public GradientBooster {
       model_.Configure(cfg);
     }
     param_.UpdateAllowUnknown(cfg);
+    param_.CheckGPUSupport();
     updater_.reset(LinearUpdater::Create(param_.updater, generic_param_));
     updater_->Configure(cfg);
     monitor_.Init("GBLinear");
-    if (param_.updater == "gpu_coord_descent") {
-      common::AssertGPUSupport();
-    }
   }
 
   int32_t BoostedRounds() const override {
@@ -110,6 +119,7 @@ class GBLinear : public GradientBooster {
   void LoadConfig(Json const& in) override {
     CHECK_EQ(get<String>(in["name"]), "gblinear");
     FromJson(in["gblinear_train_param"], &param_);
+    param_.CheckGPUSupport();
     updater_.reset(LinearUpdater::Create(param_.updater, generic_param_));
     this->updater_->LoadConfig(in["updater"]);
   }

src/linear/updater_gpu_coordinate.cu

@@ -28,12 +28,6 @@ DMLC_REGISTRY_FILE_TAG(updater_gpu_coordinate);
 
 class GPUCoordinateUpdater : public LinearUpdater {  // NOLINT
  public:
-  ~GPUCoordinateUpdater() {  // NOLINT
-    if (learner_param_->gpu_id >= 0) {
-      dh::safe_cuda(cudaSetDevice(learner_param_->gpu_id));
-    }
-  }
-
   // set training parameter
   void Configure(Args const& args) override {
     tparam_.UpdateAllowUnknown(args);

tests/python-gpu/load_pickle.py

@@ -19,8 +19,15 @@ class TestLoadPickle:
         assert os.environ['CUDA_VISIBLE_DEVICES'] == '-1'
         bst = load_pickle(model_path)
         x, y = build_dataset()
+        if isinstance(bst, xgb.Booster):
             test_x = xgb.DMatrix(x)
             res = bst.predict(test_x)
+        else:
+            res = bst.predict(x)
+            assert len(res) == 10
+            bst.set_params(n_jobs=1)  # triggers a re-configuration
+            res = bst.predict(x)
+
         assert len(res) == 10
 
     def test_predictor_type_is_auto(self):

tests/python-gpu/test_gpu_pickling.py

@@ -41,13 +41,7 @@ class TestPickling:
         "-s",
         "--fulltrace"]
 
-    def test_pickling(self):
+    def run_pickling(self, bst) -> None:
-        x, y = build_dataset()
-        train_x = xgb.DMatrix(x, label=y)
-        param = {'tree_method': 'gpu_hist',
-                 'verbosity': 1}
-        bst = xgb.train(param, train_x)
-
         save_pickle(bst, model_path)
         args = [
             "pytest", "--verbose", "-s", "--fulltrace",
@@ -71,6 +65,25 @@ class TestPickling:
         assert status == 0
         os.remove(model_path)
 
+    @pytest.mark.skipif(**tm.no_sklearn())
+    def test_pickling(self):
+        x, y = build_dataset()
+        train_x = xgb.DMatrix(x, label=y)
+
+        param = {'tree_method': 'gpu_hist', "gpu_id": 0}
+        bst = xgb.train(param, train_x)
+        self.run_pickling(bst)
+
+        bst = xgb.XGBRegressor(**param).fit(x, y)
+        self.run_pickling(bst)
+
+        param = {"booster": "gblinear", "updater": "gpu_coord_descent", "gpu_id": 0}
+        bst = xgb.train(param, train_x)
+        self.run_pickling(bst)
+
+        bst = xgb.XGBRegressor(**param).fit(x, y)
+        self.run_pickling(bst)
+
     @pytest.mark.mgpu
     def test_wrap_gpu_id(self):
         X, y = build_dataset()