add FLAGS_enable_api_kernel_fallback (#44706)

* add FLAGS_enable_api_kernel_fallback

* deal with more cases

* add ut for coverage

paddle/fluid/platform/flags.cc

@@ -184,6 +184,19 @@ PADDLE_DEFINE_EXPORTED_string(
     "please refer to the documents");
 #endif
 
+/*
+ * Kernel related FLAG
+ * Name: FLAGS_enable_api_kernel_fallback
+ * Since Version: 2.4
+ * Value Range: bool, default=true
+ * Example: FLAGS_enable_api_kernel_fallback=true would allow kernel of current
+ * backend fallback to CPU one when not found
+ */
+PADDLE_DEFINE_EXPORTED_bool(
+    enable_api_kernel_fallback,
+    true,
+    "Whether enable api kernel fallback to CPU one when not found");
+
 #if defined(PADDLE_WITH_CUDA) || defined(PADDLE_WITH_HIP)
 /**
  * CUDNN related FLAG

paddle/phi/core/kernel_factory.cc

@@ -17,6 +17,8 @@
 #include "glog/logging.h"
 #include "paddle/phi/core/enforce.h"
 
+DECLARE_bool(enable_api_kernel_fallback);
+
 namespace phi {
 
 const static Kernel empty_kernel;  // NOLINT
@@ -120,8 +122,15 @@ KernelResult KernelFactory::SelectKernelOrThrowError(
     kernel_iter = iter->second.find(any_layout_kernel_key);
   }
 
-  bool has_fallback_cpu = false;
-  if (kernel_iter == iter->second.end()) {
+  PADDLE_ENFORCE_NE(
+      kernel_iter == iter->second.end() && kernel_key.backend() == Backend::CPU,
+      true,
+      phi::errors::NotFound(
+          "The kernel with key %s of kernel `%s` is not registered.",
+          kernel_key,
+          kernel_name));
+
+  if (FLAGS_enable_api_kernel_fallback && kernel_iter == iter->second.end()) {
     // Fallback CPU backend
     phi::KernelKey cpu_kernel_key(
         phi::Backend::CPU, kernel_key.layout(), kernel_key.dtype());
@@ -132,18 +141,35 @@ KernelResult KernelFactory::SelectKernelOrThrowError(
           phi::Backend::CPU, phi::DataLayout::ALL_LAYOUT, kernel_key.dtype());
       kernel_iter = iter->second.find(any_layout_kernel_key);
     }
-    has_fallback_cpu = true;
+
+    PADDLE_ENFORCE_NE(
+        kernel_iter,
+        iter->second.end(),
+        phi::errors::NotFound(
+            "The kernel with key %s of kernel `%s` is not registered and"
+            " fail to fallback to CPU one.",
+            kernel_key,
+            kernel_name));
+
+    VLOG(3) << "missing " << kernel_key.backend() << " kernel: " << kernel_name
+            << ", expected_kernel_key:" << kernel_key
+            << ", fallbacking to CPU one!";
+
+    return {kernel_iter->second, true};
   }
 
   PADDLE_ENFORCE_NE(
       kernel_iter,
       iter->second.end(),
       phi::errors::NotFound(
-          "The kernel with key %s of kernel `%s` is not registered.",
+          "The kernel with key %s of kernel `%s` is not registered and"
+          " the current value of FLAGS_enable_api_kernel_fallback(bool,"
+          " default true) is false. If you want to fallback this kernel"
+          " to CPU one, please set the flag true before run again.",
           kernel_key,
           kernel_name));
 
-  return {kernel_iter->second, has_fallback_cpu};
+  return {kernel_iter->second, false};
 }
 
 const KernelArgsDef& KernelFactory::GetFirstKernelArgsDef(

python/paddle/fluid/tests/custom_runtime/test_custom_cpu_plugin.py

@@ -40,6 +40,7 @@ class TestCustomCPUPlugin(unittest.TestCase):
             self._test_custom_device_mnist()
             self._test_eager_backward_api()
             self._test_eager_copy_to()
+            self._test_fallback_kernel()
         self._test_custom_device_dataloader()
         self._test_custom_device_mnist()
 
@@ -160,6 +161,15 @@ class TestCustomCPUPlugin(unittest.TestCase):
         self.assertTrue(np.array_equal(another_custom_cpu_tensor, x))
         self.assertTrue(another_custom_cpu_tensor.place.is_custom_place())
 
+    def _test_fallback_kernel(self):
+        # using (custom_cpu, add, int16) which is not registered
+        import paddle
+        r = np.array([6, 6, 6], 'int16')
+        x = paddle.to_tensor([5, 4, 3], 'int16')
+        y = paddle.to_tensor([1, 2, 3], 'int16')
+        z = paddle.add(x, y)
+        self.assertTrue(np.array_equal(z, r))
+
     def tearDown(self):
         del os.environ['CUSTOM_DEVICE_ROOT']