Support ddp_fork strategy with native AMP by attempting NVML-based CUDA availability assessment

src/lightning_lite/accelerators/cuda.py

@@ -13,8 +13,9 @@
 # limitations under the License.
 import os
 import warnings
+from contextlib import contextmanager
 from functools import lru_cache
-from typing import Dict, List, Optional, Set, Union
+from typing import Dict, Generator, List, Optional, Set, Union
 
 import torch
 
@@ -77,6 +78,23 @@ def _get_all_available_cuda_gpus() -> List[int]:
     return list(range(num_cuda_devices()))
 
 
+@contextmanager
+def _patch_cuda_is_available() -> Generator:
+    """Context manager that safely patches :func:`torch.cuda.is_available` with its NVML-based version if
+    possible."""
+    if hasattr(torch._C, "_cuda_getDeviceCount") and _device_count_nvml() >= 0:
+        # we can safely patch is_available if both torch has CUDA compiled and the NVML count is succeeding
+        # otherwise, patching is_available could lead to attribute errors or infinite recursion
+        orig_check = torch.cuda.is_available
+        torch.cuda.is_available = is_cuda_available
+        try:
+            yield
+        finally:
+            torch.cuda.is_available = orig_check
+    else:
+        yield
+
+
 @lru_cache(1)
 def num_cuda_devices() -> int:
     """Returns the number of available CUDA devices.

src/lightning_lite/lite.py

@@ -26,9 +26,9 @@
 from torch.optim import Optimizer
 from torch.utils.data import BatchSampler, DataLoader, DistributedSampler
 
+from lightning_lite.plugins import Precision  # avoid circular imports: # isort: split
 from lightning_lite.accelerators.accelerator import Accelerator
 from lightning_lite.connector import _Connector, _PLUGIN_INPUT, _PRECISION_INPUT
-from lightning_lite.plugins import Precision
 from lightning_lite.strategies import DeepSpeedStrategy, Strategy, XLAStrategy
 from lightning_lite.strategies.strategy import TBroadcast
 from lightning_lite.utilities import move_data_to_device

src/lightning_lite/plugins/precision/native_amp.py

@@ -20,6 +20,7 @@
 from torch.optim import LBFGS
 from typing_extensions import Literal
 
+from lightning_lite.accelerators.cuda import _patch_cuda_is_available
 from lightning_lite.plugins.precision.precision import Precision
 from lightning_lite.plugins.precision.utils import _convert_fp_tensor
 from lightning_lite.utilities.imports import _TORCH_GREATER_EQUAL_1_10
@@ -47,7 +48,9 @@ def __init__(
         if precision == "bf16" and not _TORCH_GREATER_EQUAL_1_10:
             raise ImportError("To use bfloat16 with native amp you must install torch greater or equal to 1.10.")
         if scaler is None and precision == 16:
-            scaler = torch.cuda.amp.GradScaler()
+            with _patch_cuda_is_available():
+                # if possible, we defer CUDA initialization to support strategies that will attempt forks
+                scaler = torch.cuda.amp.GradScaler()
         if scaler is not None and precision == "bf16":
             raise ValueError(f"`precision='bf16'` does not use a scaler, found {scaler}.")
         self.precision = precision

src/pytorch_lightning/CHANGELOG.md

@@ -9,6 +9,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Added
 
+- Added native AMP support for `ddp_fork` (and associated alias strategies) with CUDA GPUs ([#14983](https://github.com/Lightning-AI/lightning/pull/14983))
+
+
 - Added `BatchSizeFinder` callback ([#11089](https://github.com/PyTorchLightning/pytorch-lightning/pull/11089))
 
 

src/pytorch_lightning/plugins/precision/native_amp.py

@@ -19,6 +19,7 @@
 from torch.optim import LBFGS
 
 import pytorch_lightning as pl
+from lightning_lite.accelerators.cuda import _patch_cuda_is_available
 from lightning_lite.utilities.types import Optimizable
 from pytorch_lightning.plugins.precision.precision_plugin import PrecisionPlugin
 from pytorch_lightning.utilities import _TORCH_GREATER_EQUAL_1_10, AMPType
@@ -50,7 +51,9 @@ def __init__(
                 "To use bfloat16 with native amp you must install torch greater or equal to 1.10."
             )
         if scaler is None and precision == 16:
-            scaler = torch.cuda.amp.GradScaler()
+            with _patch_cuda_is_available():
+                # if possible, we defer CUDA initialization to support strategies that will attempt forks
+                scaler = torch.cuda.amp.GradScaler()
         if scaler is not None and precision == "bf16":
             raise MisconfigurationException(f"`precision='bf16'` does not use a scaler, found {scaler}.")
         self.precision = precision

tests/tests_lite/helpers/runif.py

@@ -21,6 +21,7 @@
 from pkg_resources import get_distribution
 
 from lightning_lite.accelerators import TPUAccelerator
+from lightning_lite.accelerators.cuda import num_cuda_devices
 from lightning_lite.accelerators.mps import MPSAccelerator
 from lightning_lite.strategies.deepspeed import _DEEPSPEED_AVAILABLE
 from lightning_lite.strategies.fairscale import _FAIRSCALE_AVAILABLE
@@ -74,7 +75,7 @@ def __new__(
         reasons = []
 
         if min_cuda_gpus:
-            conditions.append(torch.cuda.device_count() < min_cuda_gpus)
+            conditions.append(num_cuda_devices() < min_cuda_gpus)
             reasons.append(f"GPUs>={min_cuda_gpus}")
             # used in conftest.py::pytest_collection_modifyitems
             kwargs["min_cuda_gpus"] = True

tests/tests_pytorch/helpers/runif.py

@@ -20,6 +20,7 @@
 from packaging.version import Version
 from pkg_resources import get_distribution
 
+from lightning_lite.accelerators.cuda import num_cuda_devices
 from lightning_lite.strategies.fairscale import _FAIRSCALE_AVAILABLE
 from pytorch_lightning.accelerators.mps import MPSAccelerator
 from pytorch_lightning.accelerators.tpu import TPUAccelerator
@@ -124,7 +125,7 @@ def __new__(
         reasons = []
 
         if min_cuda_gpus:
-            conditions.append(torch.cuda.device_count() < min_cuda_gpus)
+            conditions.append(num_cuda_devices() < min_cuda_gpus)
             reasons.append(f"GPUs>={min_cuda_gpus}")
             # used in conftest.py::pytest_collection_modifyitems
             kwargs["min_cuda_gpus"] = True

tests/tests_pytorch/models/test_ddp_fork_amp.py

@@ -0,0 +1,30 @@
+# Copyright The PyTorch Lightning team.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import multiprocessing
+
+import torch
+
+from pytorch_lightning.plugins import NativeMixedPrecisionPlugin
+from tests_pytorch.helpers.runif import RunIf
+
+
+# needs to be standalone to avoid other processes initializing CUDA
+@RunIf(min_cuda_gpus=1, skip_windows=True, standalone=True)
+def test_amp_gpus_ddp_fork():
+    """Ensure the use of native AMP with `ddp_fork` (or associated alias strategies) does not generate CUDA
+    initialization errors."""
+    _ = NativeMixedPrecisionPlugin(precision=16, device="cuda")
+    with multiprocessing.get_context("fork").Pool(1) as pool:
+        in_bad_fork = pool.apply(torch.cuda._is_in_bad_fork)
+    assert not in_bad_fork