[Fix] use `torch.inference_mode()` instead of `torch.no_grad()` (#4372)

python/sglang/srt/managers/scheduler.py

@@ -101,6 +101,7 @@ from sglang.srt.server_args import PortArgs, ServerArgs
 from sglang.srt.speculative.spec_info import SpeculativeAlgorithm
 from sglang.srt.torch_memory_saver_adapter import TorchMemorySaverAdapter
 from sglang.srt.utils import (
+    DynamicGradMode,
     broadcast_pyobj,
     configure_logger,
     crash_on_warnings,
@@ -487,7 +488,7 @@ class Scheduler(SchedulerOutputProcessorMixin):
                 },
             )
 
-    @torch.no_grad()
+    @DynamicGradMode()
     def event_loop_normal(self):
         """A normal scheduler loop."""
         while True:
@@ -507,7 +508,7 @@ class Scheduler(SchedulerOutputProcessorMixin):
 
             self.last_batch = batch
 
-    @torch.no_grad()
+    @DynamicGradMode()
     def event_loop_overlap(self):
         """A scheduler loop that overlaps the CPU processing and GPU computation."""
         self.result_queue = deque()

python/sglang/srt/managers/tp_worker_overlap_thread.py

@@ -33,7 +33,7 @@ from sglang.srt.managers.io_struct import (
 from sglang.srt.managers.schedule_batch import ModelWorkerBatch
 from sglang.srt.managers.tp_worker import TpModelWorker
 from sglang.srt.server_args import ServerArgs
-from sglang.srt.utils import get_compiler_backend
+from sglang.srt.utils import DynamicGradMode, get_compiler_backend
 from sglang.utils import get_exception_traceback
 
 logger = logging.getLogger(__name__)
@@ -115,7 +115,7 @@ class TpModelWorkerClient:
             logger.error(f"TpModelWorkerClient hit an exception: {traceback}")
             self.parent_process.send_signal(signal.SIGQUIT)
 
-    @torch.no_grad()
+    @DynamicGradMode()
     def forward_thread_func_(self):
         batch_pt = 0
         batch_lists = [None] * 2

python/sglang/srt/utils.py

@@ -61,6 +61,7 @@ from torch import nn
 from torch.func import functional_call
 from torch.library import Library
 from torch.profiler import ProfilerActivity, profile, record_function
+from torch.utils._contextlib import _DecoratorContextManager
 from torch.utils.cpp_extension import CUDA_HOME
 from triton.runtime.cache import (
     FileCacheManager,
@@ -127,6 +128,63 @@ def is_cuda_available():
     return is_cuda()
 
 
+_ENABLE_TORCH_INFERENCE_MODE = os.getenv(
+    "SGLANG_ENABLE_TORCH_INFERENCE_MODE", "false"
+).lower() in ("true", "1")
+
+
+class DynamicGradMode(_DecoratorContextManager):
+    """
+    A combination of torch.no_grad and torch.inference_mode,
+    with their behavior controlled by an environment variable. Just refer to them.
+    """
+
+    @staticmethod
+    def set_inference_mode(mode: bool):
+        if isinstance(mode, bool):
+            global _ENABLE_TORCH_INFERENCE_MODE
+
+            _ENABLE_TORCH_INFERENCE_MODE = mode
+        else:
+            logger.warning("mode is not a boolean object")
+
+    def __init__(self, mode=True):
+        if not torch._jit_internal.is_scripting():
+            super().__init__()
+        if _ENABLE_TORCH_INFERENCE_MODE:
+            self.mode = mode
+        else:
+            self.prev = False
+
+    def __new__(cls, mode_or_orig_func=True if _ENABLE_TORCH_INFERENCE_MODE else None):
+        if mode_or_orig_func is None or isinstance(mode_or_orig_func, bool):
+            return super().__new__(cls)
+        return cls()(mode_or_orig_func)
+
+    def __enter__(self) -> None:
+        if _ENABLE_TORCH_INFERENCE_MODE:
+            self._inference_mode_context = torch._C._InferenceMode(self.mode)
+            self._inference_mode_context.__enter__()
+        else:
+            self.prev = torch.is_grad_enabled()
+            torch.set_grad_enabled(False)
+
+    def __exit__(self, exc_type: Any, exc_value: Any, traceback: Any) -> None:
+        if _ENABLE_TORCH_INFERENCE_MODE:
+            self._inference_mode_context.__exit__(exc_type, exc_value, traceback)
+        else:
+            torch.set_grad_enabled(self.prev)
+
+    def clone(self) -> "DynamicGradMode":
+        r"""
+        Create a copy of this class
+        """
+        if _ENABLE_TORCH_INFERENCE_MODE:
+            return self.__class__(self.mode)
+        else:
+            return self.__class__()
+
+
 def enable_show_time_cost():
     global show_time_cost
     show_time_cost = True

python/sglang/test/test_dynamic_grad_mode.py

+import unittest
+
+import torch
+
+from sglang.srt.utils import DynamicGradMode
+
+
+class TestDynamicGradMode(unittest.TestCase):
+    def test_inference(self):
+        # Test inference_mode
+        DynamicGradMode.set_inference_mode(True)
+
+        @DynamicGradMode()
+        def create_tensor_x():
+            return torch.empty(0)
+
+        X = create_tensor_x()
+        self.assertTrue(not X.requires_grad and X.is_inference())
+
+    def test_no_grad(self):
+        # Test no_grad
+        DynamicGradMode.set_inference_mode(False)
+
+        @DynamicGradMode()
+        def create_tensor_y():
+            return torch.empty(0)
+
+        Y = create_tensor_y()
+        self.assertTrue(not Y.requires_grad and not Y.is_inference())
+
+    def test_nested_inference(self):
+        # Test no_grad nested inference_mode, inference_mode should has higher priority
+        DynamicGradMode.set_inference_mode(False)
+
+        @DynamicGradMode()
+        def create_tensor_z():
+            with torch.inference_mode():
+                return torch.empty(0)
+
+        Z = create_tensor_z()
+        self.assertTrue(not Z.requires_grad and Z.is_inference())
+
+    def test_nested_no_grad(self):
+        # Test inference_mode nested no_grad, inference_mode should has higher priority
+        DynamicGradMode.set_inference_mode(True)
+
+        @DynamicGradMode()
+        def create_tensor_w():
+            with torch.no_grad():
+                return torch.empty(0)
+
+        W = create_tensor_w()
+        self.assertTrue(not W.requires_grad and W.is_inference())
+
+
+if __name__ == "__main__":
+    unittest.main(verbosity=2)