Stepwise LR scheduler

src/lightning/pytorch/loops/training_epoch_loop.py

@@ -443,7 +443,7 @@ def _update_learning_rates(self, interval: str, update_plateau_schedulers: bool)
             if update_plateau_schedulers ^ config.reduce_on_plateau:
                 continue
 
-            current_idx = self.batch_idx if interval == "step" else trainer.current_epoch
+            current_idx = self.total_batch_idx if interval == "step" else trainer.current_epoch
             current_idx += 1  # account for both batch and epoch starts from 0
             # Take step if call to update_learning_rates matches the interval key and
             # the current step modulo the schedulers frequency is zero

tests/tests_pytorch/trainer/optimization/test_optimizers.py

@@ -12,11 +12,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 from unittest import mock
-from unittest.mock import call
+from unittest.mock import call, patch
 
 import pytest
 import torch
-from lightning.pytorch import Trainer
+from lightning.pytorch import LightningModule, Trainer
 from lightning.pytorch.callbacks import ModelCheckpoint
 from lightning.pytorch.core.optimizer import (
     _configure_optimizers,
@@ -27,6 +27,7 @@
 from lightning.pytorch.utilities.exceptions import MisconfigurationException
 from lightning.pytorch.utilities.types import LRSchedulerConfig
 from torch import optim
+from torch.utils.data import DataLoader, TensorDataset
 
 from tests_pytorch.helpers.runif import RunIf
 
@@ -657,3 +658,53 @@ def lr_scheduler_step(*_): ...
     else:
         with pytest.raises(MisconfigurationException, match="CustomScheduler` doesn't follow"):
             _init_optimizers_and_lr_schedulers(model)
+
+
+@patch("torch.optim.lr_scheduler.StepLR.step")
+def test_lr_scheduler_step_across_epoch_boundaries(mocked_sched, tmp_path):
+    class StepAcrossEpochsModel(LightningModule):
+        def __init__(self):
+            super().__init__()
+            self.layer = torch.nn.Linear(32, 2)
+
+        def forward(self, x):
+            return self.layer(x)
+
+        def training_step(self, batch, batch_idx):
+            return {"loss": torch.tensor(0.1, requires_grad=True)}
+
+        def train_dataloader(self):
+            x = torch.randn(21, 32)
+            y = torch.randn(21, 2)
+            return DataLoader(TensorDataset(x, y), batch_size=3)
+
+        def configure_optimizers(self):
+            optimizer = torch.optim.SGD(self.layer.parameters(), lr=0.1)
+            scheduler = torch.optim.lr_scheduler.StepLR(optimizer, step_size=1)
+            return {
+                "optimizer": optimizer,
+                "lr_scheduler": {
+                    "scheduler": scheduler,
+                    "interval": "step",
+                    "frequency": 5,  # Scheduler steps every 5 iterations
+                },
+            }
+
+    model = StepAcrossEpochsModel()
+
+    # Trainer configuration for cross-epoch testing
+    trainer = Trainer(
+        default_root_dir=tmp_path,
+        limit_train_batches=7,  # More than `frequency` iterations per epoch
+        max_epochs=3,  # Test across multiple epochs
+    )
+
+    # Fit the model
+    trainer.fit(model)
+
+    # Calculate the total number of steps (iterations) and expected scheduler calls
+    total_steps = 7 * 3  # Total iterations (7 batches per epoch * 3 epochs)
+    expected_steps = (total_steps - 1) // 5  # Scheduler steps every 5 iterations
+
+    # Assert that the scheduler was called the expected number of times
+    assert mocked_sched.call_count == expected_steps