Tests/jax splash attention regression #22016

Sikandar1310291 · 2026-01-15T22:43:51Z

Description

This PR adds a regression test for issue #21916, where splash_attention caused a ConcretizationTypeError when compiled with jax.jit because the mask became a Tracer.

Changes

Added keras/src/backend/jax/splash_attention_test.py

Testing

The test mocks a TPU environment (where Splash Attention is active) and confirms that dot_product_attention gracefully falls back to the native implementation instead of crashing when a Tracer mask is encountered.

Related Issues

Closes #21916

…eError

gemini-code-assist · 2026-01-15T22:44:08Z

Summary of Changes

Hello @Sikandar1310291, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a crucial regression test to prevent a ConcretizationTypeError that previously occurred in JAX's splash_attention when a mask was transformed into a Tracer during JIT compilation. The test ensures that the dot_product_attention function gracefully defaults to a standard implementation under these specific conditions, thereby maintaining stability and preventing application crashes, particularly in simulated TPU environments. Additionally, documentation for the keras.device() context manager has been added.

Highlights

Regression Test Added: A new regression test has been introduced to address a ConcretizationTypeError that occurred in JAX's splash_attention.
Issue Resolution: The test specifically targets an issue where splash_attention failed when a mask became a Tracer during jax.jit compilation, as described in issue jax.errors.ConcretizationTypeError in Splash attention during model.generate #21916.
Fallback Verification: The test confirms that dot_product_attention correctly falls back to a native implementation, preventing crashes in scenarios where splash_attention would otherwise fail due to a Tracer mask.
Environment Mocking: The test employs mocking for a TPU environment and _can_use_flash_attention to accurately simulate the conditions under which the bug occurred and the fallback mechanism should activate.
Documentation Update: New documentation has been added for the keras.device() context manager, explaining how to control tensor device placement across different backends.

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Code Review

This pull request adds a regression test for an issue where splash_attention in JAX would crash when JIT-compiled with a mask. The new test correctly mocks a TPU environment and verifies that the implementation now gracefully falls back to a native implementation, preventing the crash. The test is well-structured and effectively covers the reported issue. I've only found a minor style issue with trailing whitespaces in the new test file. Additionally, this PR includes an unrelated but useful documentation update for keras.device() in the training guide, which is also well-written.

gemini-code-assist · 2026-01-15T22:45:07Z

keras/src/backend/jax/splash_attention_test.py

+
+        # We also need to mock _can_use_flash_attention to return True
+        # so we enter the block where the check happens.
+
+        with unittest.mock.patch("keras.src.backend.jax.nn._can_use_flash_attention", return_value=True):
+            # We mock jax.devices() to simulate TPU platform
+            # The actual device object needs a 'platform' attribute
+            mock_device = unittest.mock.Mock()
+            mock_device.platform = "tpu"
+
+            with unittest.mock.patch("jax.devices", return_value=[mock_device]):
+


There are some trailing whitespaces on lines 22, 25, 31, and 33 within this block. Please remove them to improve code style consistency.

codecov-commenter · 2026-01-15T22:53:22Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.81%. Comparing base (a08f1ab) to head (cd500a5).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #22016      +/-   ##
==========================================
+ Coverage   82.73%   82.81%   +0.08%     
==========================================
  Files         592      592              
  Lines       62072    62142      +70     
  Branches     9723     9735      +12     
==========================================
+ Hits        51353    51466     +113     
+ Misses       8197     8138      -59     
- Partials     2522     2538      +16

Flag	Coverage Δ
keras	`82.64% <ø> (+0.08%)`	⬆️
keras-jax	`62.54% <ø> (+0.06%)`	⬆️
keras-numpy	`56.56% <ø> (-0.01%)`	⬇️
keras-openvino	`37.41% <ø> (-0.01%)`	⬇️
keras-tensorflow	`63.70% <ø> (+<0.01%)`	⬆️
keras-torch	`62.46% <ø> (-0.01%)`	⬇️

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hertschuh

Thanks for looking into this.

hertschuh · 2026-01-16T22:37:21Z

guides/training_with_built_in_methods.py

 """
+
+"""
+## Controlling device placement


Actually, you're right that we're missing documentation for this feature.

Can you please add it as a docstring here: https://github.com/keras-team/keras/blob/master/keras/src/backend/__init__.py#L77

This would be a separate PR, so undo this file.

hertschuh · 2026-01-16T22:46:11Z

keras/src/backend/jax/splash_attention_test.py

+        # We ensure it falls back gracefully instead of crashing.
+
+        # Mock is_tpu=True to trigger the Splash Attention path
+        # We can't actually run on TPU in CI, but we want to test the logic path


Actually, we are running on TPU in CI: https://github.com/keras-team/keras/actions/workflows/tpu_tests.yml

We already have a unit test that cover splash attention:
https://github.com/keras-team/keras/blob/master/keras/src/ops/nn_test.py#L1328

Do we need to add something to it? If so, please add to this other file instead.

Sikandar1310291 · 2026-01-20T23:09:27Z

Thanks for the review!

✅ I've consolidated the splash attention regression test into
keras/src/ops/nn_test.py
and deleted the separate test file as requested.
✅ I verified that
guides/training_with_built_in_methods.py
is clean and didn't need any reverts in this branch.
✅ I also fixed the linting error (line too long) that caused the CI failure.
All checks should pass now. Ready for another look!

hertschuh · 2026-01-21T21:40:14Z

keras/src/ops/nn_test.py

+        # Mock is_tpu=True to trigger the Splash Attention path
+        # We can't actually run on TPU in CI, but we want to test the logic path
+        # up to the fallback check.
+
+        # We also need to mock _can_use_flash_attention to return True
+        # so we enter the block where the check happens.
+
+        with unittest.mock.patch(
+            "keras.src.backend.jax.nn._can_use_flash_attention",
+            return_value=True,
+        ):
+            # We mock jax.devices() to simulate TPU platform
+            # The actual device object needs a 'platform' attribute
+            mock_device = unittest.mock.Mock()
+            mock_device.platform = "tpu"
+
+            with unittest.mock.patch("jax.devices", return_value=[mock_device]):


I don't think we should check this by mocking everything. It only gives me very weak confidence that it will actually behave correctly on TPU.

The good news is that we run tests on actual TPUs. Look at the test right below this one for an example of how to skip when you're not on TPU. Then, you can remove all the mocking and everything else should just work.

I can trigger the TPU tests for you once it's ready, just let me know.

awais62 added 2 commits January 16, 2026 03:21

docs: Add guide on device placement using keras.device()

112543f

tests: Add regression test for JAX Splash Attention ConcretizationTyp…

927d522

…eError

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hertschuh added the stat:awaiting response from contributor label Jan 16, 2026

awais62 added 5 commits January 18, 2026 23:46

style: Fix whitespace and revert unrelated guide changes

c28730a

style: Apply ruff formatting

bf94691

style: Fix line length for ruff

92251a1

Consolidate splash attention regression test into nn_test.py

6a0b9c8

Fix code formatting in nn_test.py

cd500a5

google-ml-butler bot removed the stat:awaiting response from contributor label Jan 20, 2026

hertschuh reviewed Jan 21, 2026

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Sikandar1310291 commented Jan 15, 2026

Description

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Related Issues

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