test: Mark test_fp8_prefill.py as xfail on SM90

[bkryu]

📌 Description

test_fp8_prefill.py is currently failing on SM90, but consumes too much time to run/fail, causing unit-tests to time out.

--Current PR marks it as xfail so that unit tests can progress forward.--

Update: Root cause of failure is because mixed precision attention is not available on fa3 backend, but the attention prefill wrapper automatically selects backend='fa3' on SM90.

Fix is to explicitly specify the backend='fa2' so that fa2 is always used.

Status after fix:

$ pytest tests/attention/test_fp8_prefill.py
=================================================================================================================================================== test session starts ===================================================================================================================================================
...
collected 768 items                                                                                                                                                                                                                                                                                                       

tests/attention/test_fp8_prefill.py ............................................................................................................................................................................................................................................................................... [ 35%]
................................................................................................................................................................................................................................................................................................................... [ 75%]
..............................................................................................................................................................................................                                                                                                                      [100%]
======================================================================================================================================= 768 passed, 1 warning in 131.42s (0:02:11) ========================================================================================================================================

🔍 Related Issues

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Reviewer Notes

Summary by CodeRabbit

• Tests

  • Adjusted FP8/FP16 attention test configuration to explicitly select a backend during prefill/decoding, stabilizing test behavior across environments.

• Public API

  • Constructors now accept an explicit backend parameter to allow selecting the backend used for KV cache operations.

[coderabbitai]

Note

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CodeRabbit has detected other AI code review bot(s) in this pull request and will avoid duplicating their findings in the review comments. This may lead to a less comprehensive review.

Walkthrough

Adds an explicit backend="fa2" keyword to many test instantiations of BatchPrefillWithPagedKVCacheWrapper and BatchDecodeWithPagedKVCacheWrapper, and updates the constructors in the flashinfer module to accept a backend parameter.

Changes

Cohort / File(s)	Summary
Tests updated tests/attention/test_fp8_prefill.py	Adds backend="fa2" argument to constructor calls of BatchPrefillWithPagedKVCacheWrapper and BatchDecodeWithPagedKVCacheWrapper in multiple test cases (FP16 and FP8 calibration/decoding). No other test logic changed.
Public API (flashinfer) flashinfer/...	Constructor signatures updated to accept backend=None: BatchPrefillWithPagedKVCacheWrapper(workspace_buffer, kv_layout, backend=None) and BatchDecodeWithPagedKVCacheWrapper(workspace_buffer, kv_layout, backend=None).

Sequence Diagram(s)

(omitted — changes are simple argument additions and signature updates, not control-flow changes)

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

• Check constructor signature changes in the flashinfer module match all call sites.
• Verify tests pass with the explicit backend="fa2" on CI configurations and that no other places require the new parameter.

Suggested reviewers

• cyx-6
• nvmbreughe
• Anerudhan
• yzh119

Poem

🐰 I hopped through code with nimble feet,
Added a backend to make things neat,
Constructors now accept my cue,
Tests stay tidy — hop, woohoo! 🥕✨

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Title check	⚠️ Warning	The title mentions marking tests as xfail, but the actual change fixes the tests by specifying backend='fa2', which allows them to pass rather than fail.	Update the title to reflect the actual change, e.g., 'test: Fix test_fp8_prefill.py on SM90 by explicitly setting backend to fa2'

✅ Passed checks (1 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description is complete with clear explanation of the problem, root cause analysis, the applied fix, and test results confirming all 768 tests pass.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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• tests/attention/test_fp8_prefill.py (4 hunks)

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• GitHub Check: Deploy Docs

🔇 Additional comments (1)

tests/attention/test_fp8_prefill.py (1)

68-70: Verify that backend="fa2" is intentional for FP8 test isolation, or add runtime SM90 detection.

The codebase already implements SM90-aware backend selection in determine_attention_backend() (utils.py:409), which automatically selects fa3 on SM90 devices with CUDA ≥12.3 when fa3 is supported. By hardcoding backend="fa2" across all four wrapper instantiations in this test, you bypass this logic.

If this is intentional test isolation for FP8 (forcing consistent backend behavior), add an explanatory comment. Otherwise, consider:

1. Adding SM90 runtime detection and conditionally setting backend
2. Let auto-selection handle it by removing the backend parameter

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[gemini-code-assist]

Summary of Changes

Hello @bkryu, 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 implements a targeted fix for persistent unit test failures and timeouts occurring on SM90 (Hopper architecture) GPUs. By programmatically identifying the compute capability, two specific FP8 prefill tests are now marked as expected to fail, preventing them from blocking the entire test suite's execution and allowing for continued development and testing on other platforms.

Highlights

• Test Marking: Two tests within test_fp8_prefill.py are now conditionally marked as xfail (expected to fail) when executed on GPUs with SM90 compute capability.
• Utility Import: The get_compute_capability utility function from flashinfer.utils has been imported to determine the GPU's compute capability at runtime.
• Issue Mitigation: This change addresses a problem where these specific FP8 prefill tests were failing and causing unit test suites to time out on SM90 hardware.

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[gemini-code-assist]

Code Review

This pull request correctly marks failing tests in test_fp8_prefill.py as xfail on SM90 to unblock the CI pipeline. The implementation is functional, but it introduces duplicated code in two separate test functions. I have added a review comment suggesting a refactoring to improve code maintainability by centralizing the check.

[gemini-code-assist]

This logic to xfail tests on SM90 is duplicated in test_batch_decode_with_prefill_with_paged_kv_cache. To improve maintainability and adhere to the Don't Repeat Yourself (DRY) principle, consider extracting this check into a shared helper function or a custom pytest marker. This would centralize the logic, making it easier to manage and update in the future.

[coderabbitai]

Actionable comments posted: 0

♻️ Duplicate comments (1)

tests/attention/test_fp8_prefill.py (1)

139-141: Same refactoring suggestions apply here.

This block duplicates the xfail logic from lines 44-46. See the previous comment for suggested improvements regarding the device instantiation, message wording, and extracting the common logic.

🧹 Nitpick comments (1)

tests/attention/test_fp8_prefill.py (1)

44-46: Consider simplifying the device instantiation and reducing duplication.

The xfail logic correctly prevents SM90 tests from running and timing out. However, there are a few minor improvements to consider:

1. The device= keyword argument is redundant: torch.device("cuda") is sufficient
2. The message contains redundant phrasing: "currently" and "at this moment" convey the same meaning
3. This identical 3-line block is duplicated in both tests (here and lines 139-141)

Suggested improvements:

-    compute_capability = get_compute_capability(torch.device(device="cuda"))
+    compute_capability = get_compute_capability(torch.device("cuda"))
     if compute_capability[0] == 9:
-        pytest.xfail("SM90 tests are currently not passing at this moment")
+        pytest.xfail("SM90 tests are currently not passing")

To reduce duplication, consider extracting this to a pytest fixture or helper function that both tests can use:

def skip_if_sm90():
    compute_capability = get_compute_capability(torch.device("cuda"))
    if compute_capability[0] == 9:
        pytest.xfail("SM90 tests are currently not passing")

Then call skip_if_sm90() at the start of each test function.

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📒 Files selected for processing (1)

• tests/attention/test_fp8_prefill.py (3 hunks)

🧰 Additional context used

🧬 Code graph analysis (1)

tests/attention/test_fp8_prefill.py (1)

flashinfer/utils.py (1)

• get_compute_capability (252-255)

⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (1)

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🔇 Additional comments (1)

tests/attention/test_fp8_prefill.py (1)

21-21: LGTM! Import is correct.

The import of get_compute_capability from flashinfer.utils is appropriate and matches the utility function defined in the codebase.

[yzh119]

The reason of failure in this UT is that it tests mixed precision attention which is not available in FA3 template on hopper, another way to fix it is using the "fa2" backend always.

[nvmbreughe]

As @yzh119 indicated, the test fails because the mixed precision is not supported on SM90. Should we instead add a check in BatchPrefillWithPagedKVCacheWrapper for FP8 and SM90, and mark the test as skip for fp8?

[bkryu]

Based on the comments above, explicitly specifying backend='fa2' which should be supported instead of skipping seems like a better idea. Made this change.

[bkryu]

The reason of failure in this UT is that it tests mixed precision attention which is not available in FA3 template on hopper, another way to fix it is using the "fa2" backend always.

Thanks @yzh119, now this makes sense. I have now explicitly specified backend='fa2' instead of xfailing the tests.

[bkryu]

/bot run

[flashinfer-bot]

GitLab MR !110 has been created, and the CI pipeline #37891663 is currently running. I'll report back once the pipeline job completes.