[fix] bugfix 2856: Fix pre-allocated out shape check in trtllm_batch_decode_with_kv_cache_mla for q_len_per_req > 1#2876
[fix] bugfix 2856: Fix pre-allocated out shape check in trtllm_batch_decode_with_kv_cache_mla for q_len_per_req > 1#2876qsang-nv wants to merge 2 commits intoflashinfer-ai:mainfrom
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Signed-off-by: Qidi Sang <200703406+qsang-nv@users.noreply.github.com>
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Summary of ChangesHello, 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 resolves a critical bug in the Highlights
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Code Review
This pull request correctly fixes a bug in trtllm_batch_decode_with_kv_cache_mla where the shape check for a pre-allocated output tensor was incorrect for multi-token generation (q_len_per_req > 1). The fix unifies the output shape calculation for both the out=None and pre-allocated out cases, which also improves code clarity by removing duplication. A new test case has been added to verify the fix, ensuring that both paths produce identical results. The changes look good. I have one minor suggestion in the new test file to improve maintainability by reducing code duplication.
| global global_trtllm_gen_fmha_workspace_buffer | ||
| if global_trtllm_gen_fmha_workspace_buffer is None: | ||
| global_trtllm_gen_fmha_workspace_buffer = torch.zeros( | ||
| workspace_size, | ||
| dtype=torch.int8, | ||
| device=device, | ||
| ) | ||
| workspace = global_trtllm_gen_fmha_workspace_buffer |
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This workspace buffer initialization logic is duplicated from the trtllm_batch_decode_mla helper function in this file (lines 324-328). To improve maintainability and reduce code duplication, consider creating a pytest fixture to provide the workspace buffer. This would encapsulate the global variable and its initialization logic, making the tests cleaner.
For example, you could add a fixture like this:
@pytest.fixture(scope="module")
def trtllm_gen_fmha_workspace(device="cuda:0"):
"""Provides a zero-initialized workspace buffer for trtllm-gen MLA tests."""
global global_trtllm_gen_fmha_workspace_buffer
if global_trtllm_gen_fmha_workspace_buffer is None:
global_trtllm_gen_fmha_workspace_buffer = torch.zeros(
workspace_size, dtype=torch.int8, device=device
)
return global_trtllm_gen_fmha_workspace_bufferAnd then use it in the test signature.
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🧹 Nitpick comments (1)
tests/attention/test_trtllm_gen_mla.py (1)
887-903: Assert the preallocated buffer is actually reused.Right now the test verifies shape/value equivalence, but it does not guarantee zero extra allocation on the
out=path. Add a pointer check so the regression guard also enforces buffer reuse.Proposed test hardening
result_pre = flashinfer.decode.trtllm_batch_decode_with_kv_cache_mla( query=query, kv_cache=kv_cache, workspace_buffer=workspace, qk_nope_head_dim=qk_nope_head_dim, kv_lora_rank=kv_lora_rank, qk_rope_head_dim=qk_rope_head_dim, block_tables=block_tables, seq_lens=seq_lens, max_seq_len=max_seq_len, out=out, bmm1_scale=bmm1_scale, bmm2_scale=1.0, backend="trtllm-gen", ) + assert result_pre.data_ptr() == out.data_ptr(), "Expected kernel to write into provided `out` tensor" assert result_pre.shape == expected_shape torch.testing.assert_close(result_none, result_pre, rtol=1e-3, atol=1e-3)🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed. In `@tests/attention/test_trtllm_gen_mla.py` around lines 887 - 903, The test currently checks shape and values but not that the provided preallocated buffer is reused; after calling flashinfer.decode.trtllm_batch_decode_with_kv_cache_mla(...) with the out= parameter, assert that the returned tensor reuses the same memory by comparing pointers (e.g. result_pre.data_ptr() == out.data_ptr()); keep the existing torch.testing.assert_close but add this pointer equality assertion (or capture out_ptr before the call and compare to result_pre.data_ptr() after) to enforce zero extra allocation.
🤖 Prompt for all review comments with AI agents
Verify each finding against the current code and only fix it if needed.
Nitpick comments:
In `@tests/attention/test_trtllm_gen_mla.py`:
- Around line 887-903: The test currently checks shape and values but not that
the provided preallocated buffer is reused; after calling
flashinfer.decode.trtllm_batch_decode_with_kv_cache_mla(...) with the out=
parameter, assert that the returned tensor reuses the same memory by comparing
pointers (e.g. result_pre.data_ptr() == out.data_ptr()); keep the existing
torch.testing.assert_close but add this pointer equality assertion (or capture
out_ptr before the call and compare to result_pre.data_ptr() after) to enforce
zero extra allocation.
ℹ️ Review info
⚙️ Run configuration
Configuration used: defaults
Review profile: CHILL
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Run ID: 912c5542-2ff0-4ad2-98e9-ce6f7a04f0f8
📒 Files selected for processing (2)
flashinfer/mla.pytests/attention/test_trtllm_gen_mla.py
Signed-off-by: Qidi Sang <200703406+qsang-nv@users.noreply.github.com>
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🧹 Nitpick comments (1)
tests/attention/test_trtllm_gen_mla.py (1)
856-864: Consider zeroing shared workspace on reuse to avoid test-order coupling.At Line 857, the global buffer is only zero-initialized on first allocation. If a prior test mutated it, this test can become order-dependent. Re-zeroing on reuse makes this test self-contained.
Proposed tweak
global global_trtllm_gen_fmha_workspace_buffer if global_trtllm_gen_fmha_workspace_buffer is None: global_trtllm_gen_fmha_workspace_buffer = torch.zeros( workspace_size, dtype=torch.int8, device=device, ) + else: + global_trtllm_gen_fmha_workspace_buffer.zero_() workspace = global_trtllm_gen_fmha_workspace_buffer🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed. In `@tests/attention/test_trtllm_gen_mla.py` around lines 856 - 864, The global shared buffer global_trtllm_gen_fmha_workspace_buffer may contain leftovers from prior tests; when reusing it (after the existing allocation check using workspace_size and device), explicitly zero it before assigning workspace (e.g., call its in-place zeroing method such as zero_() or fill_(0) on global_trtllm_gen_fmha_workspace_buffer) so the test does not depend on prior test mutations.
🤖 Prompt for all review comments with AI agents
Verify each finding against the current code and only fix it if needed.
Nitpick comments:
In `@tests/attention/test_trtllm_gen_mla.py`:
- Around line 856-864: The global shared buffer
global_trtllm_gen_fmha_workspace_buffer may contain leftovers from prior tests;
when reusing it (after the existing allocation check using workspace_size and
device), explicitly zero it before assigning workspace (e.g., call its in-place
zeroing method such as zero_() or fill_(0) on
global_trtllm_gen_fmha_workspace_buffer) so the test does not depend on prior
test mutations.
ℹ️ Review info
⚙️ Run configuration
Configuration used: defaults
Review profile: CHILL
Plan: Pro
Run ID: 125aae67-88e7-4de3-ab3d-021b4ba04c17
📒 Files selected for processing (1)
tests/attention/test_trtllm_gen_mla.py
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/bot run |
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[FAILED] Pipeline #46925815: 13/20 passed |
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LGTM, should be ready to merge as long as all CI passed.
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/bot run |
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[FAILED] Pipeline #46953779: 9/20 passed |
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📌 Description
This PR fixes #2856.
trtllm_batch_decode_with_kv_cache_mla rejects a correctly-shaped pre-allocated out tensor when q_len_per_req > 1 (speculative decoding / MTP). The out is None path correctly infers a 4D output shape [B, q_len, H, kv_lora_rank] via query.shape[:-1] + (kv_lora_rank,), but the out is not None path hardcodes a 3D expected shape [B, H, kv_lora_rank], missing the q_len dimension.
The fix unifies both paths to use query.shape[:-1] + (kv_lora_rank,) as the expected output shape.
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