[12/n] final _C library kernel migration by cleonard530 · Pull Request #45415 · vllm-project/vllm

cleonard530 · 2026-06-12T15:11:07Z

Purpose

This PR continues the libtorch stable ABI migration (see #26946) for vLLM and is the final _C library kernels to move to the _C_stable_libtorch library. The PR moves csrc/quantization/activation_kernels.cu to csrc/libtorch_stable/quantization/activation_kernels.cu, along with the weak_ref_tensor (defined in ops.h), silu_and_mul_quant, and persistent_masked_m_silu_mul_quant kernels.

Test Plan

pytest tests/kernels/test_fused_quant_activation.py
pytest tests/kernels/moe/test_silu_mul_fp8_quant_deep_gemm.py
pytest tests/compile/passes/test_silu_mul_quant_fusion.py
pytest tests/compile/passes/test_functionalization.py

Test Result

Essential Elements of an Effective PR Description Checklist

The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
The test plan, such as providing test command.
The test results, such as pasting the results comparison before and after, or e2e results
(Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Bundled libs: 0
  -- extensions --
    [STABLE  ] [abi3-ok               ] _C_stable_libtorch.abi3.so  (stable_shim=87, unstable=0)
    [UNSTABLE] [abi3-ok               ] _flashmla_C.abi3.so  (stable_shim=0, unstable=72)
    [UNSTABLE] [abi3-ok               ] _flashmla_extension_C.abi3.so  (stable_shim=0, unstable=68)
    [STABLE  ] [abi3-ok               ] _moe_C_stable_libtorch.abi3.so  (stable_shim=72, unstable=0)
    [NO-TORCH] [abi3-ok               ] cumem_allocator.abi3.so
    [NO-TORCH] [abi3-ok               ] spinloop.abi3.so
    [UNSTABLE] [uses-private-api      ] third_party/deep_gemm/_C.cpython-312-x86_64-linux-gnu.so  (stable_shim=0, unstable=57)
    [UNSTABLE] [abi3-ok               ] vllm_flash_attn/_vllm_fa2_C.abi3.so  (stable_shim=0, unstable=84)
    [UNSTABLE] [abi3-ok               ] vllm_flash_attn/_vllm_fa3_C.abi3.so  (stable_shim=0, unstable=80)

Note, no more C_.abi.so library! It has all been moved to _C_stable_libtorch.abi3.so (similarly for _moe_C).

cleonard530 · 2026-06-12T15:33:07Z

+  VLLM_STABLE_DISPATCH_FLOATING_TYPES(                                    \
+      input.scalar_type(), "act_and_mul_kernel", [&] {                    \
+        VLLM_STABLE_DISPATCH_FP8_TYPES(                                   \
+            out.scalar_type(), "act_and_mul_quant_kernel_fp8_type", [&] { \


Note, this kernel string was renamed to indicate what kernel was actually being called.

how did this work before?

cleonard530 · 2026-06-12T15:39:48Z

+  void* data_ptr = const_cast<void*>(tensor.mutable_data_ptr());
+
+  // Create a new tensor from the raw data pointer
+  return torch::stable::from_blob(data_ptr, tensor.sizes(), tensor.strides(),


Can pass tensor.sizes() and tensor.strides() straight into stable::from_blob without having to convert them to std::vectors first

cleonard530 · 2026-06-12T15:40:45Z

+  // Create a new tensor from the raw data pointer
+  return torch::stable::from_blob(data_ptr, tensor.sizes(), tensor.strides(),
+                                  tensor.device(), tensor.scalar_type(),
+                                  [base = tensor](void*) {});


Note, this only works because we are on torch >= 2.11, this would not build on torch 2.10

cleonard530 · 2026-06-12T15:44:48Z

+  // See the following document for more info on custom types and ops that use
+  // custom types:
+  // https://docs.google.com/document/d/18fBMPuOJ0fY5ZQ6YyrHUppw9FA332CpNtgB6SOIgyuA
+


This comment should have been moved over during the marlin/machete migrations

cleonard530 · 2026-06-12T15:47:26Z

-  // kernels from being torch.compile'd.
-  // See the following document for more info on custom types and ops that use
-  // custom types:
-  // https://docs.google.com/document/d/18fBMPuOJ0fY5ZQ6YyrHUppw9FA332CpNtgB6SOIgyuA


After the marlin/machete migrations (#45176 and #45304) this #ifndef block should have been removed.

cleonard530 · 2026-06-12T15:48:27Z


 #
-# _C extension
+# Legacy _C extension (ROCm only — CUDA ops migrated to _C_stable_libtorch)


updated comment to encourage others to use _C_stable_libtorch for cuda kernels,

cleonard530 · 2026-06-12T18:13:24Z

@Harry-Chen, reminder to add the rocm tag

Harry-Chen · 2026-06-13T04:50:14Z

Please fix the pre-commit format error, thanks!

mergify · 2026-06-14T05:26:24Z

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @cleonard530.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

Harry-Chen · 2026-06-14T05:29:11Z

I think my PR #45277 has created some conflicts -- sorry for that, could you rebase / merge again?

cleonard530 · 2026-06-15T12:17:23Z

I think my PR #45277 has created some conflicts -- sorry for that, could you rebase / merge again?

Done!

Harry-Chen · 2026-06-15T12:34:49Z

Seems qutlass is expecting a _C target to build. You could also fix that.

cleonard530 · 2026-06-15T13:42:21Z

Seems qutlass is expecting a _C target to build. You could also fix that.

Fixed. Also, there was an issue with the ROCm build since the new stable weak_ref_tensor requires torch >=2.11, so I added a guard around it to not build with ROCm and left the old weak_ref_tensor in the csrc/ops.h file.

mergify · 2026-06-15T17:36:57Z

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @cleonard530.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

Harry-Chen · 2026-06-18T05:19:14Z

I have not checked thoroughly, but there seem to be a lot of RuntimeErrors -- might still be related to the changes?

Yes, I think the same errors keep failing so I assume it has something to do with this PR. Most of the errors are OOM memory errors, like No available memory for the cache blocks. or torch.OutOfMemoryError: CUDA out of memory.

I am still not too sure why but it seems the memory from CUDA graphs are possibly not being freed between test. I am still looking into this to figure out the cause, but please let me know if you have any insight into why this might be happening.

Sorry I do not have much idea either :-(. BTW we just have #44681 merged, which removed some MoE kernels and caused some conflicts. I'm just thinking -- if you are intereseted -- you can also invetigate if there are more unused kernels during migration, so that we could just remove it instead of putting extra work into it.

cleonard530 · 2026-06-18T12:00:03Z

Sorry I do not have much idea either :-(. BTW we just have #44681 merged, which removed some MoE kernels and caused some conflicts. I'm just thinking -- if you are intereseted -- you can also invetigate if there are more unused kernels during migration, so that we could just remove it instead of putting extra work into it.

@Harry-Chen, Yes, I can definitely look into that while I am migrating over. After this PR, I am planning on doing another PR to clean up anything I missed during the migration (like moving some headers I missed over to the libtorch_stable directory) and I can check for unused kernels as well when I am doing this.

…/quantization/activation_kernels.cu Signed-off-by: Chris Leonard <chleonar@redhat.com>

…e abi stable and removed the last cuda kernels from csrc/ops.h and csrc/torch_bindings.cpp and moved them to the libtorch_stable files Signed-off-by: Chris Leonard <chleonar@redhat.com>

… to clearly indicate where _C HIP ends in CMakeLists.txt Signed-off-by: Chris Leonard <chleonar@redhat.com>

Signed-off-by: Chris Leonard <chleonar@redhat.com>

…s no longer built with cuda. Also, the new stable version of weak_ref_tensor requires torch 2.11 but ROCm is still built with 2.10, so I added the old implementation back in csrc/ops.h which is no longer built on cuda. Signed-off-by: Chris Leonard <chleonar@redhat.com>

…elow the set_gencode_flags_for_srcs to avoid conflicting sm_100 and sm_100f flags Signed-off-by: Chris Leonard <chleonar@redhat.com>

…istration. Signed-off-by: Chris Leonard <chleonar@redhat.com>

…ensions via import_kernels. Signed-off-by: Chris Leonard <chleonar@redhat.com>

…that should of been const_data_ptr. Signed-off-by: Chris Leonard <chleonar@redhat.com>

…erge conflict, removing it again Signed-off-by: Chris Leonard <chleonar@redhat.com>

…eter was saving the tensor data, causing the memory buildup to cascade until we reached OOM errors. This should fix that. Also, Since from_blob was replaced with an overload that is on torch 2.10, we no longer need the ifndef ROCM flag or the legacy weak_ref_tensor method in the non-stable ops.h file. Signed-off-by: Chris Leonard <chleonar@redhat.com>

cleonard530 · 2026-06-18T19:25:17Z

@Harry-Chen, I figure out the issue. The custom deleter in the stable::from_blob method was saving the tensor data, causing memory usage to grow until we hit OOM errors.The tensor data is no longer saved and works the same way it did in the unstable path now.

I believe the three test failures are flaky test, are you able to rerun them to check?

Harry-Chen

Good work!

Harry-Chen · 2026-06-19T14:14:23Z

Sorry I do not have much idea either :-(. BTW we just have #44681 merged, which removed some MoE kernels and caused some conflicts. I'm just thinking -- if you are intereseted -- you can also invetigate if there are more unused kernels during migration, so that we could just remove it instead of putting extra work into it.

@Harry-Chen, Yes, I can definitely look into that while I am migrating over. After this PR, I am planning on doing another PR to clean up anything I missed during the migration (like moving some headers I missed over to the libtorch_stable directory) and I can check for unused kernels as well when I am doing this.

That's really great! I think you can also fix some existing issues (e.g. constness of parameters) that were kept as-is during the whole migration. I think @janeyx99 has pointed out a lot of them in the PR series.

Signed-off-by: divineearthly <divineearthly@gmail.com>

Signed-off-by: Qiang Li <qiang.li2@amd.com>

mergify Bot added ci/build nvidia labels Jun 12, 2026

github-project-automation Bot added this to NVIDIA Jun 12, 2026

cleonard530 changed the title ~~New stable abi final c~~ [12/n] final _C library kernel migration Jun 12, 2026

cleonard530 commented Jun 12, 2026

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cleonard530 marked this pull request as ready for review June 12, 2026 18:12

cleonard530 requested review from Harry-Chen, LucasWilkinson, khluu and tlrmchlsmth as code owners June 12, 2026 18:12

Harry-Chen added rocm Related to AMD ROCm ready ONLY add when PR is ready to merge/full CI is needed labels Jun 14, 2026

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mergify Bot added the needs-rebase label Jun 14, 2026

cleonard530 force-pushed the new-stable-abi-final-_C branch from 765e48f to 3469b5c Compare June 15, 2026 12:15

mergify Bot removed the needs-rebase label Jun 15, 2026

cleonard530 added 9 commits June 18, 2026 12:22

moved csrc/quantization/activation_kernels.cu to csrc/libtorch_stable…

d0ad001

…/quantization/activation_kernels.cu Signed-off-by: Chris Leonard <chleonar@redhat.com>

migrated csrc/libtorch_stable/quantization/activation_kernels.cu to b…

c6dde94

…e abi stable and removed the last cuda kernels from csrc/ops.h and csrc/torch_bindings.cpp and moved them to the libtorch_stable files Signed-off-by: Chris Leonard <chleonar@redhat.com>

small cosmetic changes to align with legacy code before migration and…

e5aed52

… to clearly indicate where _C HIP ends in CMakeLists.txt Signed-off-by: Chris Leonard <chleonar@redhat.com>

fix pre-commit issue

c29e571

Signed-off-by: Chris Leonard <chleonar@redhat.com>

moved Expert-specialization MXFP8 block in the CMakeLists.txt to be b…

1215fa0

…elow the set_gencode_flags_for_srcs to avoid conflicting sm_100 and sm_100f flags Signed-off-by: Chris Leonard <chleonar@redhat.com>

Build QuTLASS as standalone vllm._qutlass_C extension with PyInit reg…

2c27bad

…istration. Signed-off-by: Chris Leonard <chleonar@redhat.com>

Include _qutlass_C in precompiled wheels and register CUDA kernel ext…

9e539bc

…ensions via import_kernels. Signed-off-by: Chris Leonard <chleonar@redhat.com>

Moved core/math.hpp to the libtorch_stable directory. Fixed data_ptr …

549bae4

…that should of been const_data_ptr. Signed-off-by: Chris Leonard <chleonar@redhat.com>

cleonard530 force-pushed the new-stable-abi-final-_C branch from 84ee0e3 to 549bae4 Compare June 18, 2026 12:33

mergify Bot removed the needs-rebase label Jun 18, 2026

cleonard530 added 2 commits June 18, 2026 14:10

MXFP8 was accidentally added back to CMakeLists.txt after resolving m…

9b5532f

…erge conflict, removing it again Signed-off-by: Chris Leonard <chleonar@redhat.com>

Harry-Chen approved these changes Jun 19, 2026

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github-project-automation Bot moved this to Ready in NVIDIA Jun 19, 2026

Harry-Chen enabled auto-merge (squash) June 19, 2026 07:04

WoosukKwon merged commit b9a7cd4 into vllm-project:main Jun 19, 2026
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divineearthly pushed a commit to divineearthly/vllm that referenced this pull request Jun 19, 2026

[12/n] final _C library kernel migration (vllm-project#45415)

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Signed-off-by: divineearthly <divineearthly@gmail.com>

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[12/n] final _C library kernel migration (vllm-project#45415)

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[12/n] final _C library kernel migration (vllm-project#45415)

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Signed-off-by: Qiang Li <qiang.li2@amd.com>

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