[Bugfix][Kernel] Fix persistent top-k histogram reuse after short rows

[fxfxfxfxfxfxfxfx]

Purpose

Fix a correctness bug in persistent_topk when one persistent CTA group
processes a radix row (seq_len > 32768), followed by a short or medium row
(seq_len <= 32768), and then another radix row.

The kernel previously used the outer row iteration counter to rotate its
triple-buffered radix histograms. Short rows advanced that counter without
executing radix_topk, causing the next radix row to reuse a histogram that
could still contain counts from an earlier row.

This PR introduces a separate radix_iter counter that advances only when
radix_topk executes. It also adds a CUDA regression test that constructs the
long-short-long scheduling pattern in one persistent CTA group.

Related to #41748, which addresses workspace initialization between kernel
launches. This PR fixes histogram-ring misalignment within a single launch.

Test Plan

• Compare the long-short-long case against torch.topk.
• Run a consecutive-radix case as a control.
• Repeat both cases ten times on an RTX 3080.
• Exercise the adaptive CTA-group layout used by the regression test.
• Run the checked-in regression test in CI:

python -m pytest \
  tests/kernels/test_top_k_per_row.py::test_persistent_topk_reused_group_after_short_row \
  -v

• Run static checks:

python -m py_compile tests/kernels/test_top_k_per_row.py
ruff check tests/kernels/test_top_k_per_row.py
git diff --check origin/main...HEAD
git clang-format --diff origin/main -- \
  csrc/libtorch_stable/persistent_topk.cuh

Test Result

RTX 3080 results before the fix:

middle_len=32768: match=False, set_diff=8
middle_len=32769: match=True,  set_diff=0

Results after rebuilding the production topk.cu with the fix:

middle_len=32768: failures=0/10
middle_len=32769: failures=0/10

Adaptive regression layout:

ctas_per_group=7, num_groups=9, rows=(0, 9, 18)
set_diff=0

Python compilation, Ruff, git diff --check, and clang-format checks passed.

The complete _C_stable_libtorch extension and checked-in pytest were not run
locally; they are left for CI. Local GPU validation compiled the production
topk.cu and modified persistent_topk.cuh for SM86.

No documentation update is required because this is an internal kernel
correctness fix with no public API or configuration changes.

AI assistance disclosure: OpenAI Codex was used for source analysis,
implementation, regression-test construction, and validation. I reviewed and
understand all submitted changes.

---

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• No documentation update is required for this internal kernel fix.

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[fxfxfxfxfxfxfxfx]

Gentle ping for review @dcampora @LopezCastroRoberto — this fixes a correctness bug in persistent_topk that could silently return wrong top-k results.

Context: when one persistent CTA group schedules a radix row (seq_len > 32768), then a short/medium row, then another radix row, the outer row counter rotates the triple-buffered histograms even though radix_topk never ran for the short row. The next radix row then reuses a histogram that still holds counts from an earlier row — no crash, just wrong counts, which makes it easy to miss.

Fix: a separate radix_iter counter that only advances when radix_topk actually executes, plus a CUDA regression test that builds the long-short-long pattern in one persistent CTA group (verified 10/10 pass on RTX 3080; before the fix it produced set_diff=8).

This complements #41748 (workspace init between launches) — this PR fixes histogram-ring misalignment within a single launch. @dcampora since you wrote the original kernel, and @LopezCastroRoberto since it interacts with your workspace work, your eyes on the radix logic would be especially valuable. Happy to split or rebase if that helps review.

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@mgoin

[LopezCastroRoberto]

LGTM, thanks for the fix!

[LopezCastroRoberto]

@fxfxfxfxfxfxfxfx can you please fix the pre-commit?

[github-actions]

✅ @fxfxfxfxfxfxfxfx, CI is now available for this PR.

• /ci run starts a CI build.
• /ci retry retries failed jobs in the CI build for the current PR head. If the current head has no CI build, it starts a new CI build for the current head containing only jobs that failed in the latest earlier CI build for this PR.
• /ci cancel cancels scheduled or running CI builds for this PR branch.

[mergify]

Hi @fxfxfxfxfxfxfxfx, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

[mgoin]

/ci run

[github-actions]

✅ Triggered Buildkite CI #83421 for commit 82c305ea28f9.

[fxfxfxfxfxfxfxfx]

/ci retry

[github-actions]

✅ Queued 3 failed job(s) for retry in Buildkite CI #83421.

[fxfxfxfxfxfxfxfx]

@mgoin I checked the full log, and this does not appear to be related to this PR. Both the original run and the retry crash during model warmup with CUDBG_EXCEPTION_WARP_OUT_OF_RANGE_ADDRESS in deep_gemm::sched::sm90_paged_mqa_logits_metadata<1024, 256, 132, false>, while this PR only changes persistent_topk. Could you please take a look?

[zyongye]

@mgoin I checked the full log, and this does not appear to be related to this PR. Both the original run and the retry crash during model warmup with CUDBG_EXCEPTION_WARP_OUT_OF_RANGE_ADDRESS in deep_gemm::sched::sm90_paged_mqa_logits_metadata<1024, 256, 132, false>, while this PR only changes persistent_topk. Could you please take a look?

yea it is not related. We are fixing it rn.