[Perf][Kernel] Add faster topKperRow decode kernel for DeepSeek-V3.2 sparse attention (#33680)

Signed-off-by: LopezCastroRoberto <rocastro@redhat.com>
Signed-off-by: Roberto L. Castro <38211239+LopezCastroRoberto@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

tests/kernels/test_top_k_per_row.py

@@ -275,3 +275,114 @@ def test_top_k_per_row_decode_large_vocab_size(clean_logits: bool) -> None:
     _run_top_k_per_row_decode_test(
         top_k, batch_size, next_n, vocab_size, clean_logits, data_generation
     )
+
+
+@pytest.mark.skipif(not current_platform.is_cuda(), reason="This test requires CUDA")
+@pytest.mark.parametrize("clean_logits", [True, False])
+@torch.inference_mode()
+def test_deepseek_hybrid_topk(clean_logits: bool) -> None:
+    torch.set_default_device("cuda:0")
+
+    top_k = 2048
+
+    # Test case 1: Short sequences (< 8192)
+    batch_size_short = 4
+    next_n = 1
+    num_rows_short = batch_size_short * next_n
+
+    # Create sequences with max length < 8192
+    seq_lens_short = torch.randint(
+        4000, 8000, (batch_size_short,), dtype=torch.int32, device="cuda"
+    )
+
+    row_starts_short = torch.zeros(num_rows_short, dtype=torch.int32, device="cuda")
+    row_indices_short = torch.arange(num_rows_short, device="cuda") // next_n
+    next_n_offset_short = torch.arange(num_rows_short, device="cuda") % next_n
+    row_ends_short = (
+        seq_lens_short[row_indices_short] - next_n + next_n_offset_short + 1
+    )
+
+    logits_short = create_random_logits(
+        row_starts_short, row_ends_short, torch.float32, 42, clean_logits, "random"
+    )
+
+    indices_vllm = torch.empty(
+        (num_rows_short, top_k), dtype=torch.int32, device="cuda"
+    )
+
+    # Use vllm's kernel for short sequences
+    torch.ops._C.top_k_per_row_decode(
+        logits_short,
+        next_n,
+        seq_lens_short,
+        indices_vllm,
+        num_rows_short,
+        logits_short.stride(0),
+        logits_short.stride(1),
+        top_k,
+    )
+
+    # Test case 2: Long sequences (>= 8192) - should use large_context_topk kernel
+    batch_size_long = 4
+    num_rows_long = batch_size_long * next_n
+
+    # Create sequences with max length >= 8192
+    seq_lens_long = torch.randint(
+        8192, 16384, (batch_size_long,), dtype=torch.int32, device="cuda"
+    )
+
+    row_starts_long = torch.zeros(num_rows_long, dtype=torch.int32, device="cuda")
+    row_indices_long = torch.arange(num_rows_long, device="cuda") // next_n
+    next_n_offset_long = torch.arange(num_rows_long, device="cuda") % next_n
+    row_ends_long = seq_lens_long[row_indices_long] - next_n + next_n_offset_long + 1
+
+    logits_long = create_random_logits(
+        row_starts_long, row_ends_long, torch.float32, 43, clean_logits, "random"
+    )
+
+    indices = torch.empty((num_rows_long, top_k), dtype=torch.int32, device="cuda")
+
+    # Use large_context_topk kernel for long sequences
+    if next_n == 1:
+        lengths = seq_lens_long
+    else:
+        offsets = torch.arange(next_n, device=logits_long.device, dtype=torch.int32)
+        lengths = (seq_lens_long.unsqueeze(1) - next_n + 1 + offsets).flatten()
+
+    torch.ops._C.large_context_topk(
+        logits_long,
+        indices,
+        lengths,
+        None,
+    )
+
+    torch_indices_short = torch.empty(
+        (num_rows_short, top_k), dtype=torch.int32, device="cuda"
+    )
+    for i in range(num_rows_short):
+        row_end = int(row_ends_short[i])
+        k_i = min(top_k, row_end)
+        idx = logits_short[i, :row_end].topk(k_i, dim=-1)[1]
+        torch_indices_short[i, :k_i] = idx
+
+    assert compare_top_k_results(
+        logits_short,
+        indices_vllm,
+        torch_indices_short,
+        row_starts_short,
+        row_ends_short,
+        top_k,
+    ), "top_k_per_row_decode kernel (short sequences) doesn't match torch.topk"
+
+    torch_indices_long = torch.empty(
+        (num_rows_long, top_k), dtype=torch.int32, device="cuda"
+    )
+    for i in range(num_rows_long):
+        row_end = int(row_ends_long[i])
+        k_i = min(top_k, row_end)
+        idx = logits_long[i, :row_end].topk(k_i, dim=-1)[1]
+        torch_indices_long[i, :k_i] = idx
+
+    assert compare_top_k_results(
+        logits_long, indices, torch_indices_long, row_starts_long, row_ends_long, top_k
+    ), "large_context_topk kernel (long sequences) doesn't match torch.topk"