Skip unused FP8 NSA K-cache dequant work

High cache-hit NSA sparse attention only gathers rows selected by topk_indices, but the FP8 RAGGED path dequantized the whole materialized K buffer first. Wire the syh in-place topk-only dequant path behind an explicit env gate so the sparse path can dequant referenced rows at their original row ids while keeping topk_indices unchanged. The old full-dequant path remains the default and the verification gate stays available for small correctness checks only.

Constraint: Production runs should enable only SGLANG_NSA_DEQUANT_ONLY_TOPK=1; VERIFY also runs the full-dequant reference and is too expensive for perf tests.

Rejected: Compact/remap topk buffer | data-dependent shape and remap add synchronization/aliasing risk; syh final in-place contract avoids both.

Confidence: medium

Scope-risk: moderate

Directive: Do not enable SGLANG_NSA_DEQUANT_ONLY_TOPK_VERIFY in throughput runs; use it only for small correctness probes.

Tested: python -m py_compile python/sglang/srt/environ.py python/sglang/srt/layers/attention/nsa_backend.py test/registered/unit/layers/test_nsa_dequant_only_topk.py

Tested: git diff --check

Tested: remote g0034 container PYTHONPATH=python python -m pytest -q test/registered/unit/layers/test_nsa_dequant_only_topk.py -> 2 passed

Tested: remote g0034 CUDA smoke for tai_kernel.nsa_prefill.nsa_dequant_topk_inplace topk rows matched torch reference

Not-tested: Full GSM8K or replay ETE with SGLANG_NSA_DEQUANT_ONLY_TOPK=1

python/sglang/srt/environ.py

@@ -422,6 +422,11 @@ class Envs:
 
     # NSA Backend
     SGLANG_NSA_FUSE_TOPK = EnvBool(True)
+    # Dequantize only the KV rows referenced by sparse-attention topk_indices
+    # instead of full materialized FP8 K cache. Default OFF; VERIFY runs the full
+    # dequant reference and compares gathered rows, so it is for validation only.
+    SGLANG_NSA_DEQUANT_ONLY_TOPK = EnvBool(False)
+    SGLANG_NSA_DEQUANT_ONLY_TOPK_VERIFY = EnvBool(False)
     SGLANG_NSA_ENABLE_MTP_PRECOMPUTE_METADATA = EnvBool(True)
     SGLANG_USE_FUSED_METADATA_COPY = EnvBool(True)
     SGLANG_NSA_PREFILL_DENSE_ATTN_KV_LEN_THRESHOLD = EnvInt(2048)

python/sglang/srt/layers/attention/nsa_backend.py

@@ -2641,9 +2641,17 @@ class NativeSparseAttnBackend(
                                     nvtx_source="mla.ragged_partial_current_sync",
                                 )
                             )
-                        kv_cache = dequantize_k_cache_paged(
-                            kv_cache, page_table_1_flattened
-                        )
+                        if envs.SGLANG_NSA_DEQUANT_ONLY_TOPK.get():
+                            kv_cache = self._dequant_topk_inplace(
+                                kv_cache,
+                                page_table_1_flattened,
+                                topk_indices,
+                                verify=envs.SGLANG_NSA_DEQUANT_ONLY_TOPK_VERIFY.get(),
+                            )
+                        else:
+                            kv_cache = dequantize_k_cache_paged(
+                                kv_cache, page_table_1_flattened
+                            )
                     else:
                         kv_cache = _cat([k, k_rope], dim=-1)
                     page_table_1 = topk_indices
@@ -2720,6 +2728,47 @@ class NativeSparseAttnBackend(
                 index_prefetcher.wait_attention_window()
         return attn_output
 
+    def _dequant_topk_inplace(
+        self,
+        kv_cache_fp8: torch.Tensor,
+        page_table_1_flattened: torch.Tensor,
+        topk_indices: torch.Tensor,
+        verify: bool = False,
+    ) -> torch.Tensor:
+        """Dequantize only topk-referenced FP8 KV rows at their original row ids.
+
+        The output keeps the full logical row count ``[num_kv_rows, 1, 576]`` so
+        sparse attention can keep using the unchanged ``topk_indices``.  This is
+        the final syh in-place contract, not the earlier compact/remap variant:
+        no data-dependent output shape, no topk remap, and no CPU synchronization.
+        """
+        try:
+            from tai_kernel.nsa_prefill import nsa_dequant_topk_inplace
+        except ImportError:
+            nsa_dequant_topk_inplace = None
+
+        if nsa_dequant_topk_inplace is not None:
+            out = nsa_dequant_topk_inplace(
+                kv_cache_fp8,
+                page_table_1_flattened,
+                topk_indices,
+            )
+        else:
+            out = dequantize_k_cache_paged(kv_cache_fp8, page_table_1_flattened)
+
+        if verify:
+            kv_ref = dequantize_k_cache_paged(kv_cache_fp8, page_table_1_flattened)
+            flat = topk_indices.reshape(-1)
+            valid = (flat >= 0) & (flat < int(page_table_1_flattened.shape[0]))
+            rows = flat[valid].long()
+            ref_rows = kv_ref.reshape(-1, kv_ref.shape[-1])[rows]
+            new_rows = out.reshape(-1, out.shape[-1])[rows]
+            assert torch.equal(ref_rows, new_rows), (
+                "[SGLANG_NSA_DEQUANT_ONLY_TOPK_VERIFY] in-place dequant != full "
+                f"dequant (N={int(page_table_1_flattened.shape[0])})"
+            )
+        return out
+
     def forward_decode(
         self,
         q: torch.Tensor,