[Bugfix] Fix persistent_topk cooperative deadlock at TopK=1024

csrc/topk.cu

@@ -82,18 +82,73 @@ void launch_persistent_topk(const torch::Tensor& logits,
     size_t smem_size = P::kFixedSmemLarge + chunk_size * sizeof(uint32_t);
     if (smem_size < P::kSmemMedium) smem_size = P::kSmemMedium;
 
+    // Query occupancy for the instantiation that will actually launch;
+    // overestimating it deadlocks the cooperative barrier.
     int occupancy = 1;
-    cudaOccupancyMaxActiveBlocksPerMultiprocessor(
-        &occupancy, P::persistent_topk_kernel<TopK, 4>, P::kThreadsPerBlock,
-        smem_size);
+    cudaError_t occ_err = cudaSuccess;
+    if (vec_size == 4) {
+      occ_err = cudaOccupancyMaxActiveBlocksPerMultiprocessor(
+          &occupancy, P::persistent_topk_kernel<TopK, 4>, P::kThreadsPerBlock,
+          smem_size);
+    } else if (vec_size == 2) {
+      occ_err = cudaOccupancyMaxActiveBlocksPerMultiprocessor(
+          &occupancy, P::persistent_topk_kernel<TopK, 2>, P::kThreadsPerBlock,
+          smem_size);
+    } else {
+      occ_err = cudaOccupancyMaxActiveBlocksPerMultiprocessor(
+          &occupancy, P::persistent_topk_kernel<TopK, 1>, P::kThreadsPerBlock,
+          smem_size);
+    }
+    TORCH_CHECK(occ_err == cudaSuccess,
+                "persistent_topk occupancy query failed: ",
+                cudaGetErrorString(occ_err));
     if (occupancy < 1) occupancy = 1;
 
-    uint32_t max_resident_ctas = static_cast<uint32_t>(num_sms) * occupancy;
+    // The cooperative spin-wait barrier only runs when at least one row hits
+    // the radix path (seq_len > RADIX_THRESHOLD). Below that, non-CTA-0 CTAs
+    // early-exit, so oversubscription can't deadlock and headroom is wasted.
+    const bool needs_cooperative =
+        static_cast<uint32_t>(max_seq_len) > P::RADIX_THRESHOLD;
+
+    const uint32_t hw_resident_cap =
+        static_cast<uint32_t>(num_sms) * static_cast<uint32_t>(occupancy);
+    uint32_t max_resident_ctas = hw_resident_cap;
+    if (needs_cooperative) {
+      // Reserve one CTA per SM when occupancy allows; fall back to a single
+      // CTA when occupancy == 1 (the most deadlock-prone case — any straggler
+      // kernel that takes the only slot on one SM hangs the barrier). Never
+      // drop below one full group's worth.
+      uint32_t headroom = (occupancy > 1) ? static_cast<uint32_t>(num_sms) : 1u;
+      if (max_resident_ctas >= headroom + ctas_per_group) {
+        max_resident_ctas -= headroom;
+      }
+    }
     uint32_t num_groups = std::min(max_resident_ctas / ctas_per_group,
                                    static_cast<uint32_t>(num_rows));
     if (num_groups == 0) num_groups = 1;
     uint32_t total_ctas = num_groups * ctas_per_group;
 
+    // If the cooperative launch wouldn't fit, fall back to FilteredTopK
+    // instead of deadlocking. Only relevant when needs_cooperative.
+    if (needs_cooperative && total_ctas > hw_resident_cap) {
+      TORCH_CHECK(max_smem_per_block >= 128 * 1024,
+                  "persistent_topk would oversubscribe and the FilteredTopK "
+                  "fallback requires >=128KB smem per block (have ",
+                  max_smem_per_block, "). total_ctas=", total_ctas,
+                  " > num_sms*occupancy=", hw_resident_cap, " (TopK=", TopK,
+                  ", vec_size=", vec_size, ", ctas_per_group=", ctas_per_group,
+                  ", smem=", smem_size, ").");
+      cudaError_t status =
+          vllm::FilteredTopKRaggedTransform<float, int32_t, TopK>(
+              logits.data_ptr<float>(), output.data_ptr<int32_t>(),
+              lengths.data_ptr<int32_t>(), static_cast<uint32_t>(num_rows),
+              static_cast<uint32_t>(TopK), static_cast<uint32_t>(stride),
+              stream);
+      TORCH_CHECK(status == cudaSuccess,
+                  "FilteredTopK fallback failed: ", cudaGetErrorString(status));
+      return;
+    }
+
     size_t state_bytes = num_groups * sizeof(P::RadixRowState);
     TORCH_CHECK(workspace.size(0) >= static_cast<int64_t>(state_bytes),
                 "workspace too small, need ", state_bytes, " bytes");