[NPU] Avoid device synchronization in Ascend sampling (#39404)

test/registered/unit/sampling/test_ascend_sampler.py

@@ -0,0 +1,57 @@
+"""Unit tests for the Ascend sampling dispatch path."""
+
+import unittest
+from types import SimpleNamespace
+from unittest.mock import MagicMock, patch
+
+import torch
+
+from sglang.srt.layers import sampler as sampler_module
+from sglang.test.ci.ci_register import register_cpu_ci
+
+register_cpu_ci(est_time=1, suite="base-a-test-cpu")
+
+
+class TestAscendSamplerDispatch(unittest.TestCase):
+    def test_top_k_dispatch_does_not_read_device_values_on_cpu(self):
+        logits = torch.tensor([[0.1, 0.2, 0.3, 0.4]])
+        top_ks = torch.tensor([2], dtype=torch.int32)
+        top_ps = torch.ones(1)
+        min_ps = torch.zeros(1)
+        positions = torch.zeros(1, dtype=torch.int64)
+
+        for eligible in (True, False):
+            with self.subTest(eligible=eligible):
+                npu_top_k_top_p = MagicMock(return_value=logits)
+                with (
+                    patch.object(
+                        sampler_module,
+                        "torch_npu",
+                        SimpleNamespace(npu_top_k_top_p=npu_top_k_top_p),
+                        create=True,
+                    ),
+                    patch.object(
+                        sampler_module.torch,
+                        "all",
+                        side_effect=AssertionError("device predicate was inspected"),
+                    ),
+                ):
+                    result = (
+                        sampler_module.top_k_top_p_min_p_sampling_from_logits_ascend(
+                            logits.clone(),
+                            top_ks.clone(),
+                            top_ps,
+                            min_ps,
+                            False,
+                            None,
+                            positions,
+                            npu_top_k_top_p_eligible=eligible,
+                        )
+                    )
+
+                self.assertEqual(tuple(result.shape), (1,))
+                self.assertEqual(npu_top_k_top_p.called, eligible)
+
+
+if __name__ == "__main__":
+    unittest.main()

test/registered/unit/sampling/test_sampling_batch_info.py

@@ -492,18 +492,21 @@ class TestMergeBatch(CustomTestCase):
             need_top_p_sampling=False,
             need_top_k_sampling=False,
             need_min_p_sampling=False,
+            npu_top_k_top_p_eligible=True,
         )
         info2 = _make_info(
             is_all_greedy=False,
             need_top_p_sampling=True,
             need_top_k_sampling=True,
             need_min_p_sampling=True,
+            npu_top_k_top_p_eligible=False,
         )
         info1.merge_batch(info2)
         self.assertFalse(info1.is_all_greedy)  # AND semantics
         self.assertTrue(info1.need_top_p_sampling)  # OR semantics
         self.assertTrue(info1.need_top_k_sampling)  # OR semantics
         self.assertTrue(info1.need_min_p_sampling)  # OR semantics
+        self.assertFalse(info1.npu_top_k_top_p_eligible)  # AND semantics
 
     def test_merge_with_logit_bias(self):
         """Test that merge pads missing logit_bias with zeros before concatenation."""
@@ -675,6 +678,22 @@ class TestFromScheduleBatch(CustomTestCase):
         self.assertTrue(info.need_min_p_sampling)  # 0.1 > 0
         self.assertFalse(info.is_all_greedy)  # top_k=50 > 1
 
+    def test_npu_top_k_top_p_eligibility_uses_request_params(self):
+        cases = (
+            ((1, 1024), True),
+            ((1, 1025), False),
+            ((4, TOP_K_ALL), False),
+        )
+        for top_ks, expected in cases:
+            with self.subTest(top_ks=top_ks):
+                batch = MagicMock()
+                batch.reqs = [self._make_req(top_k=top_k) for top_k in top_ks]
+                batch.device = DEVICE
+
+                info = SamplingBatchInfo.from_schedule_batch(batch, VOCAB_SIZE)
+
+                self.assertEqual(info.npu_top_k_top_p_eligible, expected)
+
     def test_no_logit_bias_when_all_none(self):
         """Test that logit_bias stays None when no request has logit_bias set."""