[AMD][DSV4] Skip the paged SWA page return under the per-request ring (#38571)

Co-authored-by: HaiShaw <hixiao@gmail.com>

python/sglang/srt/mem_cache/allocator/swa.py

@@ -485,6 +485,11 @@ class SWATokenToKVPoolAllocator(BaseTokenToKVPoolAllocator):
                 ), "swa pages do not match the mapped pages"
         self.clear_full_to_swa_mapping(mapping_indices)
 
+        if self._swa_req_ring:
+            # Ring slots are owned by the req slot, never lent by the paged
+            # allocator; returning them over-credits its available_size().
+            return
+
         if self.free_group is not None:
             # Resolve ownership now, as above.
             self.swa_page_ids_group.append(swa_pages)
@@ -547,6 +552,7 @@ class SWATokenToKVPoolAllocator(BaseTokenToKVPoolAllocator):
         if self.swa_page_ids_group:
             swa_page_ids_group = self.swa_page_ids_group
             self.swa_page_ids_group = []
+            if not self._swa_req_ring:
                 self.swa_attn_allocator.free_page_ids(torch.cat(swa_page_ids_group))
         if self.swa_free_group:
             swa_free_group = self.swa_free_group

test/registered/unit/mem_cache/test_swa_ring_page_return.py

@@ -0,0 +1,129 @@
+"""Regression for the paged SWA page return under the per-request ring.
+
+Both free paths returned SWA pages to the paged swa_attn_allocator
+unconditionally. Under the per-request ring that allocator is vestigial and its
+slots are owned by the req slot rather than lent per free, so the return
+over-credited available_size() past size and tripped the assert at the end of
+free_group_end, killing the scheduler on the first decode that frees.
+
+The mapping clear in _free_swa_pages must still run in ring mode: without it,
+translate_loc_from_full_to_swa reads stale peer indices and the failure becomes
+wrong KV instead of a crash.
+"""
+
+import unittest
+
+import torch
+
+from sglang.srt.mem_cache.allocator.swa import SWATokenToKVPoolAllocator
+from sglang.test.ci.ci_register import register_cpu_ci
+from sglang.test.test_utils import CustomTestCase
+
+register_cpu_ci(est_time=10, suite="base-a-test-cpu")
+
+PAGE_SIZE = 8
+POOL_PAGES = 4
+
+
+class _CountingPagedAllocator:
+    """Paged allocator that really tracks credit, so over-crediting is visible.
+
+    A MagicMock would only record the call; the bug is that the call moves
+    available_size() past size, which is what the production assert checks.
+    """
+
+    def __init__(self, *, size: int):
+        self.size = size
+        self.debug_mode = False
+        self._free_tokens = 0  # fully allocated to start
+
+    def available_size(self) -> int:
+        return self._free_tokens
+
+    def free_page_ids(self, page_ids: torch.Tensor) -> None:
+        self._free_tokens += int(page_ids.numel()) * PAGE_SIZE
+
+    def free_group_end(self) -> None:
+        """The full-side allocator defers its own frees; nothing to settle here."""
+
+
+def _make_self(*, swa_req_ring: bool, page_size: int = PAGE_SIZE):
+    """Build a real instance without __init__; free_group_end calls zero-arg
+    super(), which requires an instance of the class rather than a stub."""
+    alloc = object.__new__(SWATokenToKVPoolAllocator)
+
+    alloc.page_size = page_size
+    alloc._swa_req_ring = swa_req_ring
+    alloc.free_group = None
+    alloc.swa_free_group = []
+    alloc.swa_page_ids_group = []
+
+    mapping = torch.zeros(64, dtype=torch.int64)
+    # Peer pages for the rows under test; page 2 of the paged SWA pool.
+    mapping[0:page_size] = torch.arange(2 * page_size, 3 * page_size, dtype=torch.int64)
+    alloc.full_to_swa_index_mapping = mapping
+
+    alloc.swa_attn_allocator = _CountingPagedAllocator(size=page_size * POOL_PAGES)
+    alloc.full_attn_allocator = _CountingPagedAllocator(size=page_size * POOL_PAGES)
+    return alloc
+
+
+class TestSWARingPageReturn(CustomTestCase):
+    def test_group_drain_keeps_paged_credit_within_size_in_ring_mode(self):
+        """Pre-fix this over-credits and the production assert raises."""
+        alloc = _make_self(swa_req_ring=True)
+        alloc.swa_page_ids_group = [torch.arange(POOL_PAGES + 4, dtype=torch.int64)]
+
+        alloc.free_group_end()
+
+        self.assertEqual(alloc.swa_attn_allocator.available_size(), 0)
+        self.assertLessEqual(
+            alloc.swa_attn_allocator.available_size(),
+            alloc.swa_attn_allocator.size,
+        )
+        # Pile still drains, or it leaks into the next group.
+        self.assertEqual(alloc.swa_page_ids_group, [])
+
+    def test_group_drain_returns_pages_without_ring(self):
+        alloc = _make_self(swa_req_ring=False)
+        alloc.swa_page_ids_group = [torch.arange(2, dtype=torch.int64)]
+
+        alloc.free_group_end()
+
+        self.assertEqual(alloc.swa_attn_allocator.available_size(), 2 * PAGE_SIZE)
+        self.assertEqual(alloc.swa_page_ids_group, [])
+
+    def test_direct_free_keeps_paged_credit_at_zero_in_ring_mode(self):
+        alloc = _make_self(swa_req_ring=True)
+        free_index = torch.arange(0, PAGE_SIZE, dtype=torch.int64)
+
+        alloc._free_swa_pages(free_index, start_pos=0)
+
+        self.assertEqual(alloc.swa_attn_allocator.available_size(), 0)
+        # Nothing deferred either: the ring must not queue what it never returns.
+        self.assertEqual(alloc.swa_page_ids_group, [])
+
+    def test_direct_free_clears_mapping_in_ring_mode(self):
+        """Guards the early-return trap: skipping the clear leaves stale peers."""
+        alloc = _make_self(swa_req_ring=True)
+        free_index = torch.arange(0, PAGE_SIZE, dtype=torch.int64)
+        self.assertTrue(bool((alloc.full_to_swa_index_mapping[free_index] > 0).any()))
+
+        alloc._free_swa_pages(free_index, start_pos=0)
+
+        self.assertTrue(
+            bool((alloc.full_to_swa_index_mapping[free_index] == 0).all()),
+            "ring mode must still clear full_to_swa before returning",
+        )
+
+    def test_direct_free_returns_pages_without_ring(self):
+        alloc = _make_self(swa_req_ring=False)
+        free_index = torch.arange(0, PAGE_SIZE, dtype=torch.int64)
+
+        alloc._free_swa_pages(free_index, start_pos=0)
+
+        self.assertEqual(alloc.swa_attn_allocator.available_size(), PAGE_SIZE)
+
+
+if __name__ == "__main__":
+    unittest.main()