From d0730a0e8bfdc7118c571ad35015728a98780679 Mon Sep 17 00:00:00 2001
From: Cheng Wan <54331508+ch-wan@users.noreply.github.com>
Date: Fri, 18 Sep 2026 17:34:33 -0700
Subject: [PATCH] Give the attention-DP width and rank one home (#40067)

---
 python/sglang/srt/debug_utils/dumper.py       |   9 +-
 .../sglang/srt/disaggregation/common/conn.py  |   8 +-
 python/sglang/srt/layers/dp_attention.py      |  69 ++++--------
 python/sglang/srt/layers/engram.py            |   5 +-
 .../sglang/srt/managers/cache_controller.py   |   3 +-
 .../managers/scheduler_components/dp_attn.py  |   3 +-
 .../dynamic_chunk_sizer.py                    |   7 +-
 .../srt/mem_cache/storage/umbp/umbp_store.py  |  18 ++--
 python/sglang/srt/models/qwen4_exp.py         |   5 +-
 python/sglang/srt/runtime_context.py          |  60 ++++++++---
 test/registered/unit/test_runtime_context.py  | 102 ++++++++++++++----
 11 files changed, 174 insertions(+), 115 deletions(-)

diff --git a/python/sglang/srt/debug_utils/dumper.py b/python/sglang/srt/debug_utils/dumper.py
index ee4686fec..4d788b505 100644
--- a/python/sglang/srt/debug_utils/dumper.py
+++ b/python/sglang/srt/debug_utils/dumper.py
@@ -1739,7 +1739,7 @@ class _SGLangPlugin(_FrameworkPlugin):
             info["moe_tp_size"] = parallel.moe_tp_size
             info["moe_dp_rank"] = parallel.moe_dp_rank
             info["moe_dp_size"] = self._dp_attn.get_moe_cp_size()
-        except (AttributeError, AssertionError, ValueError):
+        except (AttributeError, AssertionError, ValueError, RuntimeError):
             info["distributed_error"] = True
 
         try:
@@ -1747,11 +1747,12 @@ class _SGLangPlugin(_FrameworkPlugin):
             info["enable_dp_attention"] = self._dp_attn.is_dp_attention_enabled()
             info["attn_tp_rank"] = parallel.attn_tp_rank
             info["attn_tp_size"] = parallel.attn_tp_size
-            info["attn_dp_rank"] = self._dp_attn.get_attention_dp_rank()
-            info["attn_dp_size"] = self._dp_attn.get_attention_dp_size()
+            info["attn_dp_rank"] = parallel.attn_dp_rank
+            info["attn_dp_size"] = parallel.attn_dp_size
             info["attn_cp_rank"] = parallel.attn_cp_rank
             info["attn_cp_size"] = parallel.attn_cp_size
-        except (AttributeError, AssertionError, ValueError):
+        # An unstamped topology name raises RuntimeError.
+        except (AttributeError, AssertionError, ValueError, RuntimeError):
             info["dp_attention_error"] = True
 
         return info
diff --git a/python/sglang/srt/disaggregation/common/conn.py b/python/sglang/srt/disaggregation/common/conn.py
index 417491c56..1255a2056 100644
--- a/python/sglang/srt/disaggregation/common/conn.py
+++ b/python/sglang/srt/disaggregation/common/conn.py
@@ -33,10 +33,6 @@ from sglang.srt.disaggregation.utils import (
 )
 from sglang.srt.distributed import get_pp_group, get_world_group
 from sglang.srt.environ import envs
-from sglang.srt.layers.dp_attention import (
-    get_attention_dp_rank,
-    get_attention_dp_size,
-)
 from sglang.srt.runtime_context import (
     get_disagg,
     get_parallel,
@@ -197,8 +193,8 @@ class CommonKVManager(BaseKVManager):
         self.attn_cp_rank = parallel.attn_cp_rank
         self.dcp_size = parallel.attn_dcp_size
         self.dcp_rank = parallel.attn_dcp_rank
-        self.attn_dp_size = get_attention_dp_size()
-        self.attn_dp_rank = get_attention_dp_rank()
+        self.attn_dp_size = parallel.attn_dp_size
+        self.attn_dp_rank = parallel.attn_dp_rank
         self.system_dp_size = (
             1 if get_parallel().enable_dp_attention else get_parallel().dp_size
         )
diff --git a/python/sglang/srt/layers/dp_attention.py b/python/sglang/srt/layers/dp_attention.py
index 71a937bd0..961412b7c 100644
--- a/python/sglang/srt/layers/dp_attention.py
+++ b/python/sglang/srt/layers/dp_attention.py
@@ -2,7 +2,6 @@ from __future__ import annotations
 
 import functools
 import logging
-from contextlib import contextmanager
 from enum import IntEnum, auto
 from typing import TYPE_CHECKING, List, Optional, Tuple
 
@@ -53,9 +52,6 @@ logger = logging.getLogger(__name__)
 if TYPE_CHECKING:
     from sglang.srt.model_executor.forward_batch_info import ForwardBatch
 
-_ATTN_DP_RANK: Optional[int] = None
-_ATTN_DP_SIZE: Optional[int] = None
-
 
 def world_dp_gather_enabled() -> bool:
     """Whether DP gathers should use expanded WORLD after joiner admission."""
@@ -68,10 +64,9 @@ def enable_joiner_all_gather():
 
 
 def update_dp_attention_post_scale(new_dp_size: int, new_dp_rank: int):
-    global _ATTN_DP_SIZE, _ATTN_DP_RANK
-    _ATTN_DP_SIZE = new_dp_size
-    _ATTN_DP_RANK = new_dp_rank
-    get_parallel().override_permanently(attn_dp_size=new_dp_size)
+    get_parallel().override_permanently(
+        attn_dp_size=new_dp_size, attn_dp_rank=new_dp_rank
+    )
     get_flags().dp.use_world_group_for_gather = True
     logger.debug(
         "[Elastic EP] dp_attention switched to WORLD: dp_size=%d dp_rank=%d",
@@ -101,7 +96,7 @@ class DpPaddingMode(IntEnum):
     def get_dp_padding_mode(
         cls, is_extend_in_batch, global_num_tokens: List[int]
     ) -> DpPaddingMode:
-        dp_size = get_attention_dp_size()
+        dp_size = get_parallel().attn_dp_size
 
         # (trangdough) pplx-kernels a2a is a symmetric collective: every EP rank
         # must dispatch the same number of tokens or the device-side handshake
@@ -375,7 +370,6 @@ def initialize_dp_attention(
     server_args: ServerArgs,
     model_config: ModelConfig,
 ):
-    global _ATTN_DP_RANK, _ATTN_DP_SIZE
     dp = get_flags().dp
     dp.max_len_with_idle = (
         getattr(model_config.hf_config, "hybrid_override_pattern", None) is not None
@@ -389,13 +383,12 @@ def initialize_dp_attention(
     tp_rank = get_tensor_model_parallel_rank()
     tp_size = get_tensor_model_parallel_world_size()
 
-    _, _, _ATTN_DP_RANK, _ATTN_DP_SIZE = compute_dp_attention_world_info(
+    _, _, attn_dp_rank, attn_dp_size = compute_dp_attention_world_info(
         enable_dp_attention, tp_rank, tp_size, dp_size, attn_cp_size
     )
-    get_parallel().override_permanently(attn_dp_size=_ATTN_DP_SIZE)
 
     if get_exec().moe.elastic_ep_backend is not None and get_parallel().max_ep_size:
-        _ATTN_DP_RANK = tp_rank + get_parallel().ep_join_rank_offset
+        attn_dp_rank = tp_rank + get_parallel().ep_join_rank_offset
         # Reads the resolution, not a bag: this runs under
         # `initialize_dp_attention`, which the weight-cache daemon calls from
         # `_init_distributed` -- and other callers reach it from processes
@@ -404,6 +397,13 @@ def initialize_dp_attention(
         if ep_scale_joiner_of(resolving_view(server_args)):
             dp.joiner_skip_all_gather = True
 
+    # Stamped together, after the elastic adjustment: the width and the rank
+    # describe one topology, and a reader that caught them mid-update would
+    # see this process placed in a group it is not in.
+    get_parallel().override_permanently(
+        attn_dp_size=attn_dp_size, attn_dp_rank=attn_dp_rank
+    )
+
     _DpGatheredBufferWrapper.set_metadata(
         hidden_size=model_config.hidden_size,
         dtype=model_config.dtype,
@@ -419,42 +419,9 @@ def is_allocation_symmetric() -> bool:
     return not is_dp_attention_enabled() or is_dp_max_padding()
 
 
-def get_attention_dp_rank() -> int:
-    assert _ATTN_DP_RANK is not None, "dp attention not initialized!"
-    return _ATTN_DP_RANK
-
-
-def get_attention_dp_size() -> int:
-    assert _ATTN_DP_SIZE is not None, "dp attention not initialized!"
-    return _ATTN_DP_SIZE
-
-
-@contextmanager
-def disable_dp_size():
-    """Run without DP attention until this scope ends.
-
-    This is for draft workers of speculative decoding, which run the draft model
-    at a different width from the target model's workers.
-
-    The scope replaces both the module global that ``get_attention_dp_size()``
-    reads and the derived width the runtime context answers with, so the two
-    spellings of the name cannot disagree inside it.
-    """
-    global _ATTN_DP_SIZE
-    assert _ATTN_DP_SIZE is not None, "dp attention not initialized!"
-
-    old_dp_size = _ATTN_DP_SIZE
-    _ATTN_DP_SIZE = 1
-    try:
-        with get_parallel().override(attn_dp_size=1):
-            yield
-    finally:
-        _ATTN_DP_SIZE = old_dp_size
-
-
 def get_dp_local_info(forward_batch: ForwardBatch) -> Tuple[torch.Tensor, torch.Tensor]:
     # `get_dp_local_info` is only called in global DP gather and scatter. We use global DP rank here.
-    dp_rank = get_attention_dp_rank()
+    dp_rank = get_parallel().attn_dp_rank
 
     if forward_batch.dp_local_start_pos is None:
         cumtokens = torch.cumsum(forward_batch.global_num_tokens_gpu, dim=0)
@@ -478,7 +445,7 @@ def get_dp_local_slice_cpu(
     # CPU (start, length) slice for DP-local data in a rank-padded buffer.
     # Returns Python ints (no D2H sync) and handles the cuda-graph-padded layout.
     global_num_tokens = forward_batch.global_num_tokens_cpu
-    dp_rank = get_attention_dp_rank()
+    dp_rank = get_parallel().attn_dp_rank
     local_num_tokens = global_num_tokens[dp_rank]
     if can_run_graph:
         local_start_pos = dp_rank * cuda_graph_batch
@@ -758,7 +725,7 @@ def is_dp_gatherv_active() -> bool:
         _USE_DP_GATHERV
         and not world_dp_gather_enabled()
         and get_attn_tensor_model_parallel_world_size() == 1
-        and get_tensor_model_parallel_world_size() == get_attention_dp_size()
+        and get_tensor_model_parallel_world_size() == get_parallel().attn_dp_size
         and not _DpGatheredBufferWrapper.is_dp_max_padding()
     )
 
@@ -792,7 +759,7 @@ def _dp_gather_via_all_gatherv(
     # each rank's local tensor up to sizes[rank] with zeros (matching the
     # buffer's reserved per-rank slot) so sum(sizes) == buffer rows and there
     # is no uninitialized tail for the MoE to read.
-    rank = get_attention_dp_rank()
+    rank = get_parallel().attn_dp_rank
     local_rows = sizes[rank]
     if local_tokens.shape[0] == local_rows:
         local_real = local_tokens
@@ -917,7 +884,7 @@ def dp_reduce_scatter_tensor(output: torch.Tensor, input: torch.Tensor):
         if sizes is not None:
             get_tp_group().reduce_scatterv(input, output=output, sizes=sizes)
             return
-    if get_tensor_model_parallel_world_size() == get_attention_dp_size():
+    if get_tensor_model_parallel_world_size() == get_parallel().attn_dp_size:
         get_tp_group().reduce_scatter_tensor(output, input)
     else:
         scattered_local_tokens = input.tensor_split(
diff --git a/python/sglang/srt/layers/engram.py b/python/sglang/srt/layers/engram.py
index fe836bb08..a1d499dc6 100644
--- a/python/sglang/srt/layers/engram.py
+++ b/python/sglang/srt/layers/engram.py
@@ -40,7 +40,6 @@ from sglang.srt.layers.dp_attention import (
     dp_gather_replicate,
     dp_reduce_scatter_tensor,
     dp_scatter,
-    get_attention_dp_size,
     get_global_dp_buffer_len,
     is_dp_gatherv_active,
 )
@@ -766,7 +765,7 @@ class EngramEmbedding(nn.Module):
             attn_cp_all_gather_into_tensor(all_indices, indices.contiguous())
             start = parallel.attn_cp_rank * local_rows
             return self._lookup(all_indices)[start : start + local_rows]
-        if self.tp_size > 1 and get_attention_dp_size() > 1:
+        if self.tp_size > 1 and get_parallel().attn_dp_size > 1:
             return self._dp_sharded_lookup(indices, forward_batch)
         return self._lookup(indices)
 
@@ -832,7 +831,7 @@ class EngramEmbedding(nn.Module):
         if (
             padding is not None
             and padding.is_max_len()
-            and self.tp_size == get_attention_dp_size()
+            and self.tp_size == get_parallel().attn_dp_size
             and rows == self.tp_size * local.shape[0]
         ) or is_dp_gatherv_active():
             dp_reduce_scatter_tensor(local, values)
diff --git a/python/sglang/srt/managers/cache_controller.py b/python/sglang/srt/managers/cache_controller.py
index b246d8412..0cb2f607e 100644
--- a/python/sglang/srt/managers/cache_controller.py
+++ b/python/sglang/srt/managers/cache_controller.py
@@ -37,7 +37,6 @@ if TYPE_CHECKING:
     from sglang.srt.mem_cache.pool_host import HostKVCache
 
 from sglang.srt.layers.dp_attention import (
-    get_attention_dp_rank,
     is_dp_attention_enabled,
 )
 from sglang.srt.mem_cache.l2_transfer import L2Transfer, L2TransferEngine
@@ -696,7 +695,7 @@ class HiCacheController:
         if is_dp_attention_enabled():
             self.tp_rank = get_parallel().attn_tp_rank
             self.tp_size = get_parallel().attn_tp_size
-            self.dp_rank = get_attention_dp_rank()
+            self.dp_rank = get_parallel().attn_dp_rank
         else:
             self.tp_rank = get_parallel().tp_rank
             self.tp_size = get_parallel().tp_size
diff --git a/python/sglang/srt/managers/scheduler_components/dp_attn.py b/python/sglang/srt/managers/scheduler_components/dp_attn.py
index 7edb862a9..fd14b8d04 100644
--- a/python/sglang/srt/managers/scheduler_components/dp_attn.py
+++ b/python/sglang/srt/managers/scheduler_components/dp_attn.py
@@ -58,9 +58,8 @@ def _resolve_elastic_world_dp_size(
         return dp_size
 
     from sglang.srt.elastic_ep.elastic_ep import ElasticEPStateManager
-    from sglang.srt.layers.dp_attention import get_attention_dp_size
 
-    live_dp_size = get_attention_dp_size()
+    live_dp_size = get_parallel().attn_dp_size
     effective_ep_size = ElasticEPStateManager.get_effective_ep_size()
     world_size = torch.distributed.get_world_size(group)
 
diff --git a/python/sglang/srt/managers/scheduler_components/dynamic_chunk_sizer.py b/python/sglang/srt/managers/scheduler_components/dynamic_chunk_sizer.py
index ee84cbab1..e6df01c92 100644
--- a/python/sglang/srt/managers/scheduler_components/dynamic_chunk_sizer.py
+++ b/python/sglang/srt/managers/scheduler_components/dynamic_chunk_sizer.py
@@ -12,14 +12,13 @@ from tqdm import tqdm
 
 from sglang.srt.environ import envs
 from sglang.srt.layers.dp_attention import (
-    get_attention_dp_rank,
-    get_attention_dp_size,
     is_dp_attention_enabled,
     set_is_extend_in_batch,
 )
 from sglang.srt.managers.schedule_batch import Req, ScheduleBatch
 from sglang.srt.mem_cache.common import release_kv_cache
 from sglang.srt.model_executor.forward_batch_info import ForwardBatch, PPProxyTensors
+from sglang.srt.runtime_context import get_parallel
 from sglang.srt.sampling.sampling_params import SamplingParams
 from sglang.srt.utils import broadcast_pyobj
 from sglang.srt.utils.common import get_device_module
@@ -196,9 +195,9 @@ class DynamicChunkSizer:
 
             if is_dp_attention_enabled():
                 # Profiling runs one request on this rank; other DP ranks report 0.
-                dp_size = get_attention_dp_size()
+                dp_size = get_parallel().attn_dp_size
                 global_num_tokens = [0] * dp_size
-                dp_rank = get_attention_dp_rank()
+                dp_rank = get_parallel().attn_dp_rank
                 global_num_tokens[dp_rank] = current_seq_len
                 batch.global_num_tokens = global_num_tokens
                 batch.global_num_tokens_for_logprob = global_num_tokens
diff --git a/python/sglang/srt/mem_cache/storage/umbp/umbp_store.py b/python/sglang/srt/mem_cache/storage/umbp/umbp_store.py
index eed41159d..10ec5105e 100644
--- a/python/sglang/srt/mem_cache/storage/umbp/umbp_store.py
+++ b/python/sglang/srt/mem_cache/storage/umbp/umbp_store.py
@@ -310,15 +310,14 @@ class UMBPStore(HiCacheStorage):
         if dp_rank_hint is None:
             try:
                 from sglang.srt.layers.dp_attention import (
-                    get_attention_dp_rank,
-                    get_attention_dp_size,
                     is_dp_attention_enabled,
                 )
+                from sglang.srt.runtime_context import get_parallel
 
                 if is_dp_attention_enabled():
-                    dp_rank_hint = get_attention_dp_rank()
-                    dp_size_hint = get_attention_dp_size()
-            except (ImportError, AssertionError):
+                    dp_rank_hint = get_parallel().attn_dp_rank
+                    dp_size_hint = get_parallel().attn_dp_size
+            except (ImportError, AssertionError, RuntimeError):
                 pass
 
         if local_rank_hint is not None:
@@ -807,14 +806,13 @@ class UMBPStore(HiCacheStorage):
 
         try:
             from sglang.srt.layers.dp_attention import (
-                get_attention_dp_rank,
-                get_attention_dp_size,
                 is_dp_attention_enabled,
             )
+            from sglang.srt.runtime_context import get_parallel
 
             if is_dp_attention_enabled():
-                dp_rank = get_attention_dp_rank()
-                dp_size = get_attention_dp_size()
+                dp_rank = get_parallel().attn_dp_rank
+                dp_size = get_parallel().attn_dp_size
                 dp_rank_hint = dp_rank
                 dp_size_hint = dp_size
                 if cfg.ssd.enabled:
@@ -883,7 +881,7 @@ class UMBPStore(HiCacheStorage):
                             dp_size,
                             cfg.ssd.storage_dir,
                         )
-        except (ImportError, AssertionError):
+        except (ImportError, AssertionError, RuntimeError):
             pass
 
         if (
diff --git a/python/sglang/srt/models/qwen4_exp.py b/python/sglang/srt/models/qwen4_exp.py
index 224d2b393..80bb46262 100644
--- a/python/sglang/srt/models/qwen4_exp.py
+++ b/python/sglang/srt/models/qwen4_exp.py
@@ -27,7 +27,6 @@ from sglang.srt.layers.dp_attention import (
     attn_tp_all_reduce,
     dp_gather_replicate,
     dp_scatter,
-    get_attention_dp_size,
     get_dp_global_num_tokens,
     get_global_dp_buffer,
     get_local_dp_buffer,
@@ -505,7 +504,7 @@ class Qwen4ExpNGramEmbedding(nn.Module):
         self.use_attn_tp_ngram = _use_attn_tp_ngram()
         self.gather_dp_tokens = (
             is_dp_attention_enabled()
-            and get_attention_dp_size() > 1
+            and get_parallel().attn_dp_size > 1
             and not self.use_attn_tp_ngram
         )
         ngram_prefix = f"{prefix}.ngram_embedding" if prefix else "ngram_embedding"
@@ -1360,7 +1359,7 @@ class Qwen4ExpLayerExtensionMixin:
         return hidden_states, residual
 
     def _qwen4_exp_use_dp_moe_gather(self) -> bool:
-        return get_attention_dp_size() > 1 and get_moe_a2a_backend().is_none()
+        return get_parallel().attn_dp_size > 1 and get_moe_a2a_backend().is_none()
 
     def _qwen4_exp_use_attn_tp_a2a_scatter(self) -> bool:
         return get_parallel().attn_tp_size > 1 and not get_moe_a2a_backend().is_none()
diff --git a/python/sglang/srt/runtime_context.py b/python/sglang/srt/runtime_context.py
index a2838bb89..ac872e7e9 100644
--- a/python/sglang/srt/runtime_context.py
+++ b/python/sglang/srt/runtime_context.py
@@ -290,22 +290,53 @@ class ParallelContext:
         raise AttributeError(f"ParallelContext has no {name!r}")
 
     def _v(self, name, getter):
-        overrides = self._overrides
-        return overrides[name] if name in overrides else getter()
+        """Scoped override, else the permanent stamp, else the live group.
 
-    def override_permanently(self, **widths) -> None:
-        """Permanently correct a derived width the published bag can't answer
+        One priority order for ranks and widths alike (`_derived_width`), so a
+        stamped value wins over the coordinator for both.
+        """
+        overrides = self._overrides
+        if name in overrides:
+            return overrides[name]
+        derived = self._derived
+        if name in derived:
+            return derived[name]
+        return getter()
+
+    def _stamped(self, name, why):
+        """A per-process fact no configuration implies: scoped override, else
+        the permanent stamp, else fail.
+
+        Unlike a width, this has nothing to fall back on -- the configuration
+        does not carry this process's rank, and there is no group to ask --
+        so an unstamped read is a missing initialization rather than a
+        missing override, and says so.
+        """
+        overrides = self._overrides
+        if name in overrides:
+            return overrides[name]
+        derived = self._derived
+        if name in derived:
+            return derived[name]
+        raise RuntimeError(f"parallel rank {name!r} is not available: {why}")
+
+    def override_permanently(self, **values) -> None:
+        """Permanently record a width or rank the published bag can't answer
         or no longer answers correctly -- not `RuntimeContext.override`,
-        because a derived width is not a resolved config leaf and this must
-        work with no config published at all (`multimodal_gen` lends a TP
-        group to `srt` layers with no `srt` config to publish against).
+        because neither is a resolved config leaf and this must work with no
+        config published at all (`multimodal_gen` lends a TP group to `srt`
+        layers with no `srt` config to publish against).
+
+        Widths are quotients of the configured leaves, so the bag can usually
+        answer and this only corrects it; a rank is a per-process fact the
+        configuration never carries, so for those this is the only source.
 
         Lives beside, not inside, the `@contextmanager` `override` above -- a
         name it cannot also have on this class -- because these are permanent
         for the process, not scoped to a `with` block: none of the real
         callers ever restore the value they set here.
         """
-        self._derived.update(widths)
+        self._derived.update(values)
 
     def clear_derived_widths(self) -> None:
         self._derived.clear()
@@ -401,7 +432,12 @@ class ParallelContext:
 
     @property
     def attn_dp_rank(self) -> int:
-        return self._v("attn_dp_rank", _dp().get_attention_dp_rank)
+        return self._stamped(
+            "attn_dp_rank",
+            "it is computed from this process's `tp_rank` when the attention "
+            "topology is initialized, so a process that never ran "
+            "`initialize_dp_attention` has no answer to give",
+        )
 
     @property
     def world_group(self) -> Any:
@@ -560,9 +596,9 @@ class MoeFlags(_FlagGroupBase):
 
 class DpFlags(_FlagGroupBase):
     """DP-attention runtime flags, materialized by ``initialize_dp_attention``
-    (after distributed setup; reads the model config). Topology values
-    (sizes/ranks) stay on ``layers.dp_attention`` until the parallel vertical
-    migrates them."""
+    (after distributed setup; reads the model config). The topology values it
+    also computes -- the attention-DP width and rank -- are stamped on
+    ``get_parallel()``, not kept here."""
 
     enabled: bool = False
     use_world_group_for_gather: bool = False
diff --git a/test/registered/unit/test_runtime_context.py b/test/registered/unit/test_runtime_context.py
index 16d3a59bc..31f45a2e7 100644
--- a/test/registered/unit/test_runtime_context.py
+++ b/test/registered/unit/test_runtime_context.py
@@ -65,7 +65,9 @@ _DP = "sglang.srt.layers.dp_attention"
 # anything, so there is nothing to derive them from. The quotients used to be
 # in this table and are not any more -- `attn_tp_size` and its siblings are
 # functions of the configured leaves, and `TestDerivedWidthsComeFromTheLeaves`
-# is what pins them.
+# is what pins them. `attn_dp_rank` is not here either: no group coordinator
+# knows it, so it is stamped when the attention topology is initialized and
+# `TestStampedRanks` is what pins it.
 SIZE_RANK_DELEGATIONS = [
     ("world_size", f"{_PS}.get_world_size"),
     ("world_rank", f"{_PS}.get_world_rank"),
@@ -77,7 +79,6 @@ SIZE_RANK_DELEGATIONS = [
     ("moe_tp_rank", f"{_PS}.get_moe_tensor_parallel_rank"),
     ("attn_tp_rank", f"{_PS}.get_attn_tensor_model_parallel_rank"),
     ("attn_cp_rank", f"{_PS}.get_attn_context_model_parallel_rank"),
-    ("attn_dp_rank", f"{_DP}.get_attention_dp_rank"),
 ]
 
 GROUP_DELEGATIONS = [
@@ -150,6 +151,87 @@ class TestParallelDelegation(_IsolatedOverrides):
         self.assertFalse(hasattr(ParallelContext, "local_attn_dp_size"))
 
 
+class TestStampedRanks(_IsolatedOverrides):
+    """`attn_dp_rank` comes from the stamp, and says so when there is none.
+
+    It is the one rank no group answers with: `initialize_dp_attention`
+    computes it from this process's `tp_rank`, and an elastic scale-up
+    replaces it with a rank in the expanded WORLD. Falling back to anything
+    would be inventing a placement for this process.
+    """
+
+    def setUp(self):
+        super().setUp()
+        parallel = get_parallel()
+        self._saved_derived = dict(parallel._derived)
+        parallel.clear_derived_widths()
+        self.addCleanup(
+            lambda: (
+                parallel.clear_derived_widths(),
+                parallel.override_permanently(**self._saved_derived),
+            )
+        )
+
+    def test_the_stamp_is_the_answer(self):
+        parallel = get_parallel()
+        parallel.override_permanently(attn_dp_rank=3)
+        self.assertEqual(parallel.attn_dp_rank, 3)
+        # An elastic scale-up restamps it; the newest stamp wins.
+        parallel.override_permanently(attn_dp_rank=9)
+        self.assertEqual(parallel.attn_dp_rank, 9)
+
+    def test_a_scope_still_wins_over_the_stamp(self):
+        parallel = get_parallel()
+        parallel.override_permanently(attn_dp_rank=3)
+        with parallel.override(attn_dp_rank=0):
+            self.assertEqual(parallel.attn_dp_rank, 0)
+        self.assertEqual(parallel.attn_dp_rank, 3)
+
+    def test_unstamped_names_the_cause(self):
+        with self.assertRaises(RuntimeError) as caught:
+            get_parallel().attn_dp_rank
+        self.assertIn("initialize_dp_attention", str(caught.exception))
+
+    def test_a_stated_width_reaches_the_padding_mode(self):
+        """The reason this PR exists, from a reader's side.
+
+        `get_dp_padding_mode` reads the attention-DP width. Before the width
+        had one home, a scoped `override` moved the context and left the
+        module global answering, so stating a topology moved only half the
+        runtime: this asserted `SUM_LEN` with the width stated as 1.
+        """
+        from sglang.srt.layers.dp_attention import DpPaddingMode
+
+        with get_parallel().override(attn_dp_size=1):
+            mode = DpPaddingMode.get_dp_padding_mode(
+                is_extend_in_batch=True, global_num_tokens=[3, 5]
+            )
+        self.assertIs(mode, DpPaddingMode.MAX_LEN)
+
+        # And the branch it would have taken with the target's width.
+        with get_parallel().override(attn_dp_size=2):
+            mode = DpPaddingMode.get_dp_padding_mode(
+                is_extend_in_batch=True, global_num_tokens=[3, 5]
+            )
+        self.assertIs(mode, DpPaddingMode.SUM_LEN)
+
+    def test_a_scale_up_stamps_the_width_and_the_rank_together(self):
+        """The two describe one topology; a reader that saw only one moved
+        would place this process in a group it is not in."""
+        from sglang.srt.layers.dp_attention import update_dp_attention_post_scale
+
+        # It also flips a process-wide gather flag; put it back, or every
+        # later test in this process runs as if a scale-up had happened.
+        dp_flags = get_flags().dp
+        saved_gather = dp_flags.use_world_group_for_gather
+        self.addCleanup(setattr, dp_flags, "use_world_group_for_gather", saved_gather)
+
+        parallel = get_parallel()
+        update_dp_attention_post_scale(new_dp_size=16, new_dp_rank=11)
+        self.assertEqual(parallel.attn_dp_size, 16)
+        self.assertEqual(parallel.attn_dp_rank, 11)
+
+
 class TestParallelOverride(_IsolatedOverrides):
     def test_override_takes_precedence(self):
         p = get_parallel()
@@ -1573,22 +1655,6 @@ class TestDerivedWidths(_IsolatedOverrides):
             with self.assertRaisesRegex(RuntimeError, r"derived parallel width"):
                 get_parallel().attn_tp_size
 
-    def test_a_temporary_disable_beats_the_permanent_override(self):
-        """`disable_dp_size()` runs a draft scope without DP attention. It moves
-        the module global the legacy getter reads, so it has to move the derived
-        width too -- the scoped override wins over the permanent one, and a
-        scope that left it alone would answer with the target model's width
-        for its duration."""
-        from sglang.srt.layers import dp_attention
-
-        parallel = get_parallel()
-        parallel.override_permanently(attn_dp_size=4)
-        with patch.object(dp_attention, "_ATTN_DP_SIZE", 4):
-            with dp_attention.disable_dp_size():
-                self.assertEqual(dp_attention.get_attention_dp_size(), 1)
-                self.assertEqual(parallel.attn_dp_size, 1)
-            self.assertEqual(parallel.attn_dp_size, 4)
-
     def test_the_permanent_override_is_cleared_and_reset(self):
         parallel = get_parallel()
         parallel.override_permanently(attn_dp_size=2)