[BugFix][PD Disaggregation] remove redundant block allocation of prefill tasks in decode instance

[liyonghua0910]

Motivation

In the disaggregated prefill-decode serving mode (splitwise_role != "mixed"), the decode instance incorrectly applied the mixed-role block reservation logic when determining whether to admit a new prefill request. This caused the scheduler to over-provision KV-cache blocks for running decode requests, unnecessarily blocking prefill task admission and reducing throughput. Additionally, get_new_block_nums could return a negative block count when a request already had sufficient blocks allocated, potentially leading to downstream scheduling errors.

Modifications

• fastdeploy/engine/sched/resource_manager_v1.py
  1. _get_can_schedule_prefill_threshold_block: Return num_chunk_new_block directly when enabling prefill-decode disaggregation, skipping the reserve-block estimation for running requests.
  2. get_new_block_nums: Clamp the result to non-negative, preventing negative values when num_computed_tokens + num_new_tokens fits within already-allocated blocks.

Usage or Command

No new configuration or command required. The fix applies automatically in disaggregated prefill-decode deployments.

Accuracy Tests

This PR only modifies scheduling logic (block allocation thresholds) and does not affect model forward computation or kernel outputs, so accuracy is not impacted.

Checklist

• Add at least a tag in the PR title.
  • Tag list: [[FDConfig],[APIServer],[Engine], [Scheduler], [PD Disaggregation], [Executor], [Graph Optimization], [Speculative Decoding], [RL], [Models], [Quantization], [Loader], [OP], [KVCache], [DataProcessor], [BugFix], [Docs], [CI], [Optimization], [Feature], [Benchmark], [Others], [XPU], [HPU], [GCU], [DCU], [Iluvatar], [Metax]]
  • You can add new tags based on the PR content, but the semantics must be clear.
• Format your code, run pre-commit before commit.
• Add unit tests. Please write the reason in this PR if no unit tests.
• Provide accuracy results.
• If the current PR is submitting to the release branch, make sure the PR has been submitted to the develop branch, then cherry-pick it to the release branch with the [Cherry-Pick] PR tag.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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[PaddlePaddle-bot]

🤖 Paddle-CI-Agent | pr_review | 2026-06-08 21:50:29

📋 Review 摘要

PR 概述：修复 decode 实例中 prefill 任务已预分配 block 后仍可能计算出冗余/负数新增 block，以及非 mixed 角色调度阈值不再为 running decode 预留额外 block。
变更范围：fastdeploy/engine/sched/resource_manager_v1.py
影响面 Tag：[Scheduler]

问题

级别	文件	概述
-	-	未发现阻塞性问题。PR 规范问题在下面章节报，不在这里重复。

📝 PR 规范检查

存在 PR 规范问题：当前 PR 标题包含两个官方 Tag，而 checklist §D1 要求标题必须且仅包含一个官方 Tag；PR 描述保留了模板结构，但 Motivation、Modifications、Usage or Command、Accuracy Tests 仍为空或仅为占位内容，Checklist 也未按实际情况勾选。

标题建议（可直接复制）：

• [BugFix] remove redundant block allocation of prefill tasks in decode instance

PR 描述建议（点击展开，可直接复制）

## Motivation
修复 PD Disaggregation decode 实例为 prefill 任务做资源预分配时，由调度阈值和已分配 block 计算导致的冗余 block 需求，避免已有 block_tables 多于本次 chunk 需求时继续请求额外 KV block。

## Modifications
- `fastdeploy/engine/sched/resource_manager_v1.py`: 在 `get_new_block_nums()` 中将新增 block 数量裁剪为非负值。
- `fastdeploy/engine/sched/resource_manager_v1.py`: 在 `_get_can_schedule_prefill_threshold_block()` 中让非 `mixed` splitwise 角色直接按本次 chunk 需求检查可用 block，不再叠加 mixed 模式的 running decode 预留阈值。

## Usage or Command
N/A

## Accuracy Tests
N/A

## Checklist

- [x] Add at least a tag in the PR title.
  - Tag list: [`[FDConfig]`,`[APIServer]`,`[Engine]`, `[Scheduler]`, `[PD Disaggregation]`, `[Executor]`, `[Graph Optimization]`, `[Speculative Decoding]`, `[RL]`, `[Models]`, `[Quantization]`, `[Loader]`, `[OP]`, `[KVCache]`, `[DataProcessor]`, `[BugFix]`, `[Docs]`, `[CI]`, `[Optimization]`, `[Feature]`, `[Benchmark]`, `[Others]`, `[XPU]`, `[HPU]`, `[GCU]`, `[DCU]`, `[Iluvatar]`, `[Metax]`]
  - You can add new tags based on the PR content, but the semantics must be clear.
- [ ] Format your code, run `pre-commit` before commit.
- [ ] Add unit tests. Please write the reason in this PR if no unit tests.
- [ ] Provide accuracy results.
- [x] If the current PR is submitting to the `release` branch, make sure the PR has been submitted to the `develop` branch, then cherry-pick it to the `release` branch with the `[Cherry-Pick]` PR tag.

总体评价

代码变更范围很小，针对已预分配和非 mixed 角色的 block 阈值处理与调用链语义一致；未发现需要阻塞合入的代码问题。建议补齐 PR 规范信息，尤其是单 Tag 标题和验证/测试说明。

[PaddlePaddle-bot]

🤖 Paddle-CI-Agent | ci_status_monitor | 2026-06-09 08:14:04 UTC+08:00

CI报告基于以下代码生成（30分钟更新一次）:
PR commit: ddffd8c | Merge base: f18078f (branch: develop)

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1 Required任务 : 10/10 通过

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41(0)	41	38	3	0	0	0

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