Handle capped SWA admission in allocator path

[besquared]

Purpose

Follow-up to #39866.

#39866 fixes the original head-of-line scheduler stall by capping SWA admission budgeting, but there is still an
admission/allocation mismatch in one edge case:

• get_num_blocks_to_allocate() uses the capped SWA budget
• the later allocation path can still effectively require the uncapped number of blocks
• in that case, admission succeeds, but allocation raises:

ValueError: Cannot get 1 free blocks from the pool

This PR keeps the capped SWA admission behavior from #39866, but adds an uncapped safety check before allocation so this path returns None cleanly instead of throwing.

Test Plan

Focused unit test:

python -m pytest tests/v1/core/test_prefix_caching.py -k hybrid_swa_cap_does_not_crash_allocator -q

The added test reproduces the mismatch directly at KVCacheManager.allocate_slots() with:

- one full-attention group
- one sliding-window group
- exhausted free block pool
- full group exactly satisfied
- SWA group short by one block

## Test Result

Before this change:

- allocate_slots(...) raises:
    - ValueError: Cannot get 1 free blocks from the pool

After this change:

- the same setup returns None

Focused test result on top of #39866:

- 1 passed

The added regression test is:

def test_hybrid_swa_cap_does_not_crash_allocator():
    block_size = 16
    sliding_window = 64
    num_tokens = 200
    request_id = "r"

    manager = KVCacheManager(
        kv_cache_config=KVCacheConfig(
            num_blocks=26,  # 25 usable blocks + 1 null block.
            kv_cache_tensors=[],
            kv_cache_groups=[
                KVCacheGroupSpec(
                    ["full"],
                    FullAttentionSpec(
                        block_size=block_size,
                        num_kv_heads=1,
                        head_size=1,
                        dtype=torch.float32,
                    ),
                ),
                KVCacheGroupSpec(
                    ["swa"],
                    SlidingWindowSpec(
                        block_size=block_size,
                        num_kv_heads=1,
                        head_size=1,
                        dtype=torch.float32,
                        sliding_window=sliding_window,
                    ),
                ),
            ],
        ),
        max_model_len=4096,
        hash_block_size=block_size,
        max_num_batched_tokens=32,
        enable_caching=True,
    )

    req = make_request(request_id, [1] * num_tokens, block_size, sha256)
    full_required_blocks = (num_tokens + block_size - 1) // block_size

    allocated = manager.block_pool.get_new_blocks(
        manager.block_pool.num_gpu_blocks - 1
    )
    full_mgr, swa_mgr = manager.coordinator.single_type_managers

    full_mgr.req_to_blocks[request_id] = allocated[:full_required_blocks]
    swa_mgr.req_to_blocks[request_id] = allocated[
        full_required_blocks:full_required_blocks + (full_required_blocks - 1)
    ]

    assert manager.block_pool.get_num_free_blocks() == 0

    # Expected behavior: return None, not crash.
    assert manager.allocate_slots(req, num_new_tokens=num_tokens) is None

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[gemini-code-assist]

Code Review

This pull request implements Sliding Window Attention (SWA) budget capping in the KV cache allocator to prevent over-allocation. It introduces max_num_batched_tokens across the KV cache management stack and adds a secondary safety check in allocate_slots to ensure the allocator does not fail when requests already hold blocks. Feedback suggests refactoring the allocate_slots method to use a dictionary for shared arguments in get_num_blocks_to_allocate calls, improving maintainability by adhering to the DRY principle.

[jhaotingc]

Hi @besquared so this resolves the issue you find on 39866? thanks for fixing.
(just asking so I don't need to personally test it)

[besquared]

Yes the crash doesn't happen with this applied and it's been running gemma4 MoE and dense fine for 12+ hours straight under heavy concurrency with full saturation on my RTX 6K Pro.

Current runtime stack on our side:

Base:

• b075604da — [Bugfix] Fix Gemma4 tool parser converting bare null to string "null" (#39679)

On top of that:

• #39690 — [Core] Move EAGLE drop from KV cache managers to coordinators
• #39866 — [Scheduler] Cap SWA admission budget at sliding_window + chunk_size
• #40027 — Guard uncapped SWA allocation after capped admission