Kernel-override Determinism [1/n] (#25603)

Signed-off-by: Bram Wasti <bwasti@meta.com>

csrc/core/batch_invariant.hpp

+#pragma once
+#include <cstdlib>
+#include <string>
+#include <cctype>
+
+namespace vllm {
+
+// vllm_kernel_override_batch_invariant(); returns true
+// if env VLLM_KERNEL_OVERRIDE_BATCH_INVARIANT=1
+inline bool vllm_kernel_override_batch_invariant() {
+  std::string env_key = "VLLM_KERNEL_OVERRIDE_BATCH_INVARIANT";
+  const char* val = std::getenv(env_key.c_str());
+  return (val && std::atoi(val) != 0) ? 1 : 0;
+}
+
+}  // namespace vllm

csrc/layernorm_kernels.cu

 #include "type_convert.cuh"
 #include "dispatch_utils.h"
 #include "cub_helpers.h"
+#include "core/batch_invariant.hpp"
 
 #include <torch/cuda.h>
 #include <c10/cuda/CUDAGuard.h>
@@ -413,7 +414,9 @@ void fused_add_rms_norm(torch::Tensor& input,     // [..., hidden_size]
                           wt_ptr % req_alignment_bytes == 0;
   bool offsets_are_multiple_of_vector_width =
       hidden_size % vector_width == 0 && input_stride % vector_width == 0;
-  if (ptrs_are_aligned && offsets_are_multiple_of_vector_width) {
+  bool batch_invariant_launch = vllm::vllm_kernel_override_batch_invariant();
+  if (ptrs_are_aligned && offsets_are_multiple_of_vector_width &&
+      !batch_invariant_launch) {
     LAUNCH_FUSED_ADD_RMS_NORM(8);
   } else {
     LAUNCH_FUSED_ADD_RMS_NORM(0);
@@ -459,7 +462,8 @@ void poly_norm(torch::Tensor& out,     // [..., hidden_size]
   auto inp_ptr = reinterpret_cast<std::uintptr_t>(input.data_ptr());
   auto out_ptr = reinterpret_cast<std::uintptr_t>(out.data_ptr());
   bool ptrs_are_aligned = inp_ptr % 16 == 0 && out_ptr % 16 == 0;
-  if (ptrs_are_aligned && hidden_size % 8 == 0) {
+  bool batch_invariant_launch = vllm::vllm_kernel_override_batch_invariant();
+  if (ptrs_are_aligned && hidden_size % 8 == 0 && !batch_invariant_launch) {
     LAUNCH_FUSED_POLY_NORM(8);
   } else {
     LAUNCH_FUSED_POLY_NORM(0);

csrc/layernorm_quant_kernels.cu

@@ -9,6 +9,7 @@
 #include "quantization/fp8/common.cuh"
 #include "dispatch_utils.h"
 #include "cub_helpers.h"
+#include "core/batch_invariant.hpp"
 
 #include <torch/cuda.h>
 #include <c10/cuda/CUDAGuard.h>
@@ -240,7 +241,9 @@ void fused_add_rms_norm_static_fp8_quant(
   auto wt_ptr = reinterpret_cast<std::uintptr_t>(weight.data_ptr());
   bool ptrs_are_aligned =
       inp_ptr % 16 == 0 && res_ptr % 16 == 0 && wt_ptr % 16 == 0;
-  if (ptrs_are_aligned && hidden_size % 8 == 0 && input_stride % 8 == 0) {
+  bool batch_invariant_launch = vllm::vllm_kernel_override_batch_invariant();
+  if (ptrs_are_aligned && hidden_size % 8 == 0 && input_stride % 8 == 0 &&
+      !batch_invariant_launch) {
     LAUNCH_FUSED_ADD_RMS_NORM(8);
   } else {
     LAUNCH_FUSED_ADD_RMS_NORM(0);

csrc/moe/topk_softmax_kernels.cu

@@ -21,6 +21,7 @@
 #include <c10/cuda/CUDAGuard.h>
 #include "../cuda_compat.h"
 #include "../cub_helpers.h"
+#include "../core/batch_invariant.hpp"
 
 #define MAX(a, b) ((a) > (b) ? (a) : (b))
 #define MIN(a, b) ((a) < (b) ? (a) : (b))
@@ -405,7 +406,8 @@ void topkGatingSoftmaxLauncherHelper(const float* input, const bool* finished, f
     using Constants = detail::TopkConstants<EXPERTS, BYTES_PER_LDG, WARP_SIZE_PARAM>;
     static constexpr int VPT = Constants::VPT;
     static constexpr int ROWS_PER_WARP = Constants::ROWS_PER_WARP;
-    const int num_warps = (num_rows + ROWS_PER_WARP - 1) / ROWS_PER_WARP;
+    const bool batch_invariant_launch = vllm::vllm_kernel_override_batch_invariant();
+    const int num_warps = batch_invariant_launch ? 32 : (num_rows + ROWS_PER_WARP - 1) / ROWS_PER_WARP;
     const int num_blocks = (num_warps + WARPS_PER_TB - 1) / WARPS_PER_TB;
 
     dim3 block_dim(WARP_SIZE_PARAM, WARPS_PER_TB);

tests/v1/generation/test_batch_invariance.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+import contextlib
+import os
+import random
+import string
+
+import pytest
+import torch
+
+from vllm import LLM, SamplingParams
+
+
+def _random_prompt(min_words: int = 1024, max_words: int = 1024 * 2) -> str:
+    # Lightweight random prompt generator to vary prompt lengths and content.
+    vocab = [
+        "alpha",
+        "bravo",
+        "charlie",
+        "delta",
+        "echo",
+        "foxtrot",
+        "golf",
+        "hotel",
+        "india",
+        "juliet",
+        "kilo",
+        "lima",
+        "mike",
+        "november",
+        "oscar",
+        "papa",
+        "quebec",
+        "romeo",
+        "sierra",
+        "tango",
+        "uniform",
+        "victor",
+        "whiskey",
+        "xray",
+        "yankee",
+        "zulu",
+    ]
+    n = random.randint(min_words, max_words)
+    words = random.choices(vocab, k=n)
+
+    # Add some noise and punctuation variability
+    if random.random() < 0.5:
+        words[0] = words[0].capitalize()
+    if random.random() < 0.2:
+        words.append("".join(random.choices(string.ascii_lowercase, k=5)))
+    punct = random.choice([".", "?", "!", "...", ""])
+    return " ".join(words) + punct
+
+
+@pytest.mark.timeout(1000)
+def test_v1_generation_is_deterministic_across_batch_sizes_with_needle():
+    """
+    Ensures that the same request (the 'needle' prompt) yields identical output
+    whether run alone (bs=1) or mixed into a larger batch (e.g., bs=64),
+    using the high-level v1 LLM() API only (no manual batching).
+
+    Strategy:
+    - Create two LLM engines with identical config except max_num_seqs: 1 vs N.
+    - Compute a baseline output for the needle prompt with the bs=1 engine.
+    - For many trials, generate a batch (size N) where the needle appears at a
+      random position among random filler prompts using the bs=N engine.
+    - Track how many trials match vs mismatch, and report totals at the end.
+      The test fails if any mismatches occur, but we still dump pass/fail
+      counts.
+
+    Notes:
+    - Use seeded stochastic sampling with a fixed seed to test determinism.
+    - Outputs are intentionally longer and sampled at higher temperature/top_p
+      to produce a more random-sounding phrase, yet remain deterministic by 
+      seed.
+    - Keep max_tokens and max_model_len bounded for speed and memory use.
+    """
+    random.seed(12345)
+
+    # Allow overrides from environment (useful for CI tuning)
+    # "facebook/opt-125m" is too small, doesn't reliably test determinism
+    model = os.getenv("VLLM_TEST_MODEL", "Qwen/Qwen3-1.7B")
+    num_trials = int(os.getenv("VLLM_NEEDLE_TRIALS", "5"))
+    batch_size = int(os.getenv("VLLM_NEEDLE_BATCH_SIZE", "64"))
+    assert batch_size >= 2, "Batch size should be >= 2 to mix needle."
+
+    # Keep GPU memory usage low to avoid startup allocation failures.
+    gpu_mem_util = float(os.getenv("VLLM_GPU_MEMORY_UTILIZATION", "0.3"))
+    max_model_len = int(os.getenv("VLLM_MAX_MODEL_LEN", "4096"))
+    swap_space_gb = int(os.getenv("VLLM_SWAP_SPACE_GB", "4"))
+
+    # Sampling parameters: longer outputs with a more random-sounding
+    # continuation,but still deterministic due to fixed seed.
+    temperature = float(os.getenv("VLLM_NEEDLE_TEMPERATURE", "0.0"))
+    top_p = float(os.getenv("VLLM_NEEDLE_TOP_P", "0.95"))
+    max_tokens = int(os.getenv("VLLM_NEEDLE_MAX_TOKENS", "128"))
+
+    sampling = SamplingParams(
+        temperature=temperature,
+        top_p=top_p,
+        max_tokens=max_tokens,
+        seed=20240919,
+    )
+
+    needle_prompt = ("There once was a ")
+
+    llm_bs1 = None
+    llm_bsN = None
+    try:
+        # Engine with bs=1 behavior
+        llm_bs1 = LLM_with_max_seqs(
+            model=model,
+            max_num_seqs=1,
+            gpu_memory_utilization=gpu_mem_util,
+            max_model_len=max_model_len,
+            swap_space=swap_space_gb,
+        )
+
+        # Baseline generation for the needle prompt alone.
+        baseline_out = llm_bs1.generate([needle_prompt], sampling)
+        assert len(baseline_out) == 1
+        assert len(baseline_out[0].outputs) >= 1
+        baseline_text = baseline_out[0].outputs[0].text
+
+        # Engine with larger batch limit (e.g., 64)
+        llm_bsN = LLM_with_max_seqs(
+            model=model,
+            max_num_seqs=batch_size,
+            gpu_memory_utilization=gpu_mem_util,
+            max_model_len=max_model_len,
+            swap_space=swap_space_gb,
+        )
+
+        mismatches = 0
+
+        for trial in range(num_trials):
+            # Create a batch of size `batch_size` and insert the needle at
+            # a random index
+            prompts: list[str] = []
+            needle_pos = random.randint(0, batch_size - 1)
+            for i in range(batch_size):
+                if i == needle_pos:
+                    prompts.append(needle_prompt)
+                else:
+                    prompts.append(_random_prompt())
+
+            # Generate with the larger-batch engine
+            outputs = llm_bsN.generate(prompts, sampling)
+            # Find the needle output by position
+            needle_output = outputs[needle_pos]
+            assert needle_output.prompt == needle_prompt
+            assert len(needle_output.outputs) >= 1
+            text = needle_output.outputs[0].text
+
+            if text != baseline_text:
+                mismatches += 1
+
+        passes = num_trials - mismatches
+        # Dump how many passed vs failed
+        print(f"[determinism] total={num_trials}, passed={passes}, "
+              f"failed={mismatches}, batch_size={batch_size}")
+
+        if mismatches > 0:
+            pytest.fail(
+                f"Nondeterministic outputs detected: {mismatches} failed out "
+                f"of {num_trials} trials (batch_size={batch_size}).")
+
+    finally:
+        # Ensure engines are shutdown to free GPU/VRAM across test sessions
+        if llm_bs1 is not None:
+            with contextlib.suppress(Exception):
+                llm_bs1.shutdown()
+        if llm_bsN is not None:
+            with contextlib.suppress(Exception):
+                llm_bsN.shutdown()
+
+
+def _extract_step_logprobs(request_output):
+    if getattr(request_output, "outputs", None):
+        inner = request_output.outputs[0]
+        if hasattr(inner, "logprobs") and inner.logprobs is not None:
+            t = torch.tensor(
+                [
+                    inner.logprobs[i][tid].logprob
+                    for i, tid in enumerate(inner.token_ids)
+                ],
+                dtype=torch.float32,
+            )
+            return t
+
+    return None
+
+
+@pytest.mark.skipif(
+    not torch.cuda.is_available(),
+    reason="Requires CUDA to match production inference path.",
+)
+def test_logprobs_bitwise_batch_invariance_bs1_vs_bs2():
+
+    #model_name = os.getenv("VLLM_TEST_MODEL", "facebook/opt-125m")
+    model_name = os.getenv("VLLM_TEST_MODEL", "Qwen/Qwen3-1.7B")
+    tp_size = int(os.getenv("VLLM_TEST_TP_SIZE", "1"))
+
+    # Force float32 to avoid precision-induced differences.
+    llm = LLM(
+        model=model_name,
+        tensor_parallel_size=tp_size,
+        enforce_eager=True,  # helps reduce nondeterminism from some backends
+    )
+
+    prompts = [
+        "The capital of France is",
+        "The capital of Germany is",
+    ]
+
+    sp = SamplingParams(
+        temperature=0.0,
+        top_p=1.0,
+        max_tokens=8,
+        # Seed shouldn't matter at temperature=0, but keeping it stable anyway.
+        seed=1234,
+        logprobs=5,
+    )
+
+    # BS=1: run prompts individually and collect logprobs per step.
+    bs1_logprobs_per_prompt = []
+    for p in prompts:
+        outs = llm.generate([p], sp, use_tqdm=False)
+        assert len(outs) == 1
+        step_logprobs = _extract_step_logprobs(outs[0])
+        if step_logprobs is None:
+            pytest.skip("Logits are not available on RequestOutput; "
+                        "enable logprobs return to run this test.")
+        bs1_logprobs_per_prompt.append(step_logprobs)
+
+    # BS=2: run prompts in a batch and collect logprobs per step for each
+    # prompt.
+    outs_batched = llm.generate(prompts, sp, use_tqdm=False)
+    assert len(outs_batched) == len(prompts)
+    bs2_logprobs_per_prompt = []
+    for o in outs_batched:
+        step_logprobs = _extract_step_logprobs(o)
+        if step_logprobs is None:
+            pytest.skip("Logits are not available on RequestOutput; "
+                        "enable logprobs return to run this test.")
+        bs2_logprobs_per_prompt.append(step_logprobs)
+
+    # Compare step-by-step logprobs for each prompt between BS=1 and BS=2 runs.
+    for i, (logprobs_bs1, logprobs_bs2) in enumerate(
+            zip(bs1_logprobs_per_prompt, bs2_logprobs_per_prompt)):
+        assert len(logprobs_bs1) == len(logprobs_bs2), (
+            f"Different number of generation steps for prompt index {i}: "
+            f"{len(logprobs_bs1)} (BS=1) vs {len(logprobs_bs2)} (BS=2)")
+        for t, (a, b) in enumerate(zip(logprobs_bs1, logprobs_bs2)):
+            assert a.shape == b.shape, (
+                f"Logits shape mismatch at prompt {i}, step {t}: "
+                f"{a.shape} vs {b.shape}")
+            # Bitwise exact equality.
+            assert torch.equal(
+                a, b), (f"Bitwise logprobs mismatch at prompt {i}, step {t} "
+                        f"(dtype={a.dtype}, shape={a.shape}).")
+
+
+def LLM_with_max_seqs(
+    model: str,
+    max_num_seqs: int,
+    gpu_memory_utilization: float,
+    max_model_len: int,
+    swap_space: int,
+) -> LLM:
+    """
+    Helper to construct an LLM with a specific max_num_seqs (batch-size limit)
+    using the high-level v1 LLM API, while constraining memory usage.
+    """
+    return LLM(
+        model=model,
+        max_num_seqs=max_num_seqs,
+        # Constrain GPU memory pool so test can run even on busy GPUs.
+        gpu_memory_utilization=gpu_memory_utilization,
+        # Keep KV cache footprint small while allowing longer outputs.
+        max_model_len=max_model_len,
+        # Allow some CPU offload if needed.
+        swap_space=swap_space,
+        # Keep things lean and CI-friendly.
+        dtype="float16",
+        # Single-GPU by default; override externally if desired.
+        tensor_parallel_size=int(os.getenv("VLLM_TP_SIZE", "1")),
+        trust_remote_code=os.getenv("VLLM_TRUST_REMOTE_CODE", "0") == "1",
+    )

vllm/model_executor/layers/batch_invariant.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+import contextlib
+import os
+from collections import namedtuple
+from collections.abc import Callable
+from typing import Any, Union
+
+import torch
+import triton
+import triton.language as tl
+
+
+def _matmul_launch_metadata(grid: Callable[..., Any], kernel: Any,
+                            args: dict[str, Any]) -> dict[str, Any]:
+    ret = {}
+    m, n, k = args["M"], args["N"], args["K"]
+    ret["name"] = f"{kernel.name} [M={m}, N={n}, K={k}]"
+    if "tiles_per_update" in args:
+        ret["name"] = (f"{kernel.name} [M={m}, N={n}, K={k}, "
+                       f"tiles_per_update={args['tiles_per_update']:02}]")
+    if "c_ptr" in args:
+        bytes_per_elem = args["c_ptr"].element_size()
+    else:
+        bytes_per_elem = 1 if args["FP8_OUTPUT"] else 2
+    ret[f"flops{bytes_per_elem * 8}"] = 2.0 * m * n * k
+    ret["bytes"] = bytes_per_elem * (m * k + n * k + m * n)
+    return ret
+
+
+@triton.jit
+def _compute_pid(tile_id, num_pid_in_group, num_pid_m, GROUP_SIZE_M, NUM_SMS):
+    group_id = tile_id // num_pid_in_group
+    first_pid_m = group_id * GROUP_SIZE_M
+    group_size_m = min(num_pid_m - first_pid_m, GROUP_SIZE_M)
+    pid_m = first_pid_m + (tile_id % group_size_m)
+    pid_n = (tile_id % num_pid_in_group) // group_size_m
+    return pid_m, pid_n
+
+
+@triton.jit(launch_metadata=_matmul_launch_metadata)
+def matmul_kernel_persistent(
+    a_ptr,
+    b_ptr,
+    c_ptr,  #
+    bias_ptr,
+    M,
+    N,
+    K,  #
+    stride_am,
+    stride_ak,
+    stride_bk,
+    stride_bn,
+    stride_cm,
+    stride_cn,
+    BLOCK_SIZE_M: tl.constexpr,  #
+    BLOCK_SIZE_N: tl.constexpr,  #
+    BLOCK_SIZE_K: tl.constexpr,  #
+    GROUP_SIZE_M: tl.constexpr,  #
+    NUM_SMS: tl.constexpr,  #
+    A_LARGE: tl.constexpr,
+    B_LARGE: tl.constexpr,
+    C_LARGE: tl.constexpr,
+    HAS_BIAS: tl.constexpr,
+):
+    start_pid = tl.program_id(axis=0)
+    num_pid_m = tl.cdiv(M, BLOCK_SIZE_M)
+    num_pid_n = tl.cdiv(N, BLOCK_SIZE_N)
+    k_tiles = tl.cdiv(K, BLOCK_SIZE_K)
+    num_tiles = num_pid_m * num_pid_n
+
+    tile_id_c = start_pid - NUM_SMS
+
+    offs_k_for_mask = tl.arange(0, BLOCK_SIZE_K)
+    num_pid_in_group = GROUP_SIZE_M * num_pid_n
+
+    for tile_id in tl.range(start_pid, num_tiles, NUM_SMS, flatten=True):
+        pid_m, pid_n = _compute_pid(tile_id, num_pid_in_group, num_pid_m,
+                                    GROUP_SIZE_M, NUM_SMS)
+        start_m = pid_m * BLOCK_SIZE_M
+        start_n = pid_n * BLOCK_SIZE_N
+        offs_am = start_m + tl.arange(0, BLOCK_SIZE_M)
+        offs_bn = start_n + tl.arange(0, BLOCK_SIZE_N)
+        if A_LARGE:
+            offs_am = offs_am.to(tl.int64)
+        if B_LARGE:
+            offs_bn = offs_bn.to(tl.int64)
+        offs_am = tl.where(offs_am < M, offs_am, 0)
+        offs_bn = tl.where(offs_bn < N, offs_bn, 0)
+        offs_am = tl.max_contiguous(tl.multiple_of(offs_am, BLOCK_SIZE_M),
+                                    BLOCK_SIZE_M)
+        offs_bn = tl.max_contiguous(tl.multiple_of(offs_bn, BLOCK_SIZE_N),
+                                    BLOCK_SIZE_N)
+
+        accumulator = tl.zeros((BLOCK_SIZE_M, BLOCK_SIZE_N), dtype=tl.float32)
+        for ki in range(k_tiles):
+            if A_LARGE or B_LARGE:
+                offs_k = ki * BLOCK_SIZE_K + tl.arange(0, BLOCK_SIZE_K).to(
+                    tl.int64)
+            else:
+                offs_k = ki * BLOCK_SIZE_K + tl.arange(0, BLOCK_SIZE_K)
+            a_ptrs = a_ptr + (offs_am[:, None] * stride_am +
+                              offs_k[None, :] * stride_ak)
+            b_ptrs = b_ptr + (offs_k[:, None] * stride_bk +
+                              offs_bn[None, :] * stride_bn)
+
+            a = tl.load(a_ptrs,
+                        mask=offs_k_for_mask[None, :] < K - ki * BLOCK_SIZE_K,
+                        other=0.0)
+            b = tl.load(b_ptrs,
+                        mask=offs_k_for_mask[:, None] < K - ki * BLOCK_SIZE_K,
+                        other=0.0)
+            accumulator = tl.dot(a, b, accumulator)
+
+        tile_id_c += NUM_SMS
+        pid_m, pid_n = _compute_pid(tile_id_c, num_pid_in_group, num_pid_m,
+                                    GROUP_SIZE_M, NUM_SMS)
+        offs_cm = pid_m * BLOCK_SIZE_M + tl.arange(0, BLOCK_SIZE_M)
+        offs_cn = pid_n * BLOCK_SIZE_N + tl.arange(0, BLOCK_SIZE_N)
+        if C_LARGE:
+            offs_cm = offs_cm.to(tl.int64)
+            offs_cn = offs_cn.to(tl.int64)
+        c_ptrs = c_ptr + stride_cm * offs_cm[:, None] + stride_cn * offs_cn[
+            None, :]
+        c_mask = (offs_cm[:, None] < M) & (offs_cn[None, :] < N)
+        if HAS_BIAS:
+            bias_ptrs = bias_ptr + offs_cn
+            bias = tl.load(bias_ptrs, mask=offs_cn < N,
+                           other=0.0).to(tl.float32)
+            accumulator += bias
+        if c_ptr.dtype.element_ty == tl.float8e4nv:
+            c = accumulator.to(tl.float8e4nv)
+        else:
+            c = accumulator.to(tl.float16)
+        tl.store(c_ptrs, c, mask=c_mask)
+
+
+def matmul_persistent(a: torch.Tensor,
+                      b: torch.Tensor,
+                      bias: Union[torch.Tensor, None] = None):
+    # Check constraints.
+    assert a.shape[1] == b.shape[0], "Incompatible dimensions"
+    assert a.dtype == b.dtype, "Incompatible dtypes"
+    assert bias is None or bias.dim() == 1, (
+        "Currently assuming bias is 1D, let Horace know if you run into this")
+    NUM_SMS = torch.cuda.get_device_properties("cuda").multi_processor_count
+    M, K = a.shape
+    K, N = b.shape
+    dtype = a.dtype
+    # Allocates output.
+    c = torch.empty((M, N), device=a.device, dtype=dtype)
+
+    # 1D launch kernel where each block gets its own program.
+    def grid(META):
+        return (min(
+            NUM_SMS,
+            triton.cdiv(M, META["BLOCK_SIZE_M"]) *
+            triton.cdiv(N, META["BLOCK_SIZE_N"])), )
+
+    configs = {
+        torch.bfloat16: {
+            "BLOCK_SIZE_M": 128,
+            "BLOCK_SIZE_N": 128,
+            "BLOCK_SIZE_K": 64,
+            "GROUP_SIZE_M": 8,
+            "num_stages": 3,
+            "num_warps": 8,
+        },
+        torch.float16: {
+            "BLOCK_SIZE_M": 128,
+            "BLOCK_SIZE_N": 256,
+            "BLOCK_SIZE_K": 64,
+            "GROUP_SIZE_M": 8,
+            "num_stages": 3,
+            "num_warps": 8,
+        },
+        torch.float32: {
+            "BLOCK_SIZE_M": 128,
+            "BLOCK_SIZE_N": 128,
+            "BLOCK_SIZE_K": 32,
+            "GROUP_SIZE_M": 8,
+            "num_stages": 3,
+            "num_warps": 8,
+        },
+    }
+    # print(a.device, b.device, c.device)
+    matmul_kernel_persistent[grid](
+        a,
+        b,
+        c,  #
+        bias,
+        M,
+        N,
+        K,  #
+        a.stride(0),
+        a.stride(1),  #
+        b.stride(0),
+        b.stride(1),  #
+        c.stride(0),
+        c.stride(1),  #
+        NUM_SMS=NUM_SMS,  #
+        A_LARGE=a.numel() > 2**31,
+        B_LARGE=b.numel() > 2**31,
+        C_LARGE=c.numel() > 2**31,
+        HAS_BIAS=bias is not None,
+        **configs[dtype],
+    )
+    return c
+
+
+@triton.jit
+def _log_softmax_kernel(
+    input_ptr,
+    output_ptr,
+    input_row_stride,
+    output_row_stride,
+    n_cols,
+    BLOCK_SIZE: tl.constexpr,
+):
+    """
+    Compute log_softmax along the last dimension of a 2D tensor.
+    Each block handles one row of the input tensor.
+    """
+    # Get the row index for this block
+    row_idx = tl.program_id(0).to(tl.int64)
+
+    # Compute base pointers for input and output rows
+    row_start_ptr = input_ptr + row_idx * input_row_stride
+    output_row_start_ptr = output_ptr + row_idx * output_row_stride
+
+    # Step 1: Find maximum value in the row for numerical stability
+    max_val = -float("inf")
+    for col_offset in range(0, n_cols, BLOCK_SIZE):
+        col_idx = col_offset + tl.arange(0, BLOCK_SIZE)
+        mask = col_idx < n_cols
+
+        # Load values
+        vals = tl.load(row_start_ptr + col_idx, mask=mask, other=-float("inf"))
+
+        # Update maximum
+        max_val = tl.max(tl.maximum(vals, max_val))
+
+    # Step 2: Compute sum of exp(x - max_val)
+    sum_exp = 0.0
+    for col_offset in range(0, n_cols, BLOCK_SIZE):
+        col_idx = col_offset + tl.arange(0, BLOCK_SIZE)
+        mask = col_idx < n_cols
+
+        # Load values
+        vals = tl.load(row_start_ptr + col_idx, mask=mask, other=0.0)
+
+        # Compute exp(x - max_val) and accumulate
+        exp_vals = tl.exp(vals - max_val)
+        sum_exp += tl.sum(tl.where(mask, exp_vals, 0.0))
+
+    # Compute log(sum_exp)
+    log_sum_exp = tl.log(sum_exp)
+
+    # Step 3: Compute final log_softmax values: x - max_val - log_sum_exp
+    for col_offset in range(0, n_cols, BLOCK_SIZE):
+        col_idx = col_offset + tl.arange(0, BLOCK_SIZE)
+        mask = col_idx < n_cols
+
+        # Load values
+        vals = tl.load(row_start_ptr + col_idx, mask=mask)
+
+        # Compute log_softmax
+        output = vals - max_val - log_sum_exp
+
+        # Store results
+        tl.store(output_row_start_ptr + col_idx, output, mask=mask)
+
+
+def log_softmax(input: torch.Tensor, dim: int = -1) -> torch.Tensor:
+    """
+    Compute log_softmax using Triton kernel.
+
+    Args:
+        input: Input tensor
+        dim: Dimension along which to compute log_softmax
+             (only -1 or last dim supported)
+    >> Stashed changes
+    Returns:
+        Tensor with log_softmax applied along the specified dimension
+    """
+    if dim != -1 and dim != input.ndim - 1:
+        raise ValueError("This implementation only supports log_softmax along "
+                         "the last dimension")
+
+    # Flatten all dimensions except the last one
+    original_shape = input.shape
+    input_2d = input.reshape(-1, input.shape[-1])
+    input_2d = input_2d.contiguous()
+
+    n_rows, n_cols = input_2d.shape
+
+    # Allocate output tensor
+    output = torch.empty_like(input_2d)
+
+    # Choose block size based on the number of columns
+    BLOCK_SIZE = 1024
+
+    # Launch kernel with one block per row
+    grid = (n_rows, )
+    _log_softmax_kernel[grid](
+        input_2d,
+        output,
+        input_2d.stride(0),
+        output.stride(0),
+        n_cols,
+        BLOCK_SIZE=BLOCK_SIZE,
+    )
+    # Reshape output back to original shape
+    return output.reshape(original_shape)
+
+
+@triton.jit
+def mean_kernel(
+    input_ptr,
+    output_ptr,
+    input_stride0,
+    input_stride1,
+    input_stride2,
+    output_stride0,
+    output_stride1,
+    M,  # size before reduction dim
+    N,  # size of reduction dim
+    K,  # size after reduction dim
+    BLOCK_SIZE: tl.constexpr,
+):
+    """
+    Kernel for computing mean along a single dimension.
+    Input is viewed as (M, N, K) where N is the dimension being reduced.
+    """
+    # Program ID gives us which output element we're computing
+    pid = tl.program_id(0)
+
+    # Compute output indices
+    m_idx = pid // K
+    k_idx = pid % K
+
+    # Bounds check
+    if m_idx >= M or k_idx >= K:
+        return
+
+    # Accumulate sum across reduction dimension
+    acc = 0.0
+    for n_start in range(0, N, BLOCK_SIZE):
+        n_offsets = n_start + tl.arange(0, BLOCK_SIZE)
+        mask = n_offsets < N
+
+        # Calculate input indices
+        input_idx = m_idx * input_stride0 + n_offsets * input_stride1 \
+            + k_idx * input_stride2
+
+        # Load and accumulate
+        vals = tl.load(input_ptr + input_idx, mask=mask, other=0.0)
+        acc += tl.sum(vals)
+
+    # Compute mean and store
+    mean_val = acc / N
+    output_idx = m_idx * output_stride0 + k_idx * output_stride1
+    tl.store(output_ptr + output_idx, mean_val)
+
+
+def mean_dim(input: torch.Tensor,
+             dim: int,
+             keepdim: bool = False,
+             dtype: Union[torch.dtype, None] = None) -> torch.Tensor:
+    """
+    Triton implementation of torch.mean with single dimension reduction.
+
+    Args:
+        input: Input tensor
+        dim: Single dimension along which to compute mean
+        keepdim: Whether to keep the reduced dimension
+        dtype: Output dtype. If None, uses input dtype
+               (or float32 for integer inputs)
+
+    Returns:
+        Tensor with mean values along specified dimension
+    """
+    # Validate inputs
+    assert input.is_cuda, "Input must be a CUDA tensor"
+    assert -input.ndim <= dim < input.ndim, (
+        f"Invalid dimension {dim} for tensor with {input.ndim} dimensions")
+
+    # Handle negative dim
+    if dim < 0:
+        dim = dim + input.ndim
+
+    # Handle dtype
+    if dtype is None:
+        if input.dtype in [torch.int8, torch.int16, torch.int32, torch.int64]:
+            dtype = torch.float32
+        else:
+            dtype = input.dtype
+
+    # Convert input to appropriate dtype if needed
+    if input.dtype != dtype:
+        input = input.to(dtype)
+
+    # Get input shape and strides
+    shape = list(input.shape)
+
+    # Calculate dimensions for kernel
+    M = 1
+    for i in range(dim):
+        M *= shape[i]
+
+    N = shape[dim]
+
+    K = 1
+    for i in range(dim + 1, len(shape)):
+        K *= shape[i]
+
+    # Reshape input to 3D view (M, N, K)
+    input_3d = input.reshape(M, N, K)
+
+    # Create output shape
+    if keepdim:
+        output_shape = shape.copy()
+        output_shape[dim] = 1
+    else:
+        output_shape = shape[:dim] + shape[dim + 1:]
+
+    # Create output tensor
+    output = torch.empty(output_shape, dtype=dtype, device=input.device)
+
+    # Reshape output for kernel
+    if keepdim:
+        output_2d = output.reshape(M, 1, K).squeeze(1)
+    else:
+        output_2d = output.reshape(M, K)
+
+    # Launch kernel
+    grid = (M * K, )
+    BLOCK_SIZE = 1024
+
+    mean_kernel[grid](
+        input_3d,
+        output_2d,
+        input_3d.stride(0),
+        input_3d.stride(1),
+        input_3d.stride(2),
+        output_2d.stride(0),
+        output_2d.stride(1) if output_2d.ndim > 1 else 0,
+        M,
+        N,
+        K,
+        BLOCK_SIZE,
+    )
+
+    return output
+
+
+def mm_batch_invariant(a, b):
+    return matmul_persistent(a, b)
+
+
+def addmm_batch_invariant(bias, a, b):
+    return matmul_persistent(a, b, bias=bias)
+
+
+def _log_softmax_batch_invariant(input, dim, _half_to_float):
+    assert not _half_to_float, "not implemented"
+    return log_softmax(input, dim=dim)
+
+
+def mean_batch_invariant(input,
+                         dim,
+                         keepdim=False,
+                         dtype: Union[torch.dtype, None] = None):
+    assert dtype is None or dtype == torch.float32, \
+        f"unsupported dtype: {dtype}"
+
+    result = input.to(torch.float32)
+
+    # Sort dimensions to reduce from largest to smallest to handle shifting dims
+    # during iterative reduction.
+    sorted_dims = sorted([d % input.ndim for d in dim], reverse=True)
+
+    # Iteratively apply a deterministic mean.
+    for d in sorted_dims:
+        result = mean_dim(result, dim=d, keepdim=True)
+
+    if not keepdim:
+        # Squeeze the reduced dimensions.
+        for d in sorted_dims:
+            result = result.squeeze(d)
+
+    return result
+
+
+_batch_invariant_MODE = False
+_batch_invariant_LIB = None
+
+
+def is_batch_invariant_mode_enabled():
+    return _batch_invariant_MODE
+
+
+def enable_batch_invariant_mode():
+    global _batch_invariant_MODE, _batch_invariant_LIB
+    if _batch_invariant_MODE:
+        return
+
+    _batch_invariant_MODE = True
+    _batch_invariant_LIB = torch.library.Library("aten", "IMPL")
+    _batch_invariant_LIB.impl("aten::mm", mm_batch_invariant, "CUDA")
+    _batch_invariant_LIB.impl("aten::addmm", addmm_batch_invariant, "CUDA")
+    _batch_invariant_LIB.impl("aten::_log_softmax",
+                              _log_softmax_batch_invariant, "CUDA")
+    _batch_invariant_LIB.impl("aten::mean.dim", mean_batch_invariant, "CUDA")
+
+
+def disable_batch_invariant_mode():
+    global _batch_invariant_MODE, _batch_invariant_LIB
+    if _batch_invariant_LIB is not None:
+        _batch_invariant_LIB._destroy()
+    _batch_invariant_MODE = False
+    _batch_invariant_LIB = None
+
+
+@contextlib.contextmanager
+def set_batch_invariant_mode(enabled: bool = True):
+    global _batch_invariant_MODE, _batch_invariant_LIB
+    old_data = (_batch_invariant_MODE, _batch_invariant_LIB)
+    if enabled:
+        enable_batch_invariant_mode()
+    else:
+        disable_batch_invariant_mode()
+    yield
+    if _batch_invariant_LIB is not None:
+        _batch_invariant_LIB._destroy()
+    _batch_invariant_MODE, _batch_invariant_LIB = old_data
+
+
+AttentionBlockSize = namedtuple("AttentionBlockSize", ["block_m", "block_n"])
+
+
+def get_batch_invariant_attention_block_size() -> AttentionBlockSize:
+    return AttentionBlockSize(block_m=16, block_n=16)
+
+
+def vllm_kernel_override_batch_invariant():
+    env_key = "VLLM_KERNEL_OVERRIDE_BATCH_INVARIANT"
+    is_overridden = False
+    val = os.getenv(env_key, "0")
+    try:
+        is_overridden = int(val) != 0
+    except ValueError:
+        is_overridden = False
+    return is_overridden
+
+
+def init_batch_invariance():
+    # this will hit all the csrc overrides as well
+    if vllm_kernel_override_batch_invariant():
+        os.environ["VLLM_ATTENTION_BACKEND"] = "FLEX_ATTENTION"
+        enable_batch_invariant_mode()

vllm/v1/attention/backends/flex_attention.py

@@ -18,6 +18,8 @@ from vllm.attention.backends.abstract import (AttentionBackend, AttentionImpl,
                                               is_quantized_kv_cache)
 from vllm.config import VllmConfig
 from vllm.logger import init_logger
+from vllm.model_executor.layers.batch_invariant import (
+    vllm_kernel_override_batch_invariant)
 from vllm.utils import cdiv, is_torch_equal_or_newer
 from vllm.v1.attention.backends.utils import (AttentionMetadataBuilder,
                                               CommonAttentionMetadata)
@@ -839,6 +841,11 @@ def get_kernel_options(query, block_m, block_n,
     kernel_options: dict[str, Union[int, bool]] = {
         "FORCE_USE_FLEX_ATTENTION": True,
     }
+    if vllm_kernel_override_batch_invariant():
+        kernel_options["BLOCK_M"] = 16
+        kernel_options["BLOCK_N"] = 16
+        kernel_options["IS_DIVISIBLE"] = False
+        return kernel_options
     if use_direct_build:
         kernel_options["BLOCK_M"] = block_m
         kernel_options["BLOCK_N"] = block_n

vllm/v1/worker/gpu_model_runner.py

@@ -192,6 +192,9 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
         from vllm.model_executor.models.utils import set_cpu_offload_max_bytes
         set_cpu_offload_max_bytes(
             int(self.cache_config.cpu_offload_gb * 1024**3))
+        from vllm.model_executor.layers.batch_invariant import (
+            init_batch_invariance)
+        init_batch_invariance()
 
         model_config = self.model_config
         cache_config = self.cache_config