[PyTorch] Fix FP8 logic related to FA2/FA3 (#1141)

* fix FP8 logic when FA3 is not installed
Signed-off-by: Charlene Yang <8636796+cyanguwa@users.noreply.github.com>

* minor tweak to make logic more explicit
Signed-off-by: Charlene Yang <8636796+cyanguwa@users.noreply.github.com>

* minor fixes
Signed-off-by: Charlene Yang <8636796+cyanguwa@users.noreply.github.com>

* limit FA3 warning to Hopper and NVTE_FLASH_ATTN=1
Signed-off-by: Charlene Yang <8636796+cyanguwa@users.noreply.github.com>

* prefer fused attn for FP8
Signed-off-by: Charlene Yang <8636796+cyanguwa@users.noreply.github.com>

---------
Signed-off-by: Charlene Yang <8636796+cyanguwa@users.noreply.github.com>

tests/pytorch/fused_attn/test_fused_attn.py

@@ -22,6 +22,7 @@ from transformer_engine.pytorch.attention import (
     get_attention_backend,
     _flash_attn_2_plus,
     _flash_attn_2_3_plus,
+    _flash_attn_3_plus,
     check_set_window_size,
     AttentionParams,
     _attention_backends,
@@ -135,7 +136,6 @@ def _get_attention_backends(
     os.environ["NVTE_FLASH_ATTN"] = "1"
     os.environ["NVTE_FUSED_ATTN"] = "1"
     os.environ["NVTE_UNFUSED_ATTN"] = "1"
-    global _attention_backends
     _attention_backends["backend_selection_requires_update"] = True
 
     alibi_slopes_shape = None
@@ -678,7 +678,6 @@ def _run_dot_product_attention(
     if backend == "FusedAttention":
         os.environ["NVTE_FUSED_ATTN"] = "1"
         os.environ["NVTE_FUSED_ATTN_FORCE_WORKSPACE_OPT"] = "1" if workspace_opt else "0"
-    global _attention_backends
     _attention_backends["backend_selection_requires_update"] = True
 
     # Create seqlens
@@ -1167,7 +1166,6 @@ def _run_transformer_layer(
         os.environ["NVTE_FLASH_ATTN"] = "1"
     if backend == "FusedAttention":
         os.environ["NVTE_FUSED_ATTN"] = "1"
-    global _attention_backends
     _attention_backends["backend_selection_requires_update"] = True
 
     # Create input tensor
@@ -1352,8 +1350,7 @@ def test_mha_fp8_vs_f16(dtype, model, qkv_format, input_layernorm, fp8_dpa_bwd,
     os.environ["NVTE_FP8_DPA_BWD"] = "1" if fp8_dpa_bwd else "0"
     config = model_configs_fp8_vs_f16[model]
 
-    global _attention_backends
-    if not is_training:
+    if _flash_attn_3_plus and not is_training:
         os.environ["NVTE_FLASH_ATTN"] = "1"
         os.environ["NVTE_FUSED_ATTN"] = "0"
         _attention_backends["backend_selection_requires_update"] = True
@@ -1379,7 +1376,7 @@ def test_mha_fp8_vs_f16(dtype, model, qkv_format, input_layernorm, fp8_dpa_bwd,
     rtol = 5e-1
     rmse_tol = 0.15
     logging.debug("========== {:^25s} ==========".format("forward output"))
-    if not is_training:
+    if _flash_attn_3_plus and not is_training:
         _error(
             flash_attn_fwd_fp8,
             fused_attn_fwd_f16,
@@ -1527,8 +1524,7 @@ def test_dpa_fp8_vs_f16(dtype, model, qkv_layout, fp8_dpa_bwd, is_training):
     os.environ["NVTE_FP8_DPA_BWD"] = "1" if fp8_dpa_bwd else "0"
     os.environ["NVTE_ALLOW_NONDETERMINISTIC_ALGO"] = "1"
 
-    global _attention_backends
-    if not is_training:
+    if _flash_attn_3_plus and not is_training:
         os.environ["NVTE_FLASH_ATTN"] = "1"
         os.environ["NVTE_FUSED_ATTN"] = "0"
         _attention_backends["backend_selection_requires_update"] = True
@@ -1555,7 +1551,7 @@ def test_dpa_fp8_vs_f16(dtype, model, qkv_layout, fp8_dpa_bwd, is_training):
     rmse_tol = 0.1
     bwd_names = ["dq", "dk", "dv"]
     logging.debug("========== {:^25s} ==========".format("forward output"))
-    if not is_training:
+    if _flash_attn_3_plus and not is_training:
         _error(
             flash_attn_fwd_fp8,
             fused_attn_fwd_f16,
@@ -1778,7 +1774,6 @@ def _run_custom_mha_fp8(dtype, config, backend):
         os.environ["NVTE_FLASH_ATTN"] = "1"
     if backend == "FusedAttention":
         os.environ["NVTE_FUSED_ATTN"] = "1"
-    global _attention_backends
     _attention_backends["backend_selection_requires_update"] = True
 
     inp = 0.0001 * torch.randint(
@@ -1833,7 +1828,6 @@ def _run_ref_mha_f16(dtype, config, backend):
         os.environ["NVTE_FLASH_ATTN"] = "1"
     if backend == "FusedAttention":
         os.environ["NVTE_FUSED_ATTN"] = "1"
-    global _attention_backends
     _attention_backends["backend_selection_requires_update"] = True
 
     inp = torch.load("qkv.pt").to(device="cuda")

transformer_engine/pytorch/attention.py

@@ -74,6 +74,9 @@ from transformer_engine.pytorch.jit import jit_fuser, no_torch_dynamo
 from transformer_engine.pytorch.graph import is_graph_capturing
 
 
+_NVTE_FLASH_ATTN = int(os.getenv("NVTE_FLASH_ATTN", "1"))
+_NVTE_FUSED_ATTN = int(os.getenv("NVTE_FUSED_ATTN", "1"))
+_NVTE_UNFUSED_ATTN = int(os.getenv("NVTE_UNFUSED_ATTN", "1"))
 _flash_attn_version = PkgVersion(get_pkg_version("flash-attn"))
 _flash_attn_version_required = PkgVersion("2.0.6")
 _flash_attn_max_version = PkgVersion("2.6.3")
@@ -89,13 +92,14 @@ try:
     _flash_attn_v3_version = PkgVersion(get_pkg_version("flashattn-hopper"))
     _flash_attn_3_plus = _flash_attn_v3_version >= PkgVersion("2.6.1")
 except PackageNotFoundError:
-    warnings.warn(
-        "To use flash-attn v3, please use the following commands to install: \n"
-        """(1) pip install "git+https://github.com/Dao-AILab/flash-attention.git#egg=flashattn-hopper&subdirectory=hopper" \n"""
-        """(2) python_path=`python -c "import site; print(site.getsitepackages()[0])"` \n"""
-        """(3) mkdir -p $python_path/flashattn_hopper \n"""
-        """(4) wget -P $python_path/flashattn_hopper https://raw.githubusercontent.com/Dao-AILab/flash-attention/main/hopper/flash_attn_interface.py"""
-    )
+    if get_device_compute_capability() == (9, 0) and _NVTE_FLASH_ATTN:
+        warnings.warn(
+            "To use flash-attn v3, please use the following commands to install: \n"
+            """(1) pip install "git+https://github.com/Dao-AILab/flash-attention.git#egg=flashattn-hopper&subdirectory=hopper" \n"""
+            """(2) python_path=`python -c "import site; print(site.getsitepackages()[0])"` \n"""
+            """(3) mkdir -p $python_path/flashattn_hopper \n"""
+            """(4) wget -P $python_path/flashattn_hopper https://raw.githubusercontent.com/Dao-AILab/flash-attention/main/hopper/flash_attn_interface.py"""
+        )
 else:
     from flashattn_hopper.flash_attn_interface import flash_attn_func as flash_attn_func_v3
     from flashattn_hopper.flash_attn_interface import (
@@ -137,10 +141,6 @@ _formatter = logging.Formatter("[%(levelname)-8s | %(name)-19s]: %(message)s")
 _stream_handler = logging.StreamHandler()
 _stream_handler.setFormatter(_formatter)
 
-_NVTE_FLASH_ATTN = int(os.getenv("NVTE_FLASH_ATTN", "1"))
-_NVTE_FUSED_ATTN = int(os.getenv("NVTE_FUSED_ATTN", "1"))
-_NVTE_UNFUSED_ATTN = int(os.getenv("NVTE_UNFUSED_ATTN", "1"))
-
 _attention_backends = {
     "attention_params": None,
     "use_flash_attention": None,
@@ -382,8 +382,13 @@ def get_attention_backend(
 
     # Filter: Execution type
     if fp8 and fp8_meta["recipe"].fp8_dpa:
-        if use_flash_attention and is_training:
-            logger.debug("Disabling FlashAttention as it does not support FP8 training")
+        if use_flash_attention and not _use_flash_attn_3:
+            logger.debug("Disabling FlashAttention as FlashAttention 2 does not support FP8")
+            use_flash_attention = False
+        if use_flash_attention and _use_flash_attn_3 and is_training:
+            logger.debug(
+                "Disabling FlashAttention as FlashAttention 3 does not support FP8 training"
+            )
             use_flash_attention = False
         if use_unfused_attention:
             logger.debug("Disabling UnfusedDotProductAttention as it does not support FP8")
@@ -826,6 +831,21 @@ def get_attention_backend(
             )
             use_flash_attention = False
 
+    # Select FusedAttention for FP8
+    # FA3 uses default scaling factors (i.e. 1) in FP8 execution, while FusedAttention takes
+    # scaling factors from `fp8_meta` and offers more accurate quantization/de-quantization
+    if (
+        use_flash_attention
+        and use_fused_attention
+        and fused_attention_backend == FusedAttnBackend["FP8"]
+        and _use_flash_attn_3
+    ):
+        logger.debug(
+            "Disabling FlashAttention 3 to give FusedAttention preference as FusedAttention "
+            "supports more accurate scaling factors in FP8 execution"
+        )
+        use_flash_attention = False
+
     # Selected backend
     if use_flash_attention:
         use_fused_attention = False