Tutorial for DebugMode (#3697)

A tutorial for DebugMode

cc @pianpwk

Co-authored-by: Svetlana Karslioglu <svekars@meta.com>
Co-authored-by: Alanna Burke <burkealanna@meta.com>

Author: 3 people

recipes_index.rst

@@ -152,6 +152,13 @@ from our full-length tutorials.
    :link: recipes/recipes/tensorboard_with_pytorch.html
    :tags: Visualization,TensorBoard
 
+.. customcarditem::
+   :header: DebugMode: Recording Dispatched Operations and Numerical Debugging
+   :card_description: Inspect dispatched ops, tensor hashes, and module boundaries to debug eager and ``torch.compile`` runs.
+   :image: _static/img/thumbnails/cropped/generic-pytorch-logo.png
+   :link: recipes/debug_mode_tutorial.html
+   :tags: Interpretability,Compiler
+
 .. Automatic Mixed Precision
 
 .. customcarditem::
@@ -354,3 +361,4 @@ from our full-length tutorials.
    recipes/distributed_device_mesh
    recipes/distributed_checkpoint_recipe
    recipes/distributed_async_checkpoint_recipe
+   recipes/debug_mode_tutorial

recipes_source/debug_mode_tutorial.py

@@ -0,0 +1,279 @@
+# -*- coding: utf-8 -*-
+
+"""
+DebugMode: Recording Dispatched Operations and Numerical Debugging
+=================================================================
+
+**Authors:** Pian Pawakapan, Shangdi Yu
+
+.. grid:: 2
+
+    .. grid-item-card:: :octicon:`mortar-board;1em;` What you will learn
+       :class-card: card-prerequisites
+
+       * How to capture dispatched ops for eager and ``torch.compile`` runs
+       * How to use tensor hashes and stack traces in DebugMode to pinpoint numerical divergence
+
+    .. grid-item-card:: :octicon:`list-unordered;1em;` Prerequisites
+       :class-card: card-prerequisites
+
+       * PyTorch 2.10 or later
+
+"""
+
+######################################################################
+# Overview
+# --------
+#
+# ``DebugMode`` (:class:`torch.utils._debug_mode.DebugMode`) is a
+# ``TorchDispatchMode`` that intercepts PyTorch runtime calls and emits a
+# hierarchical log of operations. It is particularly useful when you need to
+# understand *what* actually runs, both in eager mode and under ``torch.compile``
+# or when you need to pinpoint numerical divergence between two runs.
+#
+# Key capabilities:
+#
+# * **Runtime logging** – Records dispatched operations and TorchInductor compiled
+#   Triton kernels.
+# * **Tensor hashing** – Attaches deterministic hashes to inputs/outputs to enable
+#   diffing runs to locate numerical divergences.
+# * **Dispatch hooks** – Allows registration of custom hooks to annotate calls
+#
+# .. note::
+#
+#    This recipe describes a prototype feature. Prototype features are typically
+#    at an early stage for feedback and testing and are subject to change.
+#
+
+######################################################################
+# Quick start
+# -----------
+#
+# The snippet below captures a small eager workload and prints the debug string:
+
+from torch._inductor.decomposition import decomps_to_exclude
+import torch
+from torch.utils._debug_mode import DebugMode
+
+def run_once():
+    x = torch.randn(8, 8)
+    y = torch.randn(8, 8)
+    return torch.mm(torch.relu(x), y)
+
+with DebugMode() as debug_mode:
+    out = run_once()
+
+print("DebugMode output:")
+print(debug_mode.debug_string())
+
+
+######################################################################
+# Getting more metadata
+# -----------
+#
+# For most investigations, you'll want to enable stack traces, tensor IDs, and tensor hashing.
+# These features provide metadata to correlate operations back to model code.
+#
+# ``DebugMode.log_tensor_hashes`` decorates the log with hashes for every call.
+# The ``hash_tensor`` hash function uses ``torch.hash_tensor``, which returns 0 for tensors whose
+# elements are all the same. The ``norm`` hash function uses ``norm`` with ``p=1``.
+# With both these functions, especially ``norm``, tensor closeness in numerics is related to hash closeness,
+# so it's rather interpretable. The default ``hash_fn`` is ``norm``.
+
+with (
+    DebugMode(
+        # record_stack_trace is only supported for eager in pytorch 2.10
+        record_stack_trace=True,
+        record_ids=True,
+    ) as debug_mode,
+    DebugMode.log_tensor_hashes(
+        hash_fn=["norm"], # this is the default
+        hash_inputs=True,
+    ),
+):
+    result = run_once()
+
+print("DebugMode output with more metadata:")
+print(
+    debug_mode.debug_string(show_stack_trace=True)
+)
+
+######################################################################
+# Each line follows ``op(args) -> outputs``. When ``record_ids`` is enabled,
+# tensors are suffixed with ``$<id>`` and DTensors are labeled ``dt``.
+
+
+######################################################################
+# Log Triton kernels
+# ------------------
+#
+# Though Triton kernels are not dispatched, DebugMode has custom logic that logs their inputs and outputs.
+#
+# Inductor-generated Triton kernels show up with a ``[triton]`` prefix.
+# Pre/post hash annotations report buffer hashes around each kernel call, which
+# is helpful when isolating incorrect kernels.
+def f(x):
+    return torch.mm(torch.relu(x), x.T)
+
+x = torch.randn(3, 3, device="cuda")
+
+with (
+    DebugMode(record_output=True) as debug_mode,
+    DebugMode.log_tensor_hashes(
+        hash_inputs=True,
+    )
+):
+    a = torch.compile(f)(x)
+
+print("Triton in DebugMode logs:")
+print(debug_mode.debug_string())
+
+######################################################################
+# Numerical debugging with tensor hashes
+# --------------------------------------
+#
+# If you have numerical divergence between modes, you can use DebugMode to find where the
+# numerical divergence originates.
+# In the example below, you can see that all tensor hashes are the same for eager mode and compiled mode.
+# If any hash is different, then that's where the numerical divergence is coming from.
+
+def run_model(model, data, *, compile_with=None):
+    if compile_with is not None:
+        model = torch.compile(model, backend=compile_with)
+    with DebugMode(record_output=True) as dm, DebugMode.log_tensor_hashes(
+        hash_inputs=True,
+    ):
+        dm_out = model(*data)
+    return dm, dm_out
+
+class Toy(torch.nn.Module):
+    def forward(self, x):
+        return torch.relu(x).mm(x.T)
+
+inputs = (torch.randn(4, 4),)
+dm_eager, _ = run_model(Toy(), inputs)
+dm_compiled, _ = run_model(Toy(), inputs, compile_with="aot_eager")
+
+print("Eager mode:")
+print(dm_eager.debug_string())
+print("Compiled aot_eager mode:")
+print(dm_compiled.debug_string())
+
+###############################################################################################
+# Now let's look at an example where the tensor hashes are different.
+# I intentionally wrote a wrong decomposition that decomposes cosine to sin.
+# This will cause numerical divergence.
+
+
+from torch._dynamo.backends.common import aot_autograd
+from torch._dynamo.backends.debugging import get_nop_func
+
+def wrong_decomp(x):
+    return torch.sin(x)
+
+decomp_table = {}
+decomp_table[torch.ops.aten.cos.default] = wrong_decomp
+
+backend = aot_autograd(
+    fw_compiler=get_nop_func(),
+    bw_compiler=get_nop_func(),
+    decompositions=decomp_table
+)
+
+def f(x):
+    y = x.relu()
+    z = torch.cos(x)
+    return y + z
+
+x = torch.randn(3, 3)
+with DebugMode(record_output=True) as dm_eager, DebugMode.log_tensor_hashes(
+    hash_inputs=True,
+):
+    f(x)
+
+with DebugMode(record_output=True) as dm_compiled, DebugMode.log_tensor_hashes(
+    hash_inputs=True,
+):
+    torch.compile(f, backend=backend)(x)
+
+print("Eager:")
+print(dm_eager.debug_string(show_stack_trace=True))
+print()
+print("Compiled with wrong decomposition:")
+print(dm_compiled.debug_string())
+
+###############################################################################################
+# In the eager log, we have ``aten::cos``, but in the compiled log, we have ``aten::sin``.
+# Moreover, the output hash is different between eager and compiled mode.
+# Diffing the two logs would show that the first numerical divergence shows up in the ``aten::cos`` call.
+
+
+
+
+######################################################################
+# Custom dispatch hooks
+# ---------------------
+#
+# Hooks allow you to annotate each call with custom metadata such as GPU memory usage. ``log_hook`` returns a mapping
+# that is rendered inline with the debug string.
+
+MB = 1024 * 1024.0
+
+def memory_hook(func, types, args, kwargs, result):
+    mem = torch.cuda.memory_allocated() / MB if torch.cuda.is_available() else 0.0
+    peak = torch.cuda.max_memory_allocated() / MB if torch.cuda.is_available() else 0.0
+    torch.cuda.reset_peak_memory_stats() if torch.cuda.is_available() else None
+    return {"mem": f"{mem:.3f} MB", "peak": f"{peak:.3f} MB"}
+
+with (
+    DebugMode() as dm,
+    DebugMode.dispatch_hooks(log_hook=memory_hook),
+):
+    run_once()
+
+print("DebugMode output with memory usage:")
+print(dm.debug_string())
+
+######################################################################
+# Module boundaries
+# ----------------------------------
+#
+# ``record_nn_module=True`` inserts ``[nn.Mod]`` markers that show which
+# module executed each set of operations. As of PyTorch 2.10 it only works in eager mode,
+# but support for compiled modes is under development.
+
+class Foo(torch.nn.Module):
+        def __init__(self):
+            super().__init__()
+            self.l1 = torch.nn.Linear(4, 4)
+            self.l2 = torch.nn.Linear(4, 4)
+
+        def forward(self, x):
+            return self.l2(self.l1(x))
+
+class Bar(torch.nn.Module):
+    def __init__(self):
+        super().__init__()
+        self.abc = Foo()
+        self.xyz = torch.nn.Linear(4, 4)
+
+    def forward(self, x):
+        return self.xyz(self.abc(x))
+
+mod = Bar()
+inp = torch.randn(4, 4)
+with DebugMode(record_nn_module=True, record_output=False) as debug_mode:
+    _ = mod(inp)
+
+print("DebugMode output with stack traces and module boundaries:")
+print(debug_mode.debug_string(show_stack_trace=True))
+
+######################################################################
+# Conclusion
+# ----------
+#
+# In this tutorial, we saw how DebugMode gives you a lightweight, runtime-only
+# view of what PyTorch actually executed, whether you are running eager code or
+# compiled graphs. By layering tensor hashing, Triton logging, and custom
+# dispatch hooks you can quickly track down numerical differences. This is
+# especially helpful in debugging bit-wise equivalence between runs.