[AutoTVM] Fix Empty Config caused Crashing

[comaniac]

Problem Description

Some TOPI schedules such as depthwise_conv2d_nchw on x86 has no tuning space at all. However since depthwise_conv2d_nchw on CUDA has a tuning space, we still list topi_nn_depthwise_conv2d_nchw as an AutoTVM task. It means depthwise_conv2d_nchw on x86 will be extracted by extract_from_program and "tuned" by AutoTVM. The problem is that this will result in the following record in the history log file:

{"i": ["llvm", "topi_nn_depthwise_conv2d_nchw", [["TENSOR", [1, 192, 28, 28], "float32"], ["TENSOR", [192, 1, 3, 3], "float32"], [2, 2], [1, 1], [1, 1], "float32"], {}, ["depthwise_conv2d_nchw", [1, 192, 28, 28, "float32"], [192, 1, 3, 3, "float32"], [2, 2], [1, 1], [1, 1], "float32"], {"i": 0, "t": "direct", "c": null, "e": []}], "r": ...}

Taking a closer look, we will find an empty config:

{"i": 0, "t": "direct", "c": null, "e": []}

Although it is empty, it is still a valid record and will be loaded when ApplyHistoryBest. The problem is that when building with opt_level=3 on x86, TVM will call AlterOp pass to replace NCHW to NCHWc. During this process, it tries to apply the empty config to the NCHWc schedule template, which results in the following error:

  File "/home/ubuntu/tvm/python/tvm/_ffi/_ctypes/function.py", line 72, in cfun
    rv = local_pyfunc(*pyargs)
  File "/home/ubuntu/tvm/python/tvm/relay/op/nn/_nn.py", line 232, in alter_op_layout_conv2d
    return topi.nn.conv2d_alter_layout(attrs, inputs, tinfos, op)
  File "</usr/local/lib/python3.7/dist-packages/decorator.py:decorator-gen-36>", line 2, in conv2d_alter_layout
  File "/home/ubuntu/tvm/python/tvm/target.py", line 382, in dispatch_func
    return dispatch_dict[k](*args, **kwargs)
  File "/home/ubuntu/tvm/topi/python/topi/x86/conv2d_alter_op.py", line 98, in _alter_conv2d_layout
    ic_bn, oc_bn = cfg["tile_ic"].size[-1], cfg["tile_oc"].size[-1]
  File "/home/ubuntu/tvm/python/tvm/autotvm/task/space.py", line 806, in __getitem__
    return self._entity_map[name]
KeyError: 'tile_ic'

The most obvious case is tuning MoibleNet V2 on x86 which has lots of depthwise conv2d ops, and this problem has been reported by several topics [1][2][3][4]. One workaround suggests users replacing NCHW with NCHWc manually, but it is still confusing.

Solution

This PR proposes a quick fix to this problem by ignoring the records with an empty config when loading from the log file. I choose this solution instead of the following alternatives for reasons. The root solution is actually making up a tuning space for depthwise conv2d on x86 tho.

• Do not log empty config in callback: We may be still interested in the result of this op with this schedule even it has no config.

• Use fallback config when the history best config has no entities: Similar to what I proposed but it needs to modify more places.

cc @eqy @merrymercy @kevinthesun