src/llamafactory/model/model_utils/packing.py

# Copyright 2025 Musab Gultekin and the LlamaFactory team.
#
# This code is based on the Musab Gultekin's functionary library.
# https://github.com/MeetKai/functionary/blob/main/functionary/train/packing/monkey_patch_packing.py
#
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#     http://www.apache.org/licenses/LICENSE-2.0
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
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# MIT License
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# Copyright (c) 2023 Musab Gultekin
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from typing import TYPE_CHECKING

import torch
import torch.nn.functional as F

from ...extras import logging


if TYPE_CHECKING:
    from ...hparams import ModelArguments


logger = logging.get_logger(__name__)


def get_seqlens_in_batch(attention_mask: "torch.Tensor") -> "torch.Tensor":
    r"""Get the sequence lengths in the current batch.

    e.g.
    ```python
    # input
    [
        [1, 1, 2, 2, 2, 0],
        [1, 2, 2, 3, 3, 3],
    ]
    # output
    [2, 3, 1, 2, 3]
    ```
    """
    bsz = attention_mask.size(0)
    dtype, device = attention_mask.dtype, attention_mask.device
    max_num = torch.max(attention_mask).item()
    counts: torch.Tensor = torch.zeros((bsz, max_num), dtype=dtype, device=device)
    for i in range(max_num):
        counts[:, i] = torch.sum(attention_mask == (i + 1), dim=-1)

    counts = counts.flatten()
    seqlens = counts[counts.nonzero().squeeze(dim=-1)]
    return seqlens


def get_unpad_data(attention_mask: "torch.Tensor") -> tuple["torch.Tensor", "torch.Tensor", int]:
    r"""Prepare the indices and seqlens for flash attn varlen function.

    Returns:
        indices: indices of non-masked tokens from the flattened sequence.
        cu_seqlens: the cumulative sequence lengths in the current batch, always starts from 0.
        max_seqlen_in_batch: the largest seqlen in the current batch.

    e.g.
    ```python
    # input
    [
        [1, 1, 2, 2, 2, 0],
        [1, 2, 2, 3, 3, 3],
    ]
    # output
    [0, 1, 2, 3, 4, 6, 7, 8, 9, 10, 11]
    [0, 2, 5, 6, 8, 11]
    3
    ```

    """
    seqlens_in_batch = get_seqlens_in_batch(attention_mask)
    indices = torch.nonzero(attention_mask.flatten(), as_tuple=False).flatten()
    max_seqlen_in_batch = seqlens_in_batch.max().item()
    cu_seqlens = F.pad(torch.cumsum(seqlens_in_batch, dim=0, dtype=torch.int32), (1, 0))
    return indices, cu_seqlens, max_seqlen_in_batch


def configure_packing(model_args: "ModelArguments", is_trainable: bool) -> None:
    if not is_trainable or not model_args.block_diag_attn:
        return

    import transformers.modeling_flash_attention_utils

    transformers.modeling_flash_attention_utils._get_unpad_data = get_unpad_data
    logger.info_rank0("Using block diagonal attention for sequence packing without cross-attention.")