integrated flash attention 2 (#1035)

megatron/model/flash_attention.py

@@ -5,7 +5,7 @@
 import torch.nn.functional as F
 
 from flash_attn import flash_attn_triton
-import flash_attn_cuda
+import flash_attn_2_cuda as flash_attn_cuda # For flash_attn version 2.1.1
 
 
 def flash_attn_unpadded_unpacked_func_triton(
@@ -462,3 +462,325 @@ def flash_attn_unpadded_func_cuda(
         causal,
         return_attn_probs,
     )
+
+
+# For flash-attention 2 integration
+def _flash_attn_varlen_forward(
+    q,
+    k,
+    v,
+    cu_seqlens_q,
+    cu_seqlens_k,
+    max_seqlen_q,
+    max_seqlen_k,
+    dropout_p,
+    softmax_scale,
+    causal,
+    return_softmax,
+):
+    maybe_contiguous = lambda x: x.contiguous() if x.stride(-1) != 1 else x
+    q, k, v = [maybe_contiguous(x) for x in (q, k, v)]
+    out, q, k, v, out_padded, softmax_lse, S_dmask, rng_state = flash_attn_cuda.varlen_fwd(
+        q,
+        k,
+        v,
+        None,
+        cu_seqlens_q,
+        cu_seqlens_k,
+        max_seqlen_q,
+        max_seqlen_k,
+        dropout_p,
+        softmax_scale,
+        False,
+        causal,
+        return_softmax,
+        None,
+    )
+    # if out.isnan().any() or softmax_lse.isnan().any():
+    #     breakpoint()
+    return out, q, k, v, out_padded, softmax_lse, S_dmask, rng_state
+
+
+def _flash_attn_varlen_backward(
+    dout,
+    q,
+    k,
+    v,
+    out,
+    softmax_lse,
+    dq,
+    dk,
+    dv,
+    cu_seqlens_q,
+    cu_seqlens_k,
+    max_seqlen_q,
+    max_seqlen_k,
+    dropout_p,
+    softmax_scale,
+    causal,
+    rng_state=None,
+):
+    maybe_contiguous = lambda x: x.contiguous() if x.stride(-1) != 1 else x
+    # dq, dk, dv are allocated by us so they should already be contiguous
+    dout, q, k, v, out = [maybe_contiguous(x) for x in (dout, q, k, v, out)]
+    dq, dk, dv, softmax_d, = flash_attn_cuda.varlen_bwd(
+        dout,
+        q,
+        k,
+        v,
+        out,
+        softmax_lse,
+        dq,
+        dk,
+        dv,
+        cu_seqlens_q,
+        cu_seqlens_k,
+        max_seqlen_q,
+        max_seqlen_k,
+        dropout_p,
+        softmax_scale,
+        False,
+        causal,
+        None,
+        rng_state,
+    )
+    # if dk.isnan().any() or dk.isnan().any() or dv.isnan().any() or softmax_d.isnan().any():
+    #     breakpoint()
+    return dq, dk, dv, softmax_d
+
+
+class FlashAttnVarlenQKVPackedFunc(torch.autograd.Function):
+    @staticmethod
+    def forward(ctx, qkv, cu_seqlens, max_seqlen, dropout_p, softmax_scale, causal, return_softmax):
+        if softmax_scale is None:
+            softmax_scale = qkv.shape[-1] ** (-0.5)
+        out, q, k, v, out_padded, softmax_lse, S_dmask, rng_state = _flash_attn_varlen_forward(
+            qkv[:, 0],
+            qkv[:, 1],
+            qkv[:, 2],
+            cu_seqlens,
+            cu_seqlens,
+            max_seqlen,
+            max_seqlen,
+            dropout_p,
+            softmax_scale,
+            causal=causal,
+            return_softmax=return_softmax and dropout_p > 0,
+        )
+        ctx.save_for_backward(q, k, v, out_padded, softmax_lse, cu_seqlens, rng_state)
+        ctx.dropout_p = dropout_p
+        ctx.max_seqlen = max_seqlen
+        ctx.softmax_scale = softmax_scale
+        ctx.causal = causal
+        return out if not return_softmax else (out, softmax_lse, S_dmask)
+
+    @staticmethod
+    def backward(ctx, dout, *args):
+        q, k, v, out, softmax_lse, cu_seqlens, rng_state = ctx.saved_tensors
+        qkv_shape = q.shape[:-2] + (3, *q.shape[-2:])
+        dqkv = torch.empty(qkv_shape, dtype=q.dtype, device=q.device)
+        _flash_attn_varlen_backward(
+            dout,
+            q,
+            k,
+            v,
+            out,
+            softmax_lse,
+            dqkv[:, 0],
+            dqkv[:, 1],
+            dqkv[:, 2],
+            cu_seqlens,
+            cu_seqlens,
+            ctx.max_seqlen,
+            ctx.max_seqlen,
+            ctx.dropout_p,
+            ctx.softmax_scale,
+            ctx.causal,
+            rng_state=rng_state,
+        )
+        dqkv = dqkv[..., : dout.shape[-1]]  # We could have padded the head dimension
+        return dqkv, None, None, None, None, None, None
+
+
+def flash_attn_varlen_qkvpacked_func(
+    qkv,
+    cu_seqlens,
+    max_seqlen,
+    dropout_p=0.0,
+    softmax_scale=None,
+    causal=False,
+    return_attn_probs=False,
+):
+    """dropout_p should be set to 0.0 during evaluation
+    If Q, K, V are already stacked into 1 tensor, this function will be faster than
+    calling flash_attn_varlen_func on Q, K, V since the backward pass avoids explicit concatenation
+    of the gradients of Q, K, V.
+    For multi-query and grouped-query attention (MQA/GQA), please see
+    flash_attn_varlen_kvpacked_func and flash_attn_varlen_func.
+
+    Arguments:
+        qkv: (total, 3, nheads, headdim), where total = total number of tokens in the batch.
+        cu_seqlens: (batch_size + 1,), dtype torch.int32. The cumulative sequence lengths
+           of the sequences in the batch, used to index into qkv.
+        max_seqlen: int. Maximum sequence length in the batch.
+        dropout_p: float. Dropout probability.
+        softmax_scale: float. The scaling of QK^T before applying softmax.
+            Default to 1 / sqrt(headdim).
+        causal: bool. Whether to apply causal attention mask (e.g., for auto-regressive modeling).
+        return_attn_probs: bool. Whether to return the attention probabilities. This option is for
+           testing only. The returned probabilities are not guaranteed to be correct
+           (they might not have the right scaling).
+    Return:
+        out: (total, nheads, headdim).
+        softmax_lse [optional, if return_attn_probs=True]: (batch_size, nheads, seqlen). The
+            logsumexp of each row of the matrix QK^T * scaling (e.g., log of the softmax
+            normalization factor).
+        S_dmask [optional, if return_attn_probs=True]: (batch_size, nheads, seqlen, seqlen).
+            The output of softmax (possibly with different scaling). It also encodes the dropout
+            pattern (negative means that location was dropped, nonnegative means it was kept).
+    """
+    return FlashAttnVarlenQKVPackedFunc.apply(
+        qkv, cu_seqlens, max_seqlen, dropout_p, softmax_scale, causal, return_attn_probs
+    )
+
+
+class FlashAttnVarlenKVPackedFunc(torch.autograd.Function):
+    @staticmethod
+    def forward(
+        ctx,
+        q,
+        kv,
+        cu_seqlens_q,
+        cu_seqlens_k,
+        max_seqlen_q,
+        max_seqlen_k,
+        dropout_p,
+        softmax_scale,
+        causal,
+        return_softmax,
+    ):
+        if softmax_scale is None:
+            softmax_scale = q.shape[-1] ** (-0.5)
+        out, q, k, v, out_padded, softmax_lse, S_dmask, rng_state = _flash_attn_varlen_forward(
+            q,
+            kv[:, 0],
+            kv[:, 1],
+            cu_seqlens_q,
+            cu_seqlens_k,
+            max_seqlen_q,
+            max_seqlen_k,
+            dropout_p,
+            softmax_scale,
+            causal=causal,
+            return_softmax=return_softmax and dropout_p > 0,
+        )
+        ctx.save_for_backward(
+            q, k, v, out_padded, softmax_lse, cu_seqlens_q, cu_seqlens_k, rng_state
+        )
+        ctx.dropout_p = dropout_p
+        ctx.max_seqlen_q = max_seqlen_q
+        ctx.max_seqlen_k = max_seqlen_k
+        ctx.softmax_scale = softmax_scale
+        ctx.causal = causal
+        return out if not return_softmax else (out, softmax_lse, S_dmask)
+
+    @staticmethod
+    def backward(ctx, dout, *args):
+        q, k, v, out, softmax_lse, cu_seqlens_q, cu_seqlens_k, rng_state = ctx.saved_tensors
+        dq = torch.empty_like(q)
+        kv_shape = k.shape[:-2] + (2, *k.shape[-2:])
+        dkv = torch.empty(kv_shape, dtype=k.dtype, device=k.device)
+        _flash_attn_varlen_backward(
+            dout,
+            q,
+            k,
+            v,
+            out,
+            softmax_lse,
+            dq,
+            dkv[:, 0],
+            dkv[:, 1],
+            cu_seqlens_q,
+            cu_seqlens_k,
+            ctx.max_seqlen_q,
+            ctx.max_seqlen_k,
+            ctx.dropout_p,
+            ctx.softmax_scale,
+            ctx.causal,
+            rng_state=rng_state,
+        )
+        dq = dq[..., : dout.shape[-1]]  # We could have padded the head dimension
+        dkv = dkv[..., : dout.shape[-1]]
+        return dq, dkv, None, None, None, None, None, None, None, None
+
+
+def flash_attn_varlen_kvpacked_func(
+    q,
+    kv,
+    cu_seqlens_q,
+    cu_seqlens_k,
+    max_seqlen_q,
+    max_seqlen_k,
+    dropout_p=0.0,
+    softmax_scale=None,
+    causal=False,
+    return_attn_probs=False,
+):
+    """dropout_p should be set to 0.0 during evaluation
+    If K, V are already stacked into 1 tensor, this function will be faster than
+    calling flash_attn_func on Q, K, V since the backward pass avoids explicit concatenation
+    of the gradients of K, V.
+    Supports multi-query and grouped-query attention (MQA/GQA) by passing in KV with fewer heads
+    than Q. Note that the number of heads in Q must be divisible by the number of heads in KV.
+    For example, if Q has 6 heads and K, V have 2 heads, head 0, 1, 2 of Q will attention to head
+    0 of K, V, and head 3, 4, 5 of Q will attention to head 1 of K, V.
+
+    If causal=True, the causal mask is aligned to the bottom right corner of the attention matrix.
+    For example, if seqlen_q = 2 and seqlen_k = 5, the causal mask (1 = keep, 0 = masked out) is:
+        1 1 1 1 0
+        1 1 1 1 1
+    If seqlen_q = 5 and seqlen_k = 2, the causal mask is:
+        0 0
+        0 0
+        0 0
+        1 0
+        1 1
+    If the row of the mask is all zero, the output will be zero.
+
+    Arguments:
+        q: (total_q, nheads, headdim), where total_q = total number of query tokens in the batch.
+        kv: (total_k, 2, nheads_k, headdim), where total_k = total number of key tokens in the batch.
+        cu_seqlens_q: (batch_size + 1,), dtype torch.int32. The cumulative sequence lengths
+           of the sequences in the batch, used to index into q.
+        cu_seqlens_k: (batch_size + 1,), dtype torch.int32. The cumulative sequence lengths
+           of the sequences in the batch, used to index into kv.
+        max_seqlen_q: int. Maximum query sequence length in the batch.
+        max_seqlen_k: int. Maximum key sequence length in the batch.
+        dropout_p: float. Dropout probability.
+        softmax_scale: float. The scaling of QK^T before applying softmax.
+            Default to 1 / sqrt(headdim).
+        causal: bool. Whether to apply causal attention mask (e.g., for auto-regressive modeling).
+        return_attn_probs: bool. Whether to return the attention probabilities. This option is for
+           testing only. The returned probabilities are not guaranteed to be correct
+           (they might not have the right scaling).
+    Return:
+        out: (total, nheads, headdim).
+        softmax_lse [optional, if return_attn_probs=True]: (batch_size, nheads, seqlen). The
+            logsumexp of each row of the matrix QK^T * scaling (e.g., log of the softmax
+            normalization factor).
+        S_dmask [optional, if return_attn_probs=True]: (batch_size, nheads, seqlen, seqlen).
+            The output of softmax (possibly with different scaling). It also encodes the dropout
+            pattern (negative means that location was dropped, nonnegative means it was kept).
+    """
+    return FlashAttnVarlenKVPackedFunc.apply(
+        q,
+        kv,
+        cu_seqlens_q,
+        cu_seqlens_k,
+        max_seqlen_q,
+        max_seqlen_k,
+        dropout_p,
+        softmax_scale,
+        causal,
+        return_attn_probs,
+    )

megatron/model/transformer.py

@@ -345,14 +345,17 @@ def __init__(
         else:
             if self.use_flash_attention:
                 from megatron.model.flash_attention import (
-                    flash_attn_unpadded_qkvpacked_func_cuda,
-                    flash_attn_unpadded_kvpacked_func_cuda,
-                    flash_attn_unpadded_unpacked_func_triton,
+                    # flash_attn_unpadded_qkvpacked_func_cuda,
+                    # flash_attn_unpadded_kvpacked_func_cuda,
+                    # Change of function names going from flash attention 1 -> flash attention 2
+                    flash_attn_varlen_qkvpacked_func,
+                    flash_attn_varlen_kvpacked_func,
+                    flash_attn_unpadded_unpacked_func_triton
                 )
 
                 self.flash_triton_fn = flash_attn_unpadded_unpacked_func_triton
-                self.flash_qkv_fn = flash_attn_unpadded_qkvpacked_func_cuda
-                self.flash_kv_fn = flash_attn_unpadded_kvpacked_func_cuda
+                self.flash_qkv_fn = flash_attn_varlen_qkvpacked_func
+                self.flash_kv_fn = flash_attn_varlen_kvpacked_func
             else:
                 self.scale_mask_softmax = FusedScaleMaskSoftmax(
                     input_in_fp16=self.fp16,

requirements/requirements-flashattention.txt

@@ -1 +1 @@
-flash-attn==0.2.2
+flash-attn==2.2.1