fix(flux): cast Q/K to value dtype before core attention#914
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Q/K RMSNorm (for_qk) and RoPE can leave Q/K in fp32 while V stays bf16. AITER FlashAttention requires Q/K/V to share one dtype and otherwise raises "FlashAttention only support fp16, bf16 and fp8_e4m3". Align Q/K to value.dtype at the attention-kernel boundary in both JointSelfAttention and FluxSingleAttention forwards; no-op when dtypes already match.
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Pull request overview
This PR fixes a dtype mismatch at the Flux attention kernel boundary where Q/K could become fp32 (via for_qk RMSNorm and/or RoPE) while V remains bf16, which can crash AITER FlashAttention due to its requirement that Q, K, and V share the same dtype.
Changes:
- Cast
queryandkeytovalue.dtypeimmediately after RoPE and right before the core attention call inJointSelfAttention.forward. - Apply the same boundary cast in
FluxSingleAttention.forwardto cover the single-stream path as well.
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Summary
The Flux joint and single self-attention forwards can reach the core attention call with Q/K in fp32 while V is bf16: the Q/K RMSNorm (
for_qk) and RoPE promote Q/K to fp32, but V is left untouched. AITER FlashAttention requires Q, K and V to share one dtype and otherwise raisesFlashAttention only support fp16, bf16 and fp8_e4m3, so the Flux 12B local-spec / MXFP4 recipes cannot reach iteration 1 onrocm/primus:v26.4.Fix: cast
query/keytovalue.dtypeat the attention-kernel boundary in bothJointSelfAttention.forwardandFluxSingleAttention.forward— right after RoPE, before the core attention computation. This covers both the RMSNorm and RoPE promotion paths and is a no-op when the dtypes already match.Changed
primus/backends/megatron/core/models/diffusion/flux/attention.py— cast Q/K →value.dtypein both forwards.Test plan
flux_12b_ddp_energon_schnell_resample_local_spec_mxfp4.yamlon 8 GPUs withrocm/primus:v26.4: exitsrc=0with finite loss and no NaN; the attention forward runs (previously crashed before iteration 1).