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[model]{bug-fix}: Fix wrong log prob calculations using fused kernels and USP #3724

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[model]{bug-fix}: Fix wrong log prob calculations using fused kernels and USP #3724

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f74e767
slice labels using USP
joshyan1 Oct 10, 2025
65c4ed1
dry
joshyan1 Oct 10, 2025
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17 changes: 17 additions & 0 deletions verl/models/transformers/qwen2_vl.py
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Original file line number Diff line number Diff line change
Expand Up @@ -35,6 +35,7 @@
gather_seq_scatter_heads,
get_ulysses_sequence_parallel_group,
get_ulysses_sequence_parallel_world_size,
ulysses_pad_and_slice_inputs,
validate_ulysses_config,
)

Expand All @@ -60,6 +61,17 @@

_flash_deterministic_enabled = os.getenv("FLASH_ATTENTION_DETERMINISTIC", "0") == "1"

def slice_labels_ulysses_sp(labels: torch.Tensor):
group = get_ulysses_sequence_parallel_group()
if group is not None:
ulysses_sequence_parallel_size = get_ulysses_sequence_parallel_world_size()
labels, _, _ = ulysses_pad_and_slice_inputs(
labels,
position_ids_rmpad=None,
sp_size=ulysses_sequence_parallel_size,
)
return labels


def get_rope_index(
processor,
Expand Down Expand Up @@ -500,6 +512,9 @@ def forward_with_torch_backend(
else:
raise RuntimeError("To use forward_with_torch_backend, either labels or input_ids must be provided.")

# Slice labels if using USP
rolled_labels = slice_labels_ulysses_sp(rolled_labels)

fused_linear_for_ppo = FusedLinearForPPO()
log_probs, entropy = fused_linear_for_ppo.forward(
hidden_states=hidden_states,
Expand Down Expand Up @@ -534,6 +549,8 @@ def forward_with_triton_backend(
else:
raise RuntimeError("To use forward_with_triton_backend, either labels or input_ids must be provided.")

rolled_labels = slice_labels_ulysses_sp(rolled_labels)

log_probs, entropy = linear_cross_entropy(
hidden_states,
self.lm_head.weight,
Expand Down