[TPU] Fix tpu structured decoding in mixed batches #24458
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Signed-off-by: Chenyaaang <[email protected]>
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This pull request fixes an indexing issue for structured decoding on TPUs when mixed batches of guided and non-guided requests are present. The change correctly maps the compact grammar_bitmask to the requests in the batch. However, I've identified a critical issue with how this function handles chunked prefills (sub-batching), which can lead to incorrect behavior.
| sorted_struct_requests = sorted( | ||
| scheduler_output.structured_output_request_ids.items(), | ||
| key=lambda item: item[1]) | ||
| cumulative_mask_idx = 0 | ||
| for req_id, _ in sorted_struct_requests: | ||
| if req_id not in self.input_batch.req_id_to_index: | ||
| continue | ||
| batch_index = self.input_batch.req_id_to_index[req_id] | ||
| struct_out_indices.append(batch_index) | ||
| mask_indices.append(mask_index) | ||
| self.grammar_bitmask_cpu[struct_out_indices] = torch.from_numpy( | ||
| grammar_bitmask[mask_indices]) | ||
| # It's not guaranteed that all requests in this batch require | ||
| # structured output, so create a bool tensor to represent | ||
| # the requests that need structured output. | ||
| struct_out_indices = torch.tensor(struct_out_indices, dtype=torch.long) | ||
| self.require_structured_out_cpu[struct_out_indices] = True | ||
| self.grammar_bitmask_cpu[batch_index] = torch.from_numpy( | ||
| grammar_bitmask[cumulative_mask_idx]) | ||
| # It's not guaranteed that all requests in this batch require | ||
| # structured output, so create a bool tensor to represent | ||
| # the requests that need structured output. | ||
| self.require_structured_out_cpu[batch_index] = True | ||
| cumulative_mask_idx += 1 |
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This implementation does not correctly handle chunked prefills (sub-batching). The execute_model method processes requests in chunks, but this function seems to assume it's always processing the first chunk.
The batch_index is an absolute index within the full self.input_batch, but the logits tensor and the returned bitmask tensors are relative to the current sub-batch. When processing a sub-batch that is not the first one (i.e., when start_index > 0 in execute_model), batch_index will be larger than the sub-batch size. However, the function returns self.grammar_bitmask_cpu[:num_reqs], which is a slice from the beginning of the buffer. This will result in an incorrect (likely all-zero) bitmask for requests in subsequent chunks, leading to incorrect structured decoding.
To fix this, the function needs to be aware of the current sub-batch's start_index and end_index and use relative indexing for the output tensors.
Signed-off-by: Chenyaaang <[email protected]>
Signed-off-by: Chenyaaang <[email protected]>
Signed-off-by: Chenyaaang <[email protected]> Signed-off-by: xuebwang-amd <[email protected]>
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[TPU] Fix structured decoding in mixed batches. This should fix the TPU v1 test part2 soft fail.
When there are guided and non-guided requests in the same batch,
scheduler_output.grammar_bitmaskonly contains the entries for guided requests, sort the guided requests and use cumulative index to avoid index out of bound error.