[Hybrid] Fix and optimize block-aligned splitting in mamba cache align mode (#33706)

Signed-off-by: huanghaoyan.hhy <huanghaoyan.hhy@alibaba-inc.com>

vllm/v1/core/sched/scheduler.py

@@ -281,27 +281,30 @@ class Scheduler(SchedulerInterface):
         assert num_external_computed_tokens == 0, (
             "External KV connector is not verified yet"
         )
-        # TODO: need check for resume requests
-        if request.num_output_tokens == 0:  # prefill
+        num_computed_tokens = (
+            request.num_computed_tokens
+            + num_new_local_computed_tokens
+            + num_external_computed_tokens
+        )
+        # Perform block-aligned splitting at prefill phase, including:
+        # * non-resumed requests: num_computed_tokens < num_prompt_tokens + 0
+        # * resumed requests: num_computed_tokens < (
+        #                       num_prompt_tokens + num_output_tokens
+        #                     )
+        # NOTE: Use `request.num_tokens - 1` to bypass normal decoding.
+        if num_computed_tokens < max(request.num_prompt_tokens, request.num_tokens - 1):
             # To enable block-aligned caching of the Mamba state, `num_new_tokens`
             # must be a multiple of `block_size`.
             # As an exception, if `num_new_tokens` is less than `block_size`, the
             # state is simply not cached, requiring no special handling.
             # Additionally, when Eagle mode is enabled, FullAttn prunes the last
             # matching block. To prevent this from causing a Mamba cache miss, the
-            # last chunk must be larger than `block_size`.
+            # last chunk must be not smaller than `block_size`.
             block_size = self.cache_config.block_size
-            last_cache_position = (
-                request.num_prompt_tokens - request.num_prompt_tokens % block_size
-            )
+            last_cache_position = request.num_tokens - request.num_tokens % block_size
             # eagle prune
             if self.use_eagle:
                 last_cache_position = max(last_cache_position - block_size, 0)
-            num_computed_tokens = (
-                request.num_computed_tokens
-                + num_new_local_computed_tokens
-                + num_external_computed_tokens
-            )
             num_computed_tokens_after_sched = num_computed_tokens + num_new_tokens
             if num_computed_tokens_after_sched < last_cache_position:
                 # align to block_size