[Fix] Fix hicache backend (#8991)

python/sglang/srt/managers/scheduler.py

@@ -611,12 +611,7 @@ class Scheduler(
                     hicache_ratio=server_args.hicache_ratio,
                     hicache_size=server_args.hicache_size,
                     hicache_write_policy=server_args.hicache_write_policy,
-                    hicache_io_backend=(
-                        "direct"
-                        if server_args.attention_backend
-                        == "fa3"  # hot fix for incompatibility
-                        else server_args.hicache_io_backend
-                    ),
+                    hicache_io_backend=server_args.hicache_io_backend,
                     hicache_mem_layout=server_args.hicache_mem_layout,
                     hicache_storage_backend=server_args.hicache_storage_backend,
                     hicache_storage_prefetch_policy=server_args.hicache_storage_prefetch_policy,

python/sglang/srt/model_executor/model_runner.py

@@ -403,7 +403,6 @@ class ModelRunner:
                     is_hopper_with_cuda_12_3()
                     and is_no_spec_infer_or_topk_one(server_args)
                     and is_fa3_default_architecture(self.model_config.hf_config)
-                    and (not server_args.enable_hierarchical_cache)
                 ):
                     server_args.attention_backend = "fa3"
                 elif _is_hip:
@@ -416,9 +415,7 @@ class ModelRunner:
                     )
             else:
                 # MLA architecture
-                if is_hopper_with_cuda_12_3() and (
-                    not server_args.enable_hierarchical_cache
-                ):
+                if is_hopper_with_cuda_12_3():
                     server_args.attention_backend = "fa3"
                 elif is_sm100_supported():
                     server_args.attention_backend = "flashinfer"
@@ -506,6 +503,27 @@ class ModelRunner:
             if self.model_config.context_len > 8192:
                 self.mem_fraction_static *= 0.85
 
+        if (
+            server_args.enable_hierarchical_cache
+            and server_args.hicache_io_backend == "kernel"
+        ):
+            # fix for the compatibility issue with FlashAttention3 decoding and HiCache kernel backend
+            if server_args.decode_attention_backend is None:
+                if not self.use_mla_backend:
+                    server_args.decode_attention_backend = (
+                        "flashinfer" if is_flashinfer_available() else "triton"
+                    )
+                else:
+                    server_args.decode_attention_backend = (
+                        "flashinfer" if is_sm100_supported() else "triton"
+                    )
+            elif server_args.decode_attention_backend == "fa3":
+                server_args.hicache_io_backend = "direct"
+                logger.warning(
+                    "FlashAttention3 decode backend is not compatible with hierarchical cache. "
+                    f"Setting hicache_io_backend to vanilla I/O, which may lead to suboptimal performance with small page sizes."
+                )
+
     def init_torch_distributed(self):
         logger.info("Init torch distributed begin.")