Integrate attention sink into ET LLM export and runner

examples/models/llama/export_llama_lib.py

@@ -1781,16 +1781,17 @@ def _get_source_transforms(  # noqa
     use_attention_mask_for_custom_sdpa = use_custom_sdpa_with_attention_mask
 
     if use_sdpa_with_kv_cache:
-        transforms.append(replace_kv_cache_with_custom_kv_cache)
-        # todo: do this optionally
-        # if use attention mask instead of causal attention
-        # then create partial function that sets use_attention_mask=True
+        # Replace SDPA first, then KV cache. Order matters: the KV cache
+        # replacement sets SDPACustom.use_attention_mask=True for ring buffer
+        # models (attention sink, sliding window). If SDPA is replaced after,
+        # a new SDPACustom(use_attention_mask=False) would overwrite it.
         if use_attention_mask_for_custom_sdpa:
             transforms.append(
                 partial(replace_sdpa_with_custom_op, use_attention_mask=True)
             )
         else:
             transforms.append(replace_sdpa_with_custom_op)
+        transforms.append(replace_kv_cache_with_custom_kv_cache)
 
     if quantize_kv_cache:
         assert use_kv_cache, "quantize_kv_cache requires use_kv_cache=True"

examples/models/llama/source_transformation/custom_kv_cache.py

@@ -1,3 +1,3 @@
 # Copyright (c) Meta Platforms, Inc. and affiliates.
 # All rights reserved.
 #
@@ -371,8 +371,41 @@
 
 
 def _replace_kv_cache_with_custom_kv_cache(module):
+    # Import here to avoid circular imports
+    from executorch.examples.models.llama.source_transformation.attention_sink import (
+        KVCacheWithAttentionSink,
+    )
+
     for name, child in module.named_children():
-        if isinstance(child, KVCache):
+        if isinstance(child, KVCacheWithAttentionSink):
+            # Replace with custom op variant for performance
+            setattr(
+                module,
+                name,
+                CustomKVCacheWithAttentionSink.from_kv_cache_with_attention_sink(child),
+            )
+            # If parent has SDPACustom, enable explicit mask mode
+            sdpa = getattr(module, "SDPA", None)
+            if sdpa is not None and hasattr(sdpa, "use_attention_mask"):
+                sdpa.use_attention_mask = True
+        elif isinstance(child, RingKVCache):
+            # RingKVCache (e.g., from attention sink with sink_size=0) needs
+            # CustomRingKVCache, not plain CustomKVCache
+            setattr(
+                module,
+                name,
+                CustomRingKVCache(
+                    child.max_batch_size,
+                    child.window_size,
+                    child.n_heads,
+                    child.head_dim,
+                    dtype=child.k_cache.dtype,
+                ),
+            )
+            sdpa = getattr(module, "SDPA", None)
+            if sdpa is not None and hasattr(sdpa, "use_attention_mask"):
+                sdpa.use_attention_mask = True
+        elif isinstance(child, KVCache):
             cache_shape = child.k_cache.shape
             cache_dtype = child.k_cache.dtype
             max_batch_size, n_heads, max_context_length, head_dim = cache_shape
@@ -466,6 +499,81 @@
         )
 
 
+class CustomKVCacheWithAttentionSink(CustomKVCache):
+    """
+    CustomKVCache variant for attention sink models.
+
+    Uses the custom update_cache_with_indices op for performance while
+    supporting sink tokens (fixed slots) + ring buffer (sliding window).
+    Modeled after CustomRingKVCache but with CachePositionsManagerWithSink.
+    """
+
+    def __init__(
+        self,
+        max_batch_size,
+        n_heads,
+        head_dim,
+        window_size,
+        sink_size,
+        dtype=torch.float32,
+    ):
+        # Total cache size: sink slots + ring buffer (2x window for wrap safety)
+        total_cache_size = sink_size + window_size * 2
+        super().__init__(
+            max_batch_size, total_cache_size, n_heads, head_dim, dtype
+        )
+        from executorch.examples.models.llama.source_transformation.attention_sink import (
+            CachePositionsManagerWithSink,
+            _create_causal_mask_for_attention_sink,
+        )
+
+        self.cache_positions_manager = CachePositionsManagerWithSink(
+            total_cache_size, sink_size
+        )
+        self.is_ring_buffer = True
+        self.window_size = window_size
+        self.sink_size = sink_size
+        self._create_causal_mask_for_attention_sink = (
+            _create_causal_mask_for_attention_sink
+        )
+
+    def create_causal_mask_for_ring_buffer(self, start_pos, seq_len):
+        cache_positions = self.cache_positions_manager.cache_positions
+        if self.sink_size > 0:
+            return self._create_causal_mask_for_attention_sink(
+                cache_positions, self.window_size, self.sink_size, start_pos, seq_len
+            )
+        else:
+            return _create_causal_mask_for_ring_buffer(
+                cache_positions, self.window_size, start_pos, seq_len
+            )
+
+    def update(self, input_pos, k_val, v_val):
+        seq_len = k_val.transpose(1, 2).size(1)
+        assert seq_len <= self.k_cache.size(
+            1
+        ), f"Update sequence length({seq_len}) for kv cache must be smaller than the cache size({self.k_cache.size(1)})"
+        indices = self.cache_positions_manager.calculate_positions_and_update_indices(
+            input_pos, seq_len
+        )
+        indices = indices.unsqueeze(0)
+
+        return super().update(input_pos, k_val, v_val, indices)
+
+    @classmethod
+    def from_kv_cache_with_attention_sink(cls, kv_cache):
+        """Create from an existing KVCacheWithAttentionSink."""
+        max_batch_size, n_heads, _, head_dim = kv_cache.k_cache.shape
+        return cls(
+            max_batch_size,
+            n_heads,
+            head_dim,
+            kv_cache.window_size,
+            kv_cache.sink_size,
+            dtype=kv_cache.k_cache.dtype,
+        )
+
+
 class CustomRingKVCache(CustomKVCache):
     def __init__(
         self,

examples/models/llama/source_transformation/test_attention_sink.py

@@ -397,3 +397,48 @@ def test_beyond_context_window_basic(self):
             self.assertTrue(
                 torch.isfinite(out).all(), "Output contains non-finite values"
             )
+
+    def test_beyond_context_window_custom_sdpa(self):
+        """Generate tokens beyond context window with custom SDPA + custom KV cache."""
+        sink_size = 4
+        window_size = 16
+        args = self._make_args(max_context_len=128)
+        model = self._build_model(args, sink_size, window_size, use_custom_sdpa=True)
+
+        # Verify KV caches were replaced with CustomKVCacheWithAttentionSink
+        from executorch.examples.models.llama.source_transformation.custom_kv_cache import (
+            CustomKVCacheWithAttentionSink,
+        )
+
+        found_custom_cache = False
+        for m in model.modules():
+            if isinstance(m, CustomKVCacheWithAttentionSink):
+                found_custom_cache = True
+                break
+        self.assertTrue(
+            found_custom_cache, "Expected CustomKVCacheWithAttentionSink in model"
+        )
+
+        # Generate 80 tokens — well beyond KV cache size of 36
+        outputs = self._run_generation(model, args, num_tokens=80)
+
+        self.assertEqual(len(outputs), 77)
+        for out in outputs:
+            self.assertTrue(
+                torch.isfinite(out).all(), "Output contains non-finite values"
+            )
+
+    def test_sink_zero_custom_sdpa(self):
+        """Degenerate case: sink_size=0 with custom SDPA (pure ring buffer)."""
+        sink_size = 0
+        window_size = 16
+        args = self._make_args(max_context_len=128)
+        model = self._build_model(args, sink_size, window_size, use_custom_sdpa=True)
+
+        outputs = self._run_generation(model, args, num_tokens=60)
+
+        self.assertEqual(len(outputs), 57)
+        for out in outputs:
+            self.assertTrue(
+                torch.isfinite(out).all(), "Output contains non-finite values"
+            )

extension/llm/runner/text_llm_runner.cpp

@@ -1,3 +1,3 @@
 /*
  * Copyright (c) Meta Platforms, Inc. and affiliates.
  * All rights reserved.
@@ -110,6 +110,8 @@
   stats_->inference_start_ms = time_in_ms();
   shouldStop_ = false;
 
+  // Get max_seq_len for single prefill chunk limit
+  int64_t max_seq_len = metadata_.at(kMaxSeqLen);
   int64_t max_context_len = metadata_.at(kMaxContextLen);
 
   uint64_t cur_token = 0;
@@ -138,13 +140,26 @@
         InvalidArgument,
         "Expected at least 1 prompt token");
     ET_CHECK_OR_RETURN_ERROR(
-        pos_ + num_prompt_tokens < max_context_len,
+        num_prompt_tokens <= max_seq_len,
         InvalidArgument,
-        "pos_ %" PRId64 " + num_prompt_tokens %d >= max_context_len %" PRId64
-        ", Max seq length exceeded - please increase max seq len value in your export script",
-        pos_,
+        "num_prompt_tokens %d > max_seq_len %" PRId64
+        ", Single prefill chunk too large - please reduce prompt size or increase max_seq_len",
         num_prompt_tokens,
-        max_context_len);
+        max_seq_len);
+    // For non-sliding-window models, also check that we won't exceed
+    // KV cache capacity. Sliding window models (where max_seq_len <
+    // max_context_len) handle position wrapping internally.
+    if (max_seq_len >= max_context_len) {
+      ET_CHECK_OR_RETURN_ERROR(
+          pos_ + num_prompt_tokens < max_context_len,
+          InvalidArgument,
+          "pos_ %" PRId64 " + num_prompt_tokens %d >= max_context_len %" PRId64
+          ", Max seq length exceeded - please increase max seq len value in "
+          "your export script",
+          pos_,
+          num_prompt_tokens,
+          max_context_len);
+    }
 
     // print prompts
     if (config.echo) {
@@ -168,9 +183,10 @@
     prefill_next_token_.reset();
   }
 
-  // Resolve max_new_tokens. pos_ now reflects all occupied positions
-  // (including prompt tokens just prefilled).
-  int max_new_tokens = config.resolve_max_new_tokens(max_context_len, pos_);
+  // For sliding window models, the ring buffer recycles space — pos_ doesn't
+  // represent consumed capacity, so pass 0 to get the full budget.
+  int64_t effective_pos = (max_seq_len < max_context_len) ? 0 : pos_;
+  int max_new_tokens = config.resolve_max_new_tokens(max_context_len, effective_pos);
 
   ET_LOG(
       Info,