[Executorch][SDPA] Fix bug in sdpa

This diff fixes two bugs
1. When doing flash attention, the partical q @ k block may contain some entries that needs to be masked out. This logic had a bug. Maybe this bug also exist in PT core. I will look into that to add test and see if I can prove it.
2. Due to special handling via start_pos in SDPA it also exposed the bug in 1 when doing really long sequence prefill in chunked manner.

It is probably better to just use mask though.

Code has detail comments on the issue and fix.

Differential Revision: [D70922039](https://our.internmc.facebook.com/intern/diff/D70922039/)

ghstack-source-id: 270854238
Pull Request resolved: #9105

Author: kimishpatel

extension/llm/custom_ops/op_sdpa.cpp

@@ -382,7 +382,7 @@ void cpu_flash_attention(
       /* qk_sum */ qSplitSize +
       /* dst    */ qSplitSize * headSize;
 
-  int64_t size_bytes = size_per_thread * num_thread * query.element_size();
+  int64_t size_bytes = size_per_thread * num_thread * query.element_size() * 4;
   std::vector<char> buf_vec(size_bytes);
   void* buf = reinterpret_cast<void*>(buf_vec.data());
   // Need to double check the following
@@ -452,6 +452,7 @@ void cpu_flash_attention(
       // However, lets just fix that as well.
       int64_t num_keys =
           is_causal ? std::min(m + start_pos + qBlockSize, kvSize) : kvSize;
+      int64_t m_start_pos = m + start_pos;
       auto j_kv = j / num_reps;
       for (int64_t n = 0; n < num_keys; n += kvSplitSize) {
         int64_t kvBlockSize = std::min(kvSplitSize, kvSize - n);
@@ -471,29 +472,58 @@ void cpu_flash_attention(
             static_cast<accum_t>(0),
             qk_data,
             kvBlockSize);
-        // Apply causal mask, fill unused, i.e. future values, with -inf
-        // Say you have q @ k.T size = [16, 32]
-        // With qblock size = 4, say you are processing
-        // q seq len dim = 8:11.
-        // Say kvSplitSize = 4
-        // Then for causal mask, the entries that needs to be
-        // ignored are
-        // [8, 9:31], [9, 10:31], [10, 10:31], [11, 11:31]
-        // Following condition says that num_keys = 8 + 4 =12
-        // (num_keys - n) <= kvSplitSize
-        // num_keys <= n + kvSplitSize
-        // If n + kvSplitSize is larger than 12, then some
-        // entries need masked out. In our example n = 4
-        // will qualify for that
-        if (is_causal && num_keys - n <= kvSplitSize) {
+        // There are 4 cases that is_causal has to cover to fill not-attendable-position with -inf
+        /* 1. Everything is attended to. This happens when m_start_pos > n + kvSplitSize
+              e.g m_pos [8:15] and n_pos [0:7]. Since you must attend to all previous tokens
+              matrix is full
+        + + + + + + + +
+        + + + + + + + +
+        + + + + + + + +
+        + + + + + + + +
+        + + + + + + + +
+        + + + + + + + +
+        + + + + + + + +
+           2. Everything is not attended to. However only some tokens at the beginning dont attend
+              to everything. This happens when m_start_pos <= n + kvSplitSize but m_start_pos + qBlockSize > n + kvSplitSize
+              m_start_pos = 8 qBlockSize = 8
+              n = 4 kvSplitSize = 8
+              For example m_pos [8:15] but n_pos is [4:11]
+        + + + + + - - -
+        + + + + + + - -
+        + + + + + + + -
+        + + + + + + + +
+        + + + + + + + +
+        + + + + + + + +
+        + + + + + + + +
+        + + + + + + + +
+           3. In this case only last few tokens have something to attend to. This happens when m_start_pos < n
+              and m_start_pos + qBlockSize >= n
+              and m_start_pos + qBlockSize <= n + kvSplitSize
+              m_start_pos = 8 qBlockSize = 8
+              n = 13 kvSplitSize = 8
+              For example m_pos [8:15] but n_pos is [13:20]
+        - - - - - - - -
+        - - - - - - - -
+        - - - - - - - -
+        - - - - - - - -
+        - - - - - - - -
+        + - - - - - - -
+        + + - - - - - -
+        + + + - - - - -
+           4. In this no tokens attend to anything, but we dont really have to take care of this case because
+              the loop for (int64_t n = 0; n < num_keys; n += kvSplitSize) will exit before that.
+        */
+        if (is_causal && m_start_pos <= n + kvSplitSize) {
           // For this fn to work k_split_size > q_split_size
-          for (int32_t row = 0; row < qBlockSize; ++row) {
-            int64_t last_col = m + (row + start_pos) - n;
+          for (int32_t row = 0; row < qBlockSize && (m_start_pos + row < n + (kvSplitSize - 1)); ++row) {
+            // When last_col is 0, it means that the entire row is not attended to
+            // because m_pos is smaller than n_pos. So everything in n is for future.
+            int64_t last_col = n > (m_start_pos + row) ? 0 : row + m_start_pos + 1 - n;
             accum_t* row_ptr = qk_data + row * kvBlockSize;
             fill_stub(
-                row_ptr + last_col + 1,
+                row_ptr + last_col,
                 -std::numeric_limits<accum_t>::infinity(),
-                kvBlockSize - last_col - 1);
+                kvBlockSize - last_col);
           }
         }
         // Update attention weights with attention mask

extension/llm/custom_ops/test_sdpa_with_kv_cache.py

@@ -590,3 +590,14 @@ def test_sdpa_with_cache_seq_len_llava_example_gqa(self):
         self._test_sdpa_common(
             n_heads_kv, n_heads_q, head_dim, max_seq_len, seq_len, next_iter_seq_len
         )
+
+    def test_sdpa_to_repro_long_seq_failure(self):
+        n_heads_kv = 16
+        n_heads_q = 32
+        head_dim = 128
+        max_seq_len = 2048
+        seq_len = 508
+        next_iter_seq_len = 127
+        self._test_sdpa_common(
+            n_heads_kv, n_heads_q, head_dim, max_seq_len, seq_len, next_iter_seq_len
+        )