backend : add eval callback (llama/4935)

* backend : add eval callback

ggml-ci

* backend : group nodes in a single compute when user don't need them

* backend : clean-up the implementation

ggml-ci

* simple : do not perform tensor data copy if not needed

* simple : fix

* simple : no need for ggml_is_contiguous + fix bool parse

* llama : fix callback placement in llama_context_params

* backend : avoid double-ask callback calls

* simple : restore examples, imatrix will serve as a demo

ggml-backend.c

@@ -802,6 +802,9 @@ struct ggml_backend_sched {
     __attribute__((aligned(GGML_MEM_ALIGN)))
     #endif
     char context_buffer[GGML_MAX_SPLITS*GGML_MAX_SPLIT_INPUTS*sizeof(struct ggml_tensor) + sizeof(struct ggml_cgraph)];
+
+    ggml_backend_sched_eval_callback callback_eval;
+    void * callback_eval_user_data;
 };
 
 #define hash_id(node) ggml_hash_find_or_insert(sched->hash_set, node)
@@ -1324,9 +1327,38 @@ static void sched_compute_splits(ggml_backend_sched_t sched) {
         ggml_graph_dump_dot(split->graph, NULL, split_filename);
 #endif
 
+
         uint64_t compute_start_us = ggml_time_us();
-        ggml_backend_graph_compute(split_backend, &split->graph);
-        //ggml_backend_synchronize(split_backend); // necessary to measure compute time
+        if (!sched->callback_eval) {
+            ggml_backend_graph_compute(split_backend, &split->graph);
+          //ggml_backend_synchronize(split_backend); // necessary to measure compute time
+        } else {
+            // similar to ggml_backend_compare_graph_backend
+            for (int j0 = 0; j0 < split->graph.n_nodes; j0++) {
+                struct ggml_tensor * t = split->graph.nodes[j0];
+
+                // check if the user needs data from this node
+                bool need = sched->callback_eval(t, true, sched->callback_eval_user_data);
+
+                int j1 = j0;
+
+                // determine the range [j0, j1] of nodes that can be computed together
+                while (!need && j1 < split->graph.n_nodes - 1) {
+                    t = split->graph.nodes[++j1];
+                    need = sched->callback_eval(t, true, sched->callback_eval_user_data);
+                }
+
+                struct ggml_cgraph gv = ggml_graph_view(&split->graph, j0, j1 + 1);
+
+                ggml_backend_graph_compute(split_backend, &gv);
+
+                if (need && !sched->callback_eval(t, false, sched->callback_eval_user_data)) {
+                    break;
+                }
+
+                j0 = j1;
+            }
+        }
         uint64_t compute_end_us = ggml_time_us();
         compute_us[split_backend_id] += compute_end_us - compute_start_us;
     }
@@ -1431,6 +1463,12 @@ void ggml_backend_sched_reset(ggml_backend_sched_t sched) {
     sched_reset(sched);
 }
 
+
+void ggml_backend_sched_set_eval_callback(ggml_backend_sched_t sched, ggml_backend_sched_eval_callback callback, void * user_data) {
+    sched->callback_eval = callback;
+    sched->callback_eval_user_data = user_data;
+}
+
 int ggml_backend_sched_get_n_splits(ggml_backend_sched_t sched) {
     return sched->n_splits;
 }

ggml-backend.h

@@ -148,6 +148,14 @@ extern "C" {
     struct ggml_backend_sched;
     typedef struct ggml_backend_sched * ggml_backend_sched_t;
 
+    // when ask == true, the scheduler wants to know if the user wants to observe this node
+    // this allows the scheduler to batch nodes together in order to evaluate them in a single call
+    //
+    // when ask == false, the scheduler is passing the node tensor to the user for observation
+    // if the user returns false, the scheduler will cancel the graph compute
+    //
+    typedef bool (*ggml_backend_sched_eval_callback)(struct ggml_tensor * t, bool ask, void * user_data);
+
     // Initialize a backend scheduler
     GGML_API ggml_backend_sched_t  ggml_backend_sched_new(ggml_backend_t * backends, ggml_backend_buffer_type_t * bufts, int n_backends, size_t graph_size);
     GGML_API void                  ggml_backend_sched_free(ggml_backend_sched_t sched);
@@ -168,6 +176,9 @@ extern "C" {
     // Reset all assignments and allocators - must be called before using the sched allocators to allocate inputs
     GGML_API void                  ggml_backend_sched_reset(ggml_backend_sched_t sched);
 
+    // Set a callback to be called for each resulting node during graph compute
+    GGML_API void                  ggml_backend_sched_set_eval_callback(ggml_backend_sched_t sched, ggml_backend_sched_eval_callback callback, void * user_data);
+
     //
     // Utils
     //