[chttp2] Prioritize sending out finished requests over other in flight requests in the transport (grpc#39897)

To debug b/377705482

Closes grpc#39897

COPYBARA_INTEGRATE_REVIEW=grpc#39897 from Vignesh2208:prioritize-finished-requests a0f50f0
PiperOrigin-RevId: 772570354

Author: Vignesh2208

bazel/experiments.bzl

@@ -128,6 +128,26 @@ static void stream_list_add_tail(grpc_chttp2_transport* t,
       << "]: add to " << stream_list_id_string(id);
 }
 
+static void stream_list_add_head(grpc_chttp2_transport* t,
+                                 grpc_chttp2_stream* s,
+                                 grpc_chttp2_stream_list_id id) {
+  grpc_chttp2_stream* old_head;
+  CHECK(!s->included.is_set(id));
+  old_head = t->lists[id].head;
+  s->links[id].next = old_head;
+  s->links[id].prev = nullptr;
+  if (old_head) {
+    old_head->links[id].prev = s;
+  } else {
+    t->lists[id].tail = s;
+  }
+  t->lists[id].head = s;
+  s->included.set(id);
+  GRPC_TRACE_LOG(http2_stream_state, INFO)
+      << t << "[" << s->id << "][" << (t->is_client ? "cli" : "svr")
+      << "]: add to " << stream_list_id_string(id);
+}
+
 static bool stream_list_add(grpc_chttp2_transport* t, grpc_chttp2_stream* s,
                             grpc_chttp2_stream_list_id id) {
   if (s->included.is_set(id)) {
@@ -137,11 +157,24 @@ static bool stream_list_add(grpc_chttp2_transport* t, grpc_chttp2_stream* s,
   return true;
 }
 
+static bool stream_list_prepend(grpc_chttp2_transport* t, grpc_chttp2_stream* s,
+                                grpc_chttp2_stream_list_id id) {
+  if (s->included.is_set(id)) {
+    return false;
+  }
+  stream_list_add_head(t, s, id);
+  return true;
+}
+
 // wrappers for specializations
 
 bool grpc_chttp2_list_add_writable_stream(grpc_chttp2_transport* t,
                                           grpc_chttp2_stream* s) {
   CHECK_NE(s->id, 0u);
+  if (grpc_core::IsPrioritizeFinishedRequestsEnabled() &&
+      s->send_trailing_metadata != nullptr) {
+    return stream_list_prepend(t, s, GRPC_CHTTP2_LIST_WRITABLE);
+  }
   return stream_list_add(t, s, GRPC_CHTTP2_LIST_WRITABLE);
 }
 
@@ -186,7 +219,12 @@ void grpc_chttp2_list_remove_waiting_for_concurrency(grpc_chttp2_transport* t,
 
 void grpc_chttp2_list_add_stalled_by_transport(grpc_chttp2_transport* t,
                                                grpc_chttp2_stream* s) {
-  stream_list_add(t, s, GRPC_CHTTP2_LIST_STALLED_BY_TRANSPORT);
+  if (grpc_core::IsPrioritizeFinishedRequestsEnabled() &&
+      s->send_trailing_metadata != nullptr) {
+    stream_list_prepend(t, s, GRPC_CHTTP2_LIST_STALLED_BY_TRANSPORT);
+  } else {
+    stream_list_add(t, s, GRPC_CHTTP2_LIST_STALLED_BY_TRANSPORT);
+  }
 }
 
 bool grpc_chttp2_list_pop_stalled_by_transport(grpc_chttp2_transport* t,

src/core/ext/transport/chttp2/transport/stream_lists.cc

@@ -171,6 +171,12 @@
   test_tags: ["core_end2end_test"]
   requires: ["event_engine_client", "event_engine_listener"]
   allow_in_fuzzing_config: false
+- name: prioritize_finished_requests
+  description: Prioritize flushing out finished requests over other in-flight
+    requests during transport writes.
+  expiry: 2025/09/01
+  owner: [email protected]
+  test_tags: []
 - name: promise_based_http2_client_transport
   description:
     Use promises for the http2 client transport. We have kept client and

src/core/lib/experiments/experiments.cc

@@ -74,6 +74,8 @@
   default: true
 - name: pollset_alternative
   default: false
+- name: prioritize_finished_requests
+  default: false
 - name: promise_based_http2_client_transport
   default: false
 - name: promise_based_http2_server_transport