Avoid the thread jump in server callback APIs.

Add a utility function in iomgr to check whether the caller thread is a
worker for any background poller, and keep grpc combiner from offloading
closures to the default executor if the current thread is a worker for
any background poller.

src/core/lib/iomgr/combiner.cc

@@ -29,6 +29,7 @@
 
 #include "src/core/lib/debug/stats.h"
 #include "src/core/lib/iomgr/executor.h"
+#include "src/core/lib/iomgr/iomgr.h"
 #include "src/core/lib/profiling/timers.h"
 
 grpc_core::DebugOnlyTraceFlag grpc_combiner_trace(false, "combiner");
@@ -228,8 +229,14 @@ bool grpc_combiner_continue_exec_ctx() {
                               grpc_core::ExecCtx::Get()->IsReadyToFinish(),
                               lock->time_to_execute_final_list));
 
+  // offload only if all the following conditions are true:
+  // 1. the combiner is contended and has more than one closure to execute
+  // 2. the current execution context needs to finish as soon as possible
+  // 3. the DEFAULT executor is threaded
+  // 4. the current thread is not a worker for any background poller
   if (contended && grpc_core::ExecCtx::Get()->IsReadyToFinish() &&
-      grpc_executor_is_threaded()) {
+      grpc_executor_is_threaded() &&
+      !grpc_iomgr_is_any_background_poller_thread()) {
     GPR_TIMER_MARK("offload_from_finished_exec_ctx", 0);
     // this execution context wants to move on: schedule remaining work to be
     // picked up on the executor

src/core/lib/iomgr/ev_epoll1_linux.cc

@@ -1242,6 +1242,8 @@ static void pollset_set_del_pollset_set(grpc_pollset_set* bag,
  * Event engine binding
  */
 
+static bool is_any_background_poller_thread(void) { return false; }
+
 static void shutdown_background_closure(void) {}
 
 static void shutdown_engine(void) {
@@ -1287,6 +1289,7 @@ static const grpc_event_engine_vtable vtable = {
     pollset_set_add_fd,
     pollset_set_del_fd,
 
+    is_any_background_poller_thread,
     shutdown_background_closure,
     shutdown_engine,
 };

src/core/lib/iomgr/ev_epollex_linux.cc

@@ -1604,6 +1604,8 @@ static void pollset_set_del_pollset_set(grpc_pollset_set* bag,
  * Event engine binding
  */
 
+static bool is_any_background_poller_thread(void) { return false; }
+
 static void shutdown_background_closure(void) {}
 
 static void shutdown_engine(void) {
@@ -1644,6 +1646,7 @@ static const grpc_event_engine_vtable vtable = {
     pollset_set_add_fd,
     pollset_set_del_fd,
 
+    is_any_background_poller_thread,
     shutdown_background_closure,
     shutdown_engine,
 };

src/core/lib/iomgr/ev_poll_posix.cc

@@ -1782,6 +1782,8 @@ static void global_cv_fd_table_shutdown() {
  * event engine binding
  */
 
+static bool is_any_background_poller_thread(void) { return false; }
+
 static void shutdown_background_closure(void) {}
 
 static void shutdown_engine(void) {
@@ -1828,6 +1830,7 @@ static const grpc_event_engine_vtable vtable = {
     pollset_set_add_fd,
     pollset_set_del_fd,
 
+    is_any_background_poller_thread,
     shutdown_background_closure,
     shutdown_engine,
 };

src/core/lib/iomgr/ev_posix.cc

@@ -399,6 +399,10 @@ void grpc_pollset_set_del_fd(grpc_pollset_set* pollset_set, grpc_fd* fd) {
   g_event_engine->pollset_set_del_fd(pollset_set, fd);
 }
 
+bool grpc_is_any_background_poller_thread(void) {
+  return g_event_engine->is_any_background_poller_thread();
+}
+
 void grpc_shutdown_background_closure(void) {
   g_event_engine->shutdown_background_closure();
 }

src/core/lib/iomgr/ev_posix.h

@@ -80,6 +80,7 @@ typedef struct grpc_event_engine_vtable {
   void (*pollset_set_add_fd)(grpc_pollset_set* pollset_set, grpc_fd* fd);
   void (*pollset_set_del_fd)(grpc_pollset_set* pollset_set, grpc_fd* fd);
 
+  bool (*is_any_background_poller_thread)(void);
   void (*shutdown_background_closure)(void);
   void (*shutdown_engine)(void);
 } grpc_event_engine_vtable;
@@ -181,6 +182,9 @@ void grpc_pollset_add_fd(grpc_pollset* pollset, struct grpc_fd* fd);
 void grpc_pollset_set_add_fd(grpc_pollset_set* pollset_set, grpc_fd* fd);
 void grpc_pollset_set_del_fd(grpc_pollset_set* pollset_set, grpc_fd* fd);
 
+/* Returns true if the caller is a worker thread for any background poller. */
+bool grpc_is_any_background_poller_thread();
+
 /* Shut down all the closures registered in the background poller. */
 void grpc_shutdown_background_closure();
 

src/core/lib/iomgr/iomgr.cc

@@ -161,6 +161,10 @@ void grpc_iomgr_shutdown_background_closure() {
   grpc_iomgr_platform_shutdown_background_closure();
 }
 
+bool grpc_iomgr_is_any_background_poller_thread() {
+  return grpc_iomgr_platform_is_any_background_poller_thread();
+}
+
 void grpc_iomgr_register_object(grpc_iomgr_object* obj, const char* name) {
   obj->name = gpr_strdup(name);
   gpr_mu_lock(&g_mu);

src/core/lib/iomgr/iomgr.h

@@ -39,6 +39,9 @@ void grpc_iomgr_shutdown();
  * background poller. */
 void grpc_iomgr_shutdown_background_closure();
 
+/** Returns true if the caller is a worker thread for any background poller. */
+bool grpc_iomgr_is_any_background_poller_thread();
+
 /* Exposed only for testing */
 size_t grpc_iomgr_count_objects_for_testing();
 

src/core/lib/iomgr/iomgr_custom.cc

@@ -41,10 +41,14 @@ static void iomgr_platform_init(void) {
 static void iomgr_platform_flush(void) {}
 static void iomgr_platform_shutdown(void) { grpc_pollset_global_shutdown(); }
 static void iomgr_platform_shutdown_background_closure(void) {}
+static bool iomgr_platform_is_any_background_poller_thread(void) {
+  return false;
+}
 
 static grpc_iomgr_platform_vtable vtable = {
     iomgr_platform_init, iomgr_platform_flush, iomgr_platform_shutdown,
-    iomgr_platform_shutdown_background_closure};
+    iomgr_platform_shutdown_background_closure,
+    iomgr_platform_is_any_background_poller_thread};
 
 void grpc_custom_iomgr_init(grpc_socket_vtable* socket,
                             grpc_custom_resolver_vtable* resolver,

src/core/lib/iomgr/iomgr_internal.cc

@@ -45,3 +45,7 @@ void grpc_iomgr_platform_shutdown() { iomgr_platform_vtable->shutdown(); }
 void grpc_iomgr_platform_shutdown_background_closure() {
   iomgr_platform_vtable->shutdown_background_closure();
 }
+
+bool grpc_iomgr_platform_is_any_background_poller_thread() {
+  return iomgr_platform_vtable->is_any_background_poller_thread();
+}

src/core/lib/iomgr/iomgr_internal.h

@@ -36,6 +36,7 @@ typedef struct grpc_iomgr_platform_vtable {
   void (*flush)(void);
   void (*shutdown)(void);
   void (*shutdown_background_closure)(void);
+  bool (*is_any_background_poller_thread)(void);
 } grpc_iomgr_platform_vtable;
 
 void grpc_iomgr_register_object(grpc_iomgr_object* obj, const char* name);
@@ -56,6 +57,9 @@ void grpc_iomgr_platform_shutdown(void);
 /** shut down all the closures registered in the background poller */
 void grpc_iomgr_platform_shutdown_background_closure(void);
 
+/** return true is the caller is a worker thread for any background poller */
+bool grpc_iomgr_platform_is_any_background_poller_thread(void);
+
 bool grpc_iomgr_abort_on_leaks(void);
 
 #endif /* GRPC_CORE_LIB_IOMGR_IOMGR_INTERNAL_H */

src/core/lib/iomgr/iomgr_posix.cc

@@ -55,9 +55,14 @@ static void iomgr_platform_shutdown_background_closure(void) {
   grpc_shutdown_background_closure();
 }
 
+static bool iomgr_platform_is_any_background_poller_thread(void) {
+  return grpc_is_any_background_poller_thread();
+}
+
 static grpc_iomgr_platform_vtable vtable = {
     iomgr_platform_init, iomgr_platform_flush, iomgr_platform_shutdown,
-    iomgr_platform_shutdown_background_closure};
+    iomgr_platform_shutdown_background_closure,
+    iomgr_platform_is_any_background_poller_thread};
 
 void grpc_set_default_iomgr_platform() {
   grpc_set_tcp_client_impl(&grpc_posix_tcp_client_vtable);

src/core/lib/iomgr/iomgr_posix_cfstream.cc

@@ -58,9 +58,14 @@ static void iomgr_platform_shutdown_background_closure(void) {
   grpc_shutdown_background_closure();
 }
 
+static bool iomgr_platform_is_any_background_poller_thread(void) {
+  return grpc_is_any_background_poller_thread();
+}
+
 static grpc_iomgr_platform_vtable vtable = {
     iomgr_platform_init, iomgr_platform_flush, iomgr_platform_shutdown,
-    iomgr_platform_shutdown_background_closure};
+    iomgr_platform_shutdown_background_closure,
+    iomgr_platform_is_any_background_poller_thread};
 
 void grpc_set_default_iomgr_platform() {
   char* enable_cfstream = getenv(grpc_cfstream_env_var);

src/core/lib/iomgr/iomgr_windows.cc

@@ -73,9 +73,14 @@ static void iomgr_platform_shutdown(void) {
 
 static void iomgr_platform_shutdown_background_closure(void) {}
 
+static bool iomgr_platform_is_any_background_poller_thread(void) {
+  return false;
+}
+
 static grpc_iomgr_platform_vtable vtable = {
     iomgr_platform_init, iomgr_platform_flush, iomgr_platform_shutdown,
-    iomgr_platform_shutdown_background_closure};
+    iomgr_platform_shutdown_background_closure,
+    iomgr_platform_is_any_background_poller_thread};
 
 void grpc_set_default_iomgr_platform() {
   grpc_set_tcp_client_impl(&grpc_windows_tcp_client_vtable);

test/cpp/microbenchmarks/bm_cq_multiple_threads.cc

@@ -94,6 +94,7 @@ static const grpc_event_engine_vtable* init_engine_vtable(bool) {
   g_vtable.pollset_destroy = pollset_destroy;
   g_vtable.pollset_work = pollset_work;
   g_vtable.pollset_kick = pollset_kick;
+  g_vtable.is_any_background_poller_thread = [] { return false; };
   g_vtable.shutdown_background_closure = [] {};
   g_vtable.shutdown_engine = [] {};