bpo-40010: Optimize pending calls in multithreaded applications (GH-19091)

If a thread different than the main thread schedules a pending call
(Py_AddPendingCall()), the bytecode evaluation loop is no longer
interrupted at each bytecode instruction to check for pending calls
which cannot be executed. Only the main thread can execute pending
calls.

Previously, the bytecode evaluation loop was interrupted at each
instruction until the main thread executes pending calls.

* Add _Py_ThreadCanHandlePendingCalls() function.
* SIGNAL_PENDING_CALLS() now only sets eval_breaker to 1 if the
  current thread can execute pending calls. Only the main thread can
  execute pending calls.

1 files changed, 6 insertions, 4 deletions

diff --git a/Python/ceval.c b/Python/ceval.c
index 86aa225389..c80ee4b463 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -149,7 +149,8 @@ COMPUTE_EVAL_BREAKER(PyThreadState *tstate,
         _Py_atomic_load_relaxed(&ceval->gil_drop_request)
         | (_Py_atomic_load_relaxed(&ceval->signals_pending)
            && _Py_ThreadCanHandleSignals(tstate))
-        | _Py_atomic_load_relaxed(&ceval2->pending.calls_to_do)
+        | (_Py_atomic_load_relaxed(&ceval2->pending.calls_to_do)
+           && _Py_ThreadCanHandlePendingCalls())
         | ceval2->pending.async_exc);
 }
 
@@ -180,9 +181,10 @@ static inline void
 SIGNAL_PENDING_CALLS(PyThreadState *tstate)
 {
     assert(is_tstate_valid(tstate));
+    struct _ceval_runtime_state *ceval = &tstate->interp->runtime->ceval;
     struct _ceval_state *ceval2 = &tstate->interp->ceval;
     _Py_atomic_store_relaxed(&ceval2->pending.calls_to_do, 1);
-    _Py_atomic_store_relaxed(&ceval2->eval_breaker, 1);
+    COMPUTE_EVAL_BREAKER(tstate, ceval, ceval2);
 }
 
 
@@ -606,8 +608,8 @@ handle_signals(PyThreadState *tstate)
 static int
 make_pending_calls(PyThreadState *tstate)
 {
-    /* only service pending calls on main thread */
-    if (!_Py_IsMainThread()) {
+    /* only execute pending calls on main thread */
+    if (!_Py_ThreadCanHandlePendingCalls()) {
         return 0;
     }