homepage

When a thread gets a signal, SIGNAL_PENDING_SIGNALS() sets signals_pending to 1 and eval_breaker to 1. In this case, _PyEval_EvalFrameDefault() calls handle_signals(), but since it's not the main thread, it does nothing and signals_pending value remains 1. Moreover, eval_breaker value remains 1 which means that the following code will be called before executing *each* bytecode instruction.
 if (_Py_atomic_load_relaxed(eval_breaker)) {
 (...)
 opcode = _Py_OPCODE(*next_instr);
 if (opcode == SETUP_FINALLY || ...) {
 ...
 }
 if (_Py_atomic_load_relaxed(&ceval->signals_pending)) {
 if (handle_signals(tstate) != 0) {
 goto error;
 }
 }
 if (_Py_atomic_load_relaxed(&ceval->pending.calls_to_do)) {
 ...
 }
 if (_Py_atomic_load_relaxed(&ceval->gil_drop_request)) {
 ...
 }
 if (tstate->async_exc != NULL) {
 ...
 }
 }
This is inefficient.
I'm working on a PR modifying SIGNAL_PENDING_SIGNALS() to not set eval_breaker to 1 if the current thread is not the main thread.

I found this issue while working on PR 19066 of bpo-39984.

New changeset 5a3a71dddbe80edc75a3a74ce3b7978cf8635901 by Victor Stinner in branch 'master':
bpo-40010: Optimize signal handling in multithreaded applications (GH-19067)
https://github.com/python/cpython/commit/5a3a71dddbe80edc75a3a74ce3b7978cf8635901

New changeset a36adfa6bbf5e612a4d4639124502135690899b8 by Victor Stinner in branch 'master':
bpo-39877: 4th take_gil() fix for daemon threads (GH-19080)
https://github.com/python/cpython/commit/a36adfa6bbf5e612a4d4639124502135690899b8

New changeset da2914db4b6f786a1e9f0b424efeeb6ca9418912 by Victor Stinner in branch 'master':
bpo-40010: Pass tstate to ceval GIL functions (GH-19077)
https://github.com/python/cpython/commit/da2914db4b6f786a1e9f0b424efeeb6ca9418912

New changeset d2a8e5b42c5e9c4e745a0589043a8aebb49f8ca2 by Victor Stinner in branch 'master':
bpo-40010: COMPUTE_EVAL_BREAKER() checks for subinterpreter (GH-19087)
https://github.com/python/cpython/commit/d2a8e5b42c5e9c4e745a0589043a8aebb49f8ca2

New changeset d83168854e19d0381fa57db25fca6c622917624f by Victor Stinner in branch 'master':
bpo-40010: Optimize pending calls in multithreaded applications (GH-19091)
https://github.com/python/cpython/commit/d83168854e19d0381fa57db25fca6c622917624f

Good catch on this, Victor. Thanks for doing it.