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GH-118095: Make sure that progress is made if there are pending calls being handled. #118484

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Merged
merged 1 commit into from
May 1, 2024
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GH-118095: Make sure that progress is made if there are pending calls being handled. #118484

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2 changes: 1 addition & 1 deletion Include/internal/pycore_ceval_state.h
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Original file line number Diff line number Diff line change
Expand Up @@ -41,7 +41,7 @@ struct _pending_call {
#define MAXPENDINGCALLSLOOP_MAIN 0

struct _pending_calls {
int busy;
PyThreadState *handling_thread;
PyMutex mutex;
/* Request for running pending calls. */
int32_t npending;
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21 changes: 10 additions & 11 deletions Python/ceval_gil.c
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Expand Up @@ -877,21 +877,20 @@ make_pending_calls(PyThreadState *tstate)
/* Only one thread (per interpreter) may run the pending calls
at once. In the same way, we don't do recursive pending calls. */
PyMutex_Lock(&pending->mutex);
if (pending->busy) {
if (pending->handling_thread != NULL) {
/* A pending call was added after another thread was already
handling the pending calls (and had already "unsignaled").
Once that thread is done, it may have taken care of all the
pending calls, or there might be some still waiting.
Regardless, this interpreter's pending calls will stay
"signaled" until that first thread has finished. At that
point the next thread to trip the eval breaker will take
care of any remaining pending calls. Until then, though,
all the interpreter's threads will be tripping the eval
breaker every time it's checked. */
To avoid all threads constantly stopping on the eval breaker,
we clear the bit for this thread and make sure it is set
for the thread currently handling the pending call. */
_Py_set_eval_breaker_bit(pending->handling_thread, _PY_CALLS_TO_DO_BIT);
_Py_unset_eval_breaker_bit(tstate, _PY_CALLS_TO_DO_BIT);
Comment on lines +885 to +889
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Does this mean that only the thread that was already handling pending calls will handle any remaining ones (until the queue is empty)? Currently the next available thread handles the remaining ones, so no single thread is blocked too long handling pending calls.

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Currently, the next available thread keeps tripping the eval breaker until the current call is handled, so the _PY_CALLS_TO_DO_BIT is in effect blocking.

A non-blocking and fair mechanism would be good, but the current mechanism is neither.
This makes it non-blocking, although maybe less fair.

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That's fine. We can circle back to a fair mechanism later, if someone cares enough.

PyMutex_Unlock(&pending->mutex);
return 0;
}
pending->busy = 1;
pending->handling_thread = tstate;
PyMutex_Unlock(&pending->mutex);

/* unsignal before starting to call callbacks, so that any callback
Expand All @@ -900,7 +899,7 @@ make_pending_calls(PyThreadState *tstate)

int32_t npending;
if (_make_pending_calls(pending, &npending) != 0) {
pending->busy = 0;
pending->handling_thread = NULL;
/* There might not be more calls to make, but we play it safe. */
signal_pending_calls(tstate, interp);
return -1;
Expand All @@ -912,7 +911,7 @@ make_pending_calls(PyThreadState *tstate)

if (_Py_IsMainThread() && _Py_IsMainInterpreter(interp)) {
if (_make_pending_calls(pending_main, &npending) != 0) {
pending->busy = 0;
pending->handling_thread = NULL;
/* There might not be more calls to make, but we play it safe. */
signal_pending_calls(tstate, interp);
return -1;
Expand All @@ -923,7 +922,7 @@ make_pending_calls(PyThreadState *tstate)
}
}

pending->busy = 0;
pending->handling_thread = NULL;
return 0;
}

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