A loop split between two hot executors never calls CHECK_EVAL_BREAKER()

[gvanrossum]

When a tight for loop without calls is turned into two traces that transfer directly to each other, it seems that CHECK_EVAL_BREAKER() is never called, and the loop won't be interruptible.

See faster-cpython/ideas#669 (comment), where @brandtbucher and I reasoned that this might occur in the nbody benchmark (when the max trace length is reduced to 400).

While we didn't observe the predicted speedup, we really do believe that in this case the thread is uninterruptible.

@markshannon Any thoughts?

Linked PRs

• GH-117442: Check eval-breaker at start (rather than end) of tier 2 loops #118482

[brandtbucher]

Probably we should insert a _CHECK_PERIODIC at the beginning of a trace starting with JUMP_BACKWARD, rather than checking signals in _JUMP_TO_TOP. That way we also wouldn't need to check signals in ENTER_EXECUTOR, _JUMP_TO_TOP, and wherever else is necessary to fix this issue.