USE_STACKCHECK is a Windows-only feature which provides additional safety against C stack overflow by periodically calling PyOS_CheckStack to determine whether the current thread is too close to the end of the stack.

The way USE_STACKCHECK ensures that PyOS_CheckStack is called frequently is surprising. It does this by artificially decrementing _Py_CheckRecursionLimit with every call to Py_EnterRecursiveCall:

    #ifdef USE_STACKCHECK
    /* With USE_STACKCHECK, we artificially decrement the recursion limit in order
       to trigger regular stack checks in _Py_CheckRecursiveCall(), except if
       the "overflowed" flag is set, in which case we need the true value
       of _Py_CheckRecursionLimit for _Py_MakeEndRecCheck() to function properly.
    */
    #  define _Py_MakeRecCheck(x)  \
        (++(x) > (_Py_CheckRecursionLimit += PyThreadState_GET()->overflowed - 1))
    #else
    #  define _Py_MakeRecCheck(x)  (++(x) > _Py_CheckRecursionLimit)
    #endif

_Py_CheckRecursionLimit defaults to 1000, and is constant when USE_STACKCHECK is off. (except when changed by Py_SetRecursionLimit)

With a single thread, each call to Py_EnterRecursiveCall causes _Py_CheckRecursionLimit to decrement by 1 until it equals the current recursion depth, at which point _Py_CheckRecursiveCall is triggered, resetting _Py_CheckRecursionLimit back to the default. This could be anywhere from 500 to 1000 calls depending on the recursion depth. With multiple threads, the behavior may be more chaotic, as each thread will be decrementing _Py_CheckRecursionLimit independently.

I propose that instead, the call to PyOS_CheckStack is triggered in a predictable fashion, using a separate counter for each thread, so that it occurs every N'th call to Py_EnterRecursiveCall. (N = 64 seems reasonable, as PyOS_CheckStack guarantees 2048 * sizeof(void*) bytes remaining on the C stack).