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9b29358511
Status lines for Emacs and Vim have been added to Python sources so they'll be easier to edit using Python's preferred coding style. Some DNS helper functions have been broken up into multiple files. It's nice to have one function per file whenever possible, since that way we don't need -ffunction-sections. Another reason it's good to have small source files, is because the build will be enforcing resource limits on compilation and testing soon.
274 lines
9.7 KiB
C++
274 lines
9.7 KiB
C++
/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:4;tab-width:8;coding:utf-8 -*-│
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│vi: set net ft=c ts=4 sts=4 sw=4 fenc=utf-8 :vi│
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╞══════════════════════════════════════════════════════════════════════════════╡
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│ Python 3 │
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│ https://docs.python.org/3/license.html │
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╚─────────────────────────────────────────────────────────────────────────────*/
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#include "third_party/python/Python/condvar.h"
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/* clang-format off */
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/*
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* Implementation of the Global Interpreter Lock (GIL).
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*/
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/* First some general settings */
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/* microseconds (the Python API uses seconds, though) */
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#define DEFAULT_INTERVAL 5000
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static unsigned long gil_interval = DEFAULT_INTERVAL;
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#define INTERVAL (gil_interval >= 1 ? gil_interval : 1)
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/* Enable if you want to force the switching of threads at least every `gil_interval` */
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#undef FORCE_SWITCHING
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#define FORCE_SWITCHING
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/*
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Notes about the implementation:
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- The GIL is just a boolean variable (gil_locked) whose access is protected
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by a mutex (gil_mutex), and whose changes are signalled by a condition
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variable (gil_cond). gil_mutex is taken for short periods of time,
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and therefore mostly uncontended.
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- In the GIL-holding thread, the main loop (PyEval_EvalFrameEx) must be
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able to release the GIL on demand by another thread. A volatile boolean
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variable (gil_drop_request) is used for that purpose, which is checked
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at every turn of the eval loop. That variable is set after a wait of
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`interval` microseconds on `gil_cond` has timed out.
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[Actually, another volatile boolean variable (eval_breaker) is used
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which ORs several conditions into one. Volatile booleans are
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sufficient as inter-thread signalling means since Python is run
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on cache-coherent architectures only.]
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- A thread wanting to take the GIL will first let pass a given amount of
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time (`interval` microseconds) before setting gil_drop_request. This
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encourages a defined switching period, but doesn't enforce it since
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opcodes can take an arbitrary time to execute.
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The `interval` value is available for the user to read and modify
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using the Python API `sys.{get,set}switchinterval()`.
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- When a thread releases the GIL and gil_drop_request is set, that thread
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ensures that another GIL-awaiting thread gets scheduled.
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It does so by waiting on a condition variable (switch_cond) until
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the value of gil_last_holder is changed to something else than its
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own thread state pointer, indicating that another thread was able to
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take the GIL.
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This is meant to prohibit the latency-adverse behaviour on multi-core
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machines where one thread would speculatively release the GIL, but still
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run and end up being the first to re-acquire it, making the "timeslices"
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much longer than expected.
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(Note: this mechanism is enabled with FORCE_SWITCHING above)
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*/
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#ifndef Py_HAVE_CONDVAR
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#error You need either a POSIX-compatible or a Windows system!
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#endif
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#define MUTEX_T PyMUTEX_T
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#define MUTEX_INIT(mut) \
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if (PyMUTEX_INIT(&(mut))) { \
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Py_FatalError("PyMUTEX_INIT(" #mut ") failed"); };
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#define MUTEX_FINI(mut) \
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if (PyMUTEX_FINI(&(mut))) { \
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Py_FatalError("PyMUTEX_FINI(" #mut ") failed"); };
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#define MUTEX_LOCK(mut) \
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if (PyMUTEX_LOCK(&(mut))) { \
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Py_FatalError("PyMUTEX_LOCK(" #mut ") failed"); };
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#define MUTEX_UNLOCK(mut) \
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if (PyMUTEX_UNLOCK(&(mut))) { \
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Py_FatalError("PyMUTEX_UNLOCK(" #mut ") failed"); };
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#define COND_T PyCOND_T
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#define COND_INIT(cond) \
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if (PyCOND_INIT(&(cond))) { \
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Py_FatalError("PyCOND_INIT(" #cond ") failed"); };
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#define COND_FINI(cond) \
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if (PyCOND_FINI(&(cond))) { \
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Py_FatalError("PyCOND_FINI(" #cond ") failed"); };
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#define COND_SIGNAL(cond) \
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if (PyCOND_SIGNAL(&(cond))) { \
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Py_FatalError("PyCOND_SIGNAL(" #cond ") failed"); };
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#define COND_WAIT(cond, mut) \
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if (PyCOND_WAIT(&(cond), &(mut))) { \
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Py_FatalError("PyCOND_WAIT(" #cond ") failed"); };
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#define COND_TIMED_WAIT(cond, mut, microseconds, timeout_result) \
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{ \
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int r = PyCOND_TIMEDWAIT(&(cond), &(mut), (microseconds)); \
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if (r < 0) \
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Py_FatalError("PyCOND_WAIT(" #cond ") failed"); \
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if (r) /* 1 == timeout, 2 == impl. can't say, so assume timeout */ \
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timeout_result = 1; \
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else \
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timeout_result = 0; \
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} \
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/* Whether the GIL is already taken (-1 if uninitialized). This is atomic
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because it can be read without any lock taken in ceval.c. */
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static _Py_atomic_int gil_locked = {-1};
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/* Number of GIL switches since the beginning. */
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static unsigned long gil_switch_number = 0;
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/* Last PyThreadState holding / having held the GIL. This helps us know
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whether anyone else was scheduled after we dropped the GIL. */
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static _Py_atomic_address gil_last_holder = {0};
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/* This condition variable allows one or several threads to wait until
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the GIL is released. In addition, the mutex also protects the above
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variables. */
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static COND_T gil_cond;
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static MUTEX_T gil_mutex;
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#ifdef FORCE_SWITCHING
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/* This condition variable helps the GIL-releasing thread wait for
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a GIL-awaiting thread to be scheduled and take the GIL. */
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static COND_T switch_cond;
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static MUTEX_T switch_mutex;
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#endif
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static int gil_created(void)
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{
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return _Py_atomic_load_explicit(&gil_locked, _Py_memory_order_acquire) >= 0;
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}
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static void create_gil(void)
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{
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MUTEX_INIT(gil_mutex);
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#ifdef FORCE_SWITCHING
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MUTEX_INIT(switch_mutex);
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#endif
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COND_INIT(gil_cond);
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#ifdef FORCE_SWITCHING
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COND_INIT(switch_cond);
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#endif
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_Py_atomic_store_relaxed(&gil_last_holder, 0);
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_Py_ANNOTATE_RWLOCK_CREATE(&gil_locked);
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_Py_atomic_store_explicit(&gil_locked, 0, _Py_memory_order_release);
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}
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static void destroy_gil(void)
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{
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/* some pthread-like implementations tie the mutex to the cond
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* and must have the cond destroyed first.
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*/
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COND_FINI(gil_cond);
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MUTEX_FINI(gil_mutex);
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#ifdef FORCE_SWITCHING
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COND_FINI(switch_cond);
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MUTEX_FINI(switch_mutex);
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#endif
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_Py_atomic_store_explicit(&gil_locked, -1, _Py_memory_order_release);
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_Py_ANNOTATE_RWLOCK_DESTROY(&gil_locked);
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}
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static void recreate_gil(void)
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{
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_Py_ANNOTATE_RWLOCK_DESTROY(&gil_locked);
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/* XXX should we destroy the old OS resources here? */
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create_gil();
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}
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static void drop_gil(PyThreadState *tstate)
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{
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if (!_Py_atomic_load_relaxed(&gil_locked))
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Py_FatalError("drop_gil: GIL is not locked");
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/* tstate is allowed to be NULL (early interpreter init) */
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if (tstate != NULL) {
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/* Sub-interpreter support: threads might have been switched
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under our feet using PyThreadState_Swap(). Fix the GIL last
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holder variable so that our heuristics work. */
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_Py_atomic_store_relaxed(&gil_last_holder, (uintptr_t)tstate);
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}
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MUTEX_LOCK(gil_mutex);
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_Py_ANNOTATE_RWLOCK_RELEASED(&gil_locked, /*is_write=*/1);
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_Py_atomic_store_relaxed(&gil_locked, 0);
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COND_SIGNAL(gil_cond);
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MUTEX_UNLOCK(gil_mutex);
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#ifdef FORCE_SWITCHING
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if (_Py_atomic_load_relaxed(&gil_drop_request) && tstate != NULL) {
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MUTEX_LOCK(switch_mutex);
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/* Not switched yet => wait */
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if ((PyThreadState*)_Py_atomic_load_relaxed(&gil_last_holder) == tstate) {
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RESET_GIL_DROP_REQUEST();
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/* NOTE: if COND_WAIT does not atomically start waiting when
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releasing the mutex, another thread can run through, take
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the GIL and drop it again, and reset the condition
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before we even had a chance to wait for it. */
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COND_WAIT(switch_cond, switch_mutex);
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}
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MUTEX_UNLOCK(switch_mutex);
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}
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#endif
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}
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static void take_gil(PyThreadState *tstate)
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{
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int err;
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if (tstate == NULL)
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Py_FatalError("take_gil: NULL tstate");
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err = errno;
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MUTEX_LOCK(gil_mutex);
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if (!_Py_atomic_load_relaxed(&gil_locked))
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goto _ready;
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while (_Py_atomic_load_relaxed(&gil_locked)) {
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int timed_out = 0;
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unsigned long saved_switchnum;
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saved_switchnum = gil_switch_number;
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COND_TIMED_WAIT(gil_cond, gil_mutex, INTERVAL, timed_out);
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/* If we timed out and no switch occurred in the meantime, it is time
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to ask the GIL-holding thread to drop it. */
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if (timed_out &&
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_Py_atomic_load_relaxed(&gil_locked) &&
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gil_switch_number == saved_switchnum) {
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SET_GIL_DROP_REQUEST();
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}
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}
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_ready:
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#ifdef FORCE_SWITCHING
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/* This mutex must be taken before modifying gil_last_holder (see drop_gil()). */
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MUTEX_LOCK(switch_mutex);
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#endif
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/* We now hold the GIL */
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_Py_atomic_store_relaxed(&gil_locked, 1);
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_Py_ANNOTATE_RWLOCK_ACQUIRED(&gil_locked, /*is_write=*/1);
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if (tstate != (PyThreadState*)_Py_atomic_load_relaxed(&gil_last_holder)) {
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_Py_atomic_store_relaxed(&gil_last_holder, (uintptr_t)tstate);
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++gil_switch_number;
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}
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#ifdef FORCE_SWITCHING
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COND_SIGNAL(switch_cond);
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MUTEX_UNLOCK(switch_mutex);
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#endif
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if (_Py_atomic_load_relaxed(&gil_drop_request)) {
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RESET_GIL_DROP_REQUEST();
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}
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if (tstate->async_exc != NULL) {
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_PyEval_SignalAsyncExc();
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}
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MUTEX_UNLOCK(gil_mutex);
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errno = err;
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}
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void _PyEval_SetSwitchInterval(unsigned long microseconds)
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{
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gil_interval = microseconds;
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}
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unsigned long _PyEval_GetSwitchInterval()
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{
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return gil_interval;
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}
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