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259 lines (223 loc) · 9.04 KB
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/*
* Implementation of the Global Interpreter Lock (GIL).
*/
#include <stdlib.h>
#include <errno.h>
/* First some general settings */
#define INTERVAL (_PyRuntime.ceval.gil.interval >= 1 ? _PyRuntime.ceval.gil.interval : 1)
/*
Notes about the implementation:
- The GIL is just a boolean variable (locked) whose access is protected
by a mutex (gil_mutex), and whose changes are signalled by a condition
variable (gil_cond). gil_mutex is taken for short periods of time,
and therefore mostly uncontended.
- In the GIL-holding thread, the main loop (PyEval_EvalFrameEx) must be
able to release the GIL on demand by another thread. A volatile boolean
variable (gil_drop_request) is used for that purpose, which is checked
at every turn of the eval loop. That variable is set after a wait of
`interval` microseconds on `gil_cond` has timed out.
[Actually, another volatile boolean variable (eval_breaker) is used
which ORs several conditions into one. Volatile booleans are
sufficient as inter-thread signalling means since Python is run
on cache-coherent architectures only.]
- A thread wanting to take the GIL will first let pass a given amount of
time (`interval` microseconds) before setting gil_drop_request. This
encourages a defined switching period, but doesn't enforce it since
opcodes can take an arbitrary time to execute.
The `interval` value is available for the user to read and modify
using the Python API `sys.{get,set}switchinterval()`.
- When a thread releases the GIL and gil_drop_request is set, that thread
ensures that another GIL-awaiting thread gets scheduled.
It does so by waiting on a condition variable (switch_cond) until
the value of last_holder is changed to something else than its
own thread state pointer, indicating that another thread was able to
take the GIL.
This is meant to prohibit the latency-adverse behaviour on multi-core
machines where one thread would speculatively release the GIL, but still
run and end up being the first to re-acquire it, making the "timeslices"
much longer than expected.
(Note: this mechanism is enabled with FORCE_SWITCHING above)
*/
#include "condvar.h"
#define MUTEX_INIT(mut) \
if (PyMUTEX_INIT(&(mut))) { \
Py_FatalError("PyMUTEX_INIT(" #mut ") failed"); };
#define MUTEX_FINI(mut) \
if (PyMUTEX_FINI(&(mut))) { \
Py_FatalError("PyMUTEX_FINI(" #mut ") failed"); };
#define MUTEX_LOCK(mut) \
if (PyMUTEX_LOCK(&(mut))) { \
Py_FatalError("PyMUTEX_LOCK(" #mut ") failed"); };
#define MUTEX_UNLOCK(mut) \
if (PyMUTEX_UNLOCK(&(mut))) { \
Py_FatalError("PyMUTEX_UNLOCK(" #mut ") failed"); };
#define COND_INIT(cond) \
if (PyCOND_INIT(&(cond))) { \
Py_FatalError("PyCOND_INIT(" #cond ") failed"); };
#define COND_FINI(cond) \
if (PyCOND_FINI(&(cond))) { \
Py_FatalError("PyCOND_FINI(" #cond ") failed"); };
#define COND_SIGNAL(cond) \
if (PyCOND_SIGNAL(&(cond))) { \
Py_FatalError("PyCOND_SIGNAL(" #cond ") failed"); };
#define COND_WAIT(cond, mut) \
if (PyCOND_WAIT(&(cond), &(mut))) { \
Py_FatalError("PyCOND_WAIT(" #cond ") failed"); };
#define COND_TIMED_WAIT(cond, mut, microseconds, timeout_result) \
{ \
int r = PyCOND_TIMEDWAIT(&(cond), &(mut), (microseconds)); \
if (r < 0) \
Py_FatalError("PyCOND_WAIT(" #cond ") failed"); \
if (r) /* 1 == timeout, 2 == impl. can't say, so assume timeout */ \
timeout_result = 1; \
else \
timeout_result = 0; \
} \
#define DEFAULT_INTERVAL 5000
static void _gil_initialize(struct _gil_runtime_state *state)
{
_Py_atomic_int uninitialized = {-1};
state->locked = uninitialized;
state->interval = DEFAULT_INTERVAL;
}
static int gil_created(void)
{
return (_Py_atomic_load_explicit(&_PyRuntime.ceval.gil.locked,
_Py_memory_order_acquire)
) >= 0;
}
static void create_gil(void)
{
MUTEX_INIT(_PyRuntime.ceval.gil.mutex);
#ifdef FORCE_SWITCHING
MUTEX_INIT(_PyRuntime.ceval.gil.switch_mutex);
#endif
COND_INIT(_PyRuntime.ceval.gil.cond);
#ifdef FORCE_SWITCHING
COND_INIT(_PyRuntime.ceval.gil.switch_cond);
#endif
_Py_atomic_store_relaxed(&_PyRuntime.ceval.gil.last_holder, 0);
_Py_ANNOTATE_RWLOCK_CREATE(&_PyRuntime.ceval.gil.locked);
_Py_atomic_store_explicit(&_PyRuntime.ceval.gil.locked, 0,
_Py_memory_order_release);
}
static void destroy_gil(void)
{
/* some pthread-like implementations tie the mutex to the cond
* and must have the cond destroyed first.
*/
COND_FINI(_PyRuntime.ceval.gil.cond);
MUTEX_FINI(_PyRuntime.ceval.gil.mutex);
#ifdef FORCE_SWITCHING
COND_FINI(_PyRuntime.ceval.gil.switch_cond);
MUTEX_FINI(_PyRuntime.ceval.gil.switch_mutex);
#endif
_Py_atomic_store_explicit(&_PyRuntime.ceval.gil.locked, -1,
_Py_memory_order_release);
_Py_ANNOTATE_RWLOCK_DESTROY(&_PyRuntime.ceval.gil.locked);
}
static void recreate_gil(void)
{
_Py_ANNOTATE_RWLOCK_DESTROY(&_PyRuntime.ceval.gil.locked);
/* XXX should we destroy the old OS resources here? */
create_gil();
}
static void drop_gil(PyThreadState *tstate)
{
if (!_Py_atomic_load_relaxed(&_PyRuntime.ceval.gil.locked))
Py_FatalError("drop_gil: GIL is not locked");
/* tstate is allowed to be NULL (early interpreter init) */
if (tstate != NULL) {
/* Sub-interpreter support: threads might have been switched
under our feet using PyThreadState_Swap(). Fix the GIL last
holder variable so that our heuristics work. */
_Py_atomic_store_relaxed(&_PyRuntime.ceval.gil.last_holder,
(uintptr_t)tstate);
}
MUTEX_LOCK(_PyRuntime.ceval.gil.mutex);
_Py_ANNOTATE_RWLOCK_RELEASED(&_PyRuntime.ceval.gil.locked, /*is_write=*/1);
_Py_atomic_store_relaxed(&_PyRuntime.ceval.gil.locked, 0);
COND_SIGNAL(_PyRuntime.ceval.gil.cond);
MUTEX_UNLOCK(_PyRuntime.ceval.gil.mutex);
#ifdef FORCE_SWITCHING
if (_Py_atomic_load_relaxed(&_PyRuntime.ceval.gil_drop_request) &&
tstate != NULL)
{
MUTEX_LOCK(_PyRuntime.ceval.gil.switch_mutex);
/* Not switched yet => wait */
if (((PyThreadState*)_Py_atomic_load_relaxed(
&_PyRuntime.ceval.gil.last_holder)
) == tstate)
{
RESET_GIL_DROP_REQUEST();
/* NOTE: if COND_WAIT does not atomically start waiting when
releasing the mutex, another thread can run through, take
the GIL and drop it again, and reset the condition
before we even had a chance to wait for it. */
COND_WAIT(_PyRuntime.ceval.gil.switch_cond,
_PyRuntime.ceval.gil.switch_mutex);
}
MUTEX_UNLOCK(_PyRuntime.ceval.gil.switch_mutex);
}
#endif
}
static void take_gil(PyThreadState *tstate)
{
int err;
if (tstate == NULL)
Py_FatalError("take_gil: NULL tstate");
err = errno;
MUTEX_LOCK(_PyRuntime.ceval.gil.mutex);
if (!_Py_atomic_load_relaxed(&_PyRuntime.ceval.gil.locked))
goto _ready;
while (_Py_atomic_load_relaxed(&_PyRuntime.ceval.gil.locked)) {
int timed_out = 0;
unsigned long saved_switchnum;
saved_switchnum = _PyRuntime.ceval.gil.switch_number;
COND_TIMED_WAIT(_PyRuntime.ceval.gil.cond, _PyRuntime.ceval.gil.mutex,
INTERVAL, timed_out);
/* If we timed out and no switch occurred in the meantime, it is time
to ask the GIL-holding thread to drop it. */
if (timed_out &&
_Py_atomic_load_relaxed(&_PyRuntime.ceval.gil.locked) &&
_PyRuntime.ceval.gil.switch_number == saved_switchnum) {
SET_GIL_DROP_REQUEST();
}
}
_ready:
#ifdef FORCE_SWITCHING
/* This mutex must be taken before modifying
_PyRuntime.ceval.gil.last_holder (see drop_gil()). */
MUTEX_LOCK(_PyRuntime.ceval.gil.switch_mutex);
#endif
/* We now hold the GIL */
_Py_atomic_store_relaxed(&_PyRuntime.ceval.gil.locked, 1);
_Py_ANNOTATE_RWLOCK_ACQUIRED(&_PyRuntime.ceval.gil.locked, /*is_write=*/1);
if (tstate != (PyThreadState*)_Py_atomic_load_relaxed(
&_PyRuntime.ceval.gil.last_holder))
{
_Py_atomic_store_relaxed(&_PyRuntime.ceval.gil.last_holder,
(uintptr_t)tstate);
++_PyRuntime.ceval.gil.switch_number;
}
#ifdef FORCE_SWITCHING
COND_SIGNAL(_PyRuntime.ceval.gil.switch_cond);
MUTEX_UNLOCK(_PyRuntime.ceval.gil.switch_mutex);
#endif
if (_Py_atomic_load_relaxed(&_PyRuntime.ceval.gil_drop_request)) {
RESET_GIL_DROP_REQUEST();
}
if (tstate->async_exc != NULL) {
_PyEval_SignalAsyncExc();
}
MUTEX_UNLOCK(_PyRuntime.ceval.gil.mutex);
errno = err;
}
void _PyEval_SetSwitchInterval(unsigned long microseconds)
{
_PyRuntime.ceval.gil.interval = microseconds;
}
unsigned long _PyEval_GetSwitchInterval()
{
return _PyRuntime.ceval.gil.interval;
}