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partr.c
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// This file is a part of Julia. License is MIT: https://julialang.org/license
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <strings.h>
#include "julia.h"
#include "julia_internal.h"
#include "gc.h"
#include "threading.h"
#ifdef __cplusplus
extern "C" {
#endif
// thread sleep state
// default to DEFAULT_THREAD_SLEEP_THRESHOLD; set via $JULIA_THREAD_SLEEP_THRESHOLD
uint64_t sleep_threshold;
// thread should not be sleeping--it might need to do work.
static const int16_t not_sleeping = 0;
// it is acceptable for the thread to be sleeping.
static const int16_t sleeping = 1;
// invariant: No thread is ever asleep unless sleep_check_state is sleeping (or we have a wakeup signal pending).
// invariant: Any particular thread is not asleep unless that thread's sleep_check_state is sleeping.
// invariant: The transition of a thread state to sleeping must be followed by a check that there wasn't work pending for it.
// information: Observing thread not-sleeping is sufficient to ensure the target thread will subsequently inspect its local queue.
// information: Observing thread is-sleeping says it may be necessary to notify it at least once to wakeup. It may already be awake however for a variety of reasons.
JULIA_DEBUG_SLEEPWAKE(
uint64_t wakeup_enter;
uint64_t wakeup_leave;
uint64_t io_wakeup_enter;
uint64_t io_wakeup_leave;
);
uv_mutex_t *sleep_locks;
uv_cond_t *wake_signals;
JL_DLLEXPORT int jl_set_task_tid(jl_task_t *task, int tid) JL_NOTSAFEPOINT
{
// Try to acquire the lock on this task.
int16_t was = jl_atomic_load_relaxed(&task->tid);
if (was == tid)
return 1;
if (was == -1)
return jl_atomic_cmpswap(&task->tid, &was, tid);
return 0;
}
// GC functions used
extern int jl_gc_mark_queue_obj_explicit(jl_gc_mark_cache_t *gc_cache,
jl_gc_mark_sp_t *sp, jl_value_t *obj) JL_NOTSAFEPOINT;
// multiq
// ---
/* a task heap */
typedef struct taskheap_tag {
uv_mutex_t lock;
jl_task_t **tasks;
_Atomic(int32_t) ntasks;
_Atomic(int16_t) prio;
} taskheap_t;
/* multiqueue parameters */
static const int32_t heap_d = 8;
static const int heap_c = 2;
/* size of each heap */
static const int tasks_per_heap = 65536; // TODO: this should be smaller by default, but growable!
/* the multiqueue's heaps */
static taskheap_t *heaps;
static int32_t heap_p;
/* unbias state for the RNG */
static uint64_t cong_unbias;
static inline void multiq_init(void)
{
heap_p = heap_c * jl_n_threads;
heaps = (taskheap_t *)calloc(heap_p, sizeof(taskheap_t));
for (int32_t i = 0; i < heap_p; ++i) {
uv_mutex_init(&heaps[i].lock);
heaps[i].tasks = (jl_task_t **)calloc(tasks_per_heap, sizeof(jl_task_t*));
jl_atomic_store_relaxed(&heaps[i].ntasks, 0);
jl_atomic_store_relaxed(&heaps[i].prio, INT16_MAX);
}
unbias_cong(heap_p, &cong_unbias);
}
static inline void sift_up(taskheap_t *heap, int32_t idx)
{
if (idx > 0) {
int32_t parent = (idx-1)/heap_d;
if (heap->tasks[idx]->prio < heap->tasks[parent]->prio) {
jl_task_t *t = heap->tasks[parent];
heap->tasks[parent] = heap->tasks[idx];
heap->tasks[idx] = t;
sift_up(heap, parent);
}
}
}
static inline void sift_down(taskheap_t *heap, int32_t idx)
{
if (idx < jl_atomic_load_relaxed(&heap->ntasks)) {
for (int32_t child = heap_d*idx + 1;
child < tasks_per_heap && child <= heap_d*idx + heap_d;
++child) {
if (heap->tasks[child]
&& heap->tasks[child]->prio < heap->tasks[idx]->prio) {
jl_task_t *t = heap->tasks[idx];
heap->tasks[idx] = heap->tasks[child];
heap->tasks[child] = t;
sift_down(heap, child);
}
}
}
}
static inline int multiq_insert(jl_task_t *task, int16_t priority)
{
jl_ptls_t ptls = jl_current_task->ptls;
uint64_t rn;
task->prio = priority;
do {
rn = cong(heap_p, cong_unbias, &ptls->rngseed);
} while (uv_mutex_trylock(&heaps[rn].lock) != 0);
if (jl_atomic_load_relaxed(&heaps[rn].ntasks) >= tasks_per_heap) {
uv_mutex_unlock(&heaps[rn].lock);
// multiq insertion failed, increase #tasks per heap
return -1;
}
int32_t ntasks = jl_atomic_load_relaxed(&heaps[rn].ntasks);
jl_atomic_store_relaxed(&heaps[rn].ntasks, ntasks + 1);
heaps[rn].tasks[ntasks] = task;
sift_up(&heaps[rn], ntasks);
int16_t prio = jl_atomic_load_relaxed(&heaps[rn].prio);
if (task->prio < prio)
jl_atomic_store_relaxed(&heaps[rn].prio, task->prio);
uv_mutex_unlock(&heaps[rn].lock);
return 0;
}
static inline jl_task_t *multiq_deletemin(void)
{
jl_ptls_t ptls = jl_current_task->ptls;
uint64_t rn1 = 0, rn2;
int32_t i;
int16_t prio1, prio2;
jl_task_t *task;
retry:
jl_gc_safepoint();
for (i = 0; i < heap_p; ++i) {
rn1 = cong(heap_p, cong_unbias, &ptls->rngseed);
rn2 = cong(heap_p, cong_unbias, &ptls->rngseed);
prio1 = jl_atomic_load_relaxed(&heaps[rn1].prio);
prio2 = jl_atomic_load_relaxed(&heaps[rn2].prio);
if (prio1 > prio2) {
prio1 = prio2;
rn1 = rn2;
}
else if (prio1 == prio2 && prio1 == INT16_MAX)
continue;
if (uv_mutex_trylock(&heaps[rn1].lock) == 0) {
if (prio1 == jl_atomic_load_relaxed(&heaps[rn1].prio))
break;
uv_mutex_unlock(&heaps[rn1].lock);
}
}
if (i == heap_p)
return NULL;
task = heaps[rn1].tasks[0];
if (!jl_set_task_tid(task, ptls->tid)) {
uv_mutex_unlock(&heaps[rn1].lock);
goto retry;
}
int32_t ntasks = jl_atomic_load_relaxed(&heaps[rn1].ntasks) - 1;
jl_atomic_store_relaxed(&heaps[rn1].ntasks, ntasks);
heaps[rn1].tasks[0] = heaps[rn1].tasks[ntasks];
heaps[rn1].tasks[ntasks] = NULL;
prio1 = INT16_MAX;
if (ntasks > 0) {
sift_down(&heaps[rn1], 0);
prio1 = heaps[rn1].tasks[0]->prio;
}
jl_atomic_store_relaxed(&heaps[rn1].prio, prio1);
uv_mutex_unlock(&heaps[rn1].lock);
return task;
}
void jl_gc_mark_enqueued_tasks(jl_gc_mark_cache_t *gc_cache, jl_gc_mark_sp_t *sp)
{
int32_t i, j;
for (i = 0; i < heap_p; ++i)
for (j = 0; j < jl_atomic_load_relaxed(&heaps[i].ntasks); ++j)
jl_gc_mark_queue_obj_explicit(gc_cache, sp, (jl_value_t *)heaps[i].tasks[j]);
}
static int multiq_check_empty(void)
{
int32_t i;
for (i = 0; i < heap_p; ++i) {
if (jl_atomic_load_relaxed(&heaps[i].ntasks) != 0)
return 0;
}
return 1;
}
// parallel task runtime
// ---
// initialize the threading infrastructure
// (used only by the main thread)
void jl_init_threadinginfra(void)
{
/* initialize the synchronization trees pool and the multiqueue */
multiq_init();
sleep_threshold = DEFAULT_THREAD_SLEEP_THRESHOLD;
char *cp = getenv(THREAD_SLEEP_THRESHOLD_NAME);
if (cp) {
if (!strncasecmp(cp, "infinite", 8))
sleep_threshold = UINT64_MAX;
else
sleep_threshold = (uint64_t)strtol(cp, NULL, 10);
}
jl_ptls_t ptls = jl_current_task->ptls;
jl_install_thread_signal_handler(ptls);
int16_t tid;
sleep_locks = (uv_mutex_t*)calloc(jl_n_threads, sizeof(uv_mutex_t));
wake_signals = (uv_cond_t*)calloc(jl_n_threads, sizeof(uv_cond_t));
for (tid = 0; tid < jl_n_threads; tid++) {
uv_mutex_init(&sleep_locks[tid]);
uv_cond_init(&wake_signals[tid]);
}
}
void JL_NORETURN jl_finish_task(jl_task_t *t);
// thread function: used by all except the main thread
void jl_threadfun(void *arg)
{
jl_threadarg_t *targ = (jl_threadarg_t*)arg;
// initialize this thread (set tid, create heap, set up root task)
jl_ptls_t ptls = jl_init_threadtls(targ->tid);
void *stack_lo, *stack_hi;
jl_init_stack_limits(0, &stack_lo, &stack_hi);
// warning: this changes `jl_current_task`, so be careful not to call that from this function
jl_task_t *ct = jl_init_root_task(ptls, stack_lo, stack_hi);
JL_GC_PROMISE_ROOTED(ct);
jl_install_thread_signal_handler(ptls);
// wait for all threads
jl_gc_state_set(ptls, JL_GC_STATE_SAFE, 0);
uv_barrier_wait(targ->barrier);
// free the thread argument here
free(targ);
(void)jl_gc_unsafe_enter(ptls);
jl_finish_task(ct); // noreturn
}
// enqueue the specified task for execution
JL_DLLEXPORT int jl_enqueue_task(jl_task_t *task)
{
if (multiq_insert(task, task->prio) == -1)
return 1;
return 0;
}
int jl_running_under_rr(int recheck)
{
#ifdef _OS_LINUX_
#define RR_CALL_BASE 1000
#define SYS_rrcall_check_presence (RR_CALL_BASE + 8)
static _Atomic(int) is_running_under_rr = 0;
int rr = jl_atomic_load_relaxed(&is_running_under_rr);
if (rr == 0 || recheck) {
int ret = syscall(SYS_rrcall_check_presence, 0, 0, 0, 0, 0, 0);
if (ret == -1)
// Should always be ENOSYS, but who knows what people do for
// unknown syscalls with their seccomp filters, so just say
// that we don't have rr.
rr = 2;
else
rr = 1;
jl_atomic_store_relaxed(&is_running_under_rr, rr);
}
return rr == 1;
#else
return 0;
#endif
}
// sleep_check_after_threshold() -- if sleep_threshold ns have passed, return 1
static int sleep_check_after_threshold(uint64_t *start_cycles)
{
JULIA_DEBUG_SLEEPWAKE( return 1 ); // hammer on the sleep/wake logic much harder
/**
* This wait loop is a bit of a worst case for rr - it needs timer access,
* which are slow and it busy loops in user space, which prevents the
* scheduling logic from switching to other threads. Just don't bother
* trying to wait here
*/
if (jl_running_under_rr(0))
return 1;
if (!(*start_cycles)) {
*start_cycles = jl_hrtime();
return 0;
}
uint64_t elapsed_cycles = jl_hrtime() - (*start_cycles);
if (elapsed_cycles >= sleep_threshold) {
*start_cycles = 0;
return 1;
}
return 0;
}
static void wake_thread(int16_t tid)
{
jl_ptls_t other = jl_all_tls_states[tid];
int8_t state = sleeping;
jl_atomic_cmpswap(&other->sleep_check_state, &state, not_sleeping);
if (state == sleeping) {
uv_mutex_lock(&sleep_locks[tid]);
uv_cond_signal(&wake_signals[tid]);
uv_mutex_unlock(&sleep_locks[tid]);
}
}
static void wake_libuv(void)
{
JULIA_DEBUG_SLEEPWAKE( io_wakeup_enter = cycleclock() );
jl_wake_libuv();
JULIA_DEBUG_SLEEPWAKE( io_wakeup_leave = cycleclock() );
}
/* ensure thread tid is awake if necessary */
JL_DLLEXPORT void jl_wakeup_thread(int16_t tid)
{
jl_task_t *ct = jl_current_task;
jl_ptls_t ptls = ct->ptls;
jl_task_t *uvlock = jl_atomic_load(&jl_uv_mutex.owner);
int16_t self = jl_atomic_load_relaxed(&ct->tid);
JULIA_DEBUG_SLEEPWAKE( wakeup_enter = cycleclock() );
if (tid == self || tid == -1) {
// we're already awake, but make sure we'll exit uv_run
if (jl_atomic_load_relaxed(&ptls->sleep_check_state) == sleeping)
jl_atomic_store(&ptls->sleep_check_state, not_sleeping);
if (uvlock == ct)
uv_stop(jl_global_event_loop());
}
else {
// something added to the sticky-queue: notify that thread
wake_thread(tid);
// check if we need to notify uv_run too
jl_task_t *system_tid = jl_atomic_load_relaxed(&jl_all_tls_states[tid]->current_task);
if (uvlock != ct && jl_atomic_load(&jl_uv_mutex.owner) == system_tid)
wake_libuv();
}
// check if the other threads might be sleeping
if (tid == -1) {
// something added to the multi-queue: notify all threads
// in the future, we might want to instead wake some fraction of threads,
// and let each of those wake additional threads if they find work
for (tid = 0; tid < jl_n_threads; tid++) {
if (tid != self)
wake_thread(tid);
}
// check if we need to notify uv_run too
if (uvlock != ct && jl_atomic_load(&jl_uv_mutex.owner) != NULL)
wake_libuv();
}
JULIA_DEBUG_SLEEPWAKE( wakeup_leave = cycleclock() );
}
// get the next runnable task from the multiq
static jl_task_t *get_next_task(jl_value_t *trypoptask, jl_value_t *q)
{
jl_gc_safepoint();
jl_value_t *args[2] = { trypoptask, q };
jl_task_t *task = (jl_task_t*)jl_apply(args, 2);
if (jl_typeis(task, jl_task_type)) {
int self = jl_atomic_load_relaxed(&jl_current_task->tid);
jl_set_task_tid(task, self);
return task;
}
return multiq_deletemin();
}
static int may_sleep(jl_ptls_t ptls) JL_NOTSAFEPOINT
{
// sleep_check_state is only transitioned from not_sleeping to sleeping
// by the thread itself. As a result, if this returns false, it will
// continue returning false. If it returns true, there are no guarantees.
return jl_atomic_load_relaxed(&ptls->sleep_check_state) == sleeping;
}
extern _Atomic(unsigned) _threadedregion;
JL_DLLEXPORT jl_task_t *jl_task_get_next(jl_value_t *trypoptask, jl_value_t *q)
{
jl_task_t *ct = jl_current_task;
uint64_t start_cycles = 0;
while (1) {
jl_task_t *task = get_next_task(trypoptask, q);
if (task)
return task;
// quick, race-y check to see if there seems to be any stuff in there
jl_cpu_pause();
if (!multiq_check_empty()) {
start_cycles = 0;
continue;
}
jl_cpu_pause();
jl_ptls_t ptls = ct->ptls;
if (sleep_check_after_threshold(&start_cycles) || (!jl_atomic_load_relaxed(&_threadedregion) && ptls->tid == 0)) {
jl_atomic_store(&ptls->sleep_check_state, sleeping); // acquire sleep-check lock
if (!multiq_check_empty()) {
if (jl_atomic_load_relaxed(&ptls->sleep_check_state) != not_sleeping)
jl_atomic_store(&ptls->sleep_check_state, not_sleeping); // let other threads know they don't need to wake us
continue;
}
task = get_next_task(trypoptask, q); // WARNING: this should not yield
if (ptls != ct->ptls)
continue; // oops, get_next_task did yield--start over
if (task) {
if (jl_atomic_load_relaxed(&ptls->sleep_check_state) != not_sleeping)
jl_atomic_store(&ptls->sleep_check_state, not_sleeping); // let other threads know they don't need to wake us
return task;
}
// one thread should win this race and watch the event loop
// inside a threaded region, any thread can listen for IO messages,
// although none are allowed to create new ones
// outside of threaded regions, all IO is permitted,
// but only on thread 1
int uvlock = 0;
if (jl_atomic_load_relaxed(&_threadedregion)) {
uvlock = jl_mutex_trylock(&jl_uv_mutex);
}
else if (ptls->tid == 0) {
uvlock = 1;
JL_UV_LOCK(); // jl_mutex_lock(&jl_uv_mutex);
}
if (uvlock) {
int active = 1;
if (jl_atomic_load(&jl_uv_n_waiters) != 0) {
// but if we won the race against someone who actually needs
// the lock to do real work, we need to let them have it instead
JL_UV_UNLOCK();
}
else {
// otherwise, we may block until someone asks us for the lock
uv_loop_t *loop = jl_global_event_loop();
jl_gc_safepoint();
if (may_sleep(ptls)) {
loop->stop_flag = 0;
JULIA_DEBUG_SLEEPWAKE( ptls->uv_run_enter = cycleclock() );
active = uv_run(loop, UV_RUN_ONCE);
JULIA_DEBUG_SLEEPWAKE( ptls->uv_run_leave = cycleclock() );
}
JL_UV_UNLOCK();
// optimization: check again first if we may have work to do
if (!may_sleep(ptls)) {
assert(jl_atomic_load_relaxed(&ptls->sleep_check_state) == not_sleeping);
start_cycles = 0;
continue;
}
// otherwise, we got a spurious wakeup since some other
// thread that just wanted to steal libuv from us,
// just go right back to sleep on the other wake signal
// to let them take it from us without conflict
// TODO: this relinquishes responsibility for all event
// to the last thread to do an explicit operation,
// which may starve other threads of critical work
if (jl_atomic_load(&jl_uv_n_waiters) == 0) {
continue;
}
}
if (!jl_atomic_load_relaxed(&_threadedregion) && active && ptls->tid == 0) {
// thread 0 is the only thread permitted to run the event loop
// so it needs to stay alive
if (jl_atomic_load_relaxed(&ptls->sleep_check_state) != not_sleeping)
jl_atomic_store(&ptls->sleep_check_state, not_sleeping); // let other threads know they don't need to wake us
start_cycles = 0;
continue;
}
}
// the other threads will just wait for on signal to resume
JULIA_DEBUG_SLEEPWAKE( ptls->sleep_enter = cycleclock() );
int8_t gc_state = jl_gc_safe_enter(ptls);
uv_mutex_lock(&sleep_locks[ptls->tid]);
while (may_sleep(ptls)) {
uv_cond_wait(&wake_signals[ptls->tid], &sleep_locks[ptls->tid]);
// TODO: help with gc work here, if applicable
}
assert(jl_atomic_load_relaxed(&ptls->sleep_check_state) == not_sleeping);
uv_mutex_unlock(&sleep_locks[ptls->tid]);
JULIA_DEBUG_SLEEPWAKE( ptls->sleep_leave = cycleclock() );
jl_gc_safe_leave(ptls, gc_state); // contains jl_gc_safepoint
start_cycles = 0;
}
else {
// maybe check the kernel for new messages too
jl_process_events();
}
}
}
#ifdef __cplusplus
}
#endif