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Scheduler: Use the root task as a scheduler task (alternative 1) #57543

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Scheduler: Use the root task as a scheduler task (alternative 1) #57543

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3 changes: 3 additions & 0 deletions src/julia_threads.h
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Original file line number Diff line number Diff line change
Expand Up @@ -167,6 +167,9 @@ typedef struct _jl_tls_states_t {
struct _jl_task_t *next_task;
struct _jl_task_t *previous_task;
struct _jl_task_t *root_task;
// The scheduler sets this to instruct the task switching code (jl_switch())
// to wait in the scheduler for another task.
int8_t wait_in_scheduler;
struct _jl_timing_block_t *timing_stack;
// This is the location of our copy_stack
void *stackbase;
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13 changes: 12 additions & 1 deletion src/scheduler.c
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Expand Up @@ -378,7 +378,18 @@ JL_DLLEXPORT jl_task_t *jl_task_get_next(jl_value_t *trypoptask, jl_value_t *q,

jl_cpu_pause();
jl_ptls_t ptls = ct->ptls;
if (sleep_check_after_threshold(&start_cycles) || (ptls->tid == jl_atomic_load_relaxed(&io_loop_tid) && (!jl_atomic_load_relaxed(&_threadedregion) || wait_empty))) {
if (sleep_check_after_threshold(&start_cycles) ||
(ptls->tid == jl_atomic_load_relaxed(&io_loop_tid) &&
(!jl_atomic_load_relaxed(&_threadedregion) || wait_empty))) {
// The place seems empty and this thread is on its way to sleeping;
// switch to the root task to do that and instruct the task switching
// code to wait for another task.
if (ct != ptls->root_task) {
ptls->wait_in_scheduler = 1;
jl_switchto(&ptls->root_task);
return ct;
}

// acquire sleep-check lock
assert(jl_atomic_load_relaxed(&ptls->sleep_check_state) == not_sleeping);
jl_atomic_store_relaxed(&ptls->sleep_check_state, sleeping);
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16 changes: 15 additions & 1 deletion src/task.c
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Expand Up @@ -427,6 +427,12 @@ JL_DLLEXPORT jl_task_t *jl_get_next_task(void) JL_NOTSAFEPOINT
const char tsan_state_corruption[] = "TSAN state corrupted. Exiting HARD!\n";
#endif

// is `t` the root task?
static int is_root_task(jl_task_t *ct, jl_task_t *t)
{
return ct->ptls->root_task == t;
}

JL_NO_ASAN static void ctx_switch(jl_task_t *lastt)
{
jl_ptls_t ptls = lastt->ptls;
Expand All @@ -446,7 +452,8 @@ JL_NO_ASAN static void ctx_switch(jl_task_t *lastt)
}
#endif

int killed = jl_atomic_load_relaxed(&lastt->_state) != JL_TASK_STATE_RUNNABLE;
int killed = (jl_atomic_load_relaxed(&lastt->_state) != JL_TASK_STATE_RUNNABLE) &&
!is_root_task(t, lastt);
if (!t->ctx.started && !t->ctx.copy_stack) {
// may need to allocate the stack
if (t->ctx.stkbuf == NULL) {
Expand Down Expand Up @@ -663,6 +670,7 @@ JL_DLLEXPORT void jl_switch(void) JL_NOTSAFEPOINT_LEAVE JL_NOTSAFEPOINT_ENTER
{
jl_task_t *ct = jl_current_task;
jl_ptls_t ptls = ct->ptls;
switch_restart:
jl_task_t *t = ptls->next_task;
if (t == ct) {
return;
Expand Down Expand Up @@ -711,6 +719,12 @@ JL_DLLEXPORT void jl_switch(void) JL_NOTSAFEPOINT_LEAVE JL_NOTSAFEPOINT_ENTER
if (other_defer_signal && !defer_signal)
jl_sigint_safepoint(ptls);

if (ptls->wait_in_scheduler) {
ptls->wait_in_scheduler = 0;
jl_set_next_task(jl_task_get_next());
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@vtjnash: how would we get the various arguments needed for jl_task_get_next here?

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That seems quite tedious. I think #57544 is probably the better direction to go

goto switch_restart;
}

JL_PROBE_RT_RUN_TASK(ct);
jl_gc_unsafe_leave(ptls, gc_state);
}
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