Add JULIA_THREAD_SLEEP_THRESHOLD environment variable (JuliaLang#42981)

This patch re-introduces the code originally added in
4752908 and was removed in
93e3d28.

src/partr.c

@@ -17,6 +17,9 @@ extern "C" {
 
 // 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;
 
@@ -234,6 +237,15 @@ 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);
  uv_mutex_init(&ptls->sleep_lock);
@@ -324,7 +336,7 @@ static int sleep_check_after_threshold(uint64_t *start_cycles)
  return 0;
  }
  uint64_t elapsed_cycles = jl_hrtime() - (*start_cycles);
- if (elapsed_cycles >= DEFAULT_THREAD_SLEEP_THRESHOLD) {
+ if (elapsed_cycles >= sleep_threshold) {
  *start_cycles = 0;
  return 1;
  }