threads: further optimize scheduler wake-ups

And add some debugging aids, controlled by changing the
`JULIA_DEBUG_SLEEPWAKE()` macro in `julia_threads.h`

Eliminates the `sleep_check_state` global,
equal to `any(ptls->sleep_check_state == true)`

base/task.jl

@@ -708,7 +708,7 @@ function wait()
     W = Workqueues[Threads.threadid()]
     reftask = poptaskref(W)
     result = try_yieldto(ensure_rescheduled, reftask)
-    Sys.isjsvm() || process_events()
+    process_events()
     # return when we come out of the queue
     return result
 end

src/jl_uv.c

@@ -209,7 +209,7 @@ JL_DLLEXPORT int jl_process_events(void)
     uv_loop_t *loop = jl_io_loop;
     if (loop && (_threadedregion || ptls->tid == 0)) {
         jl_gc_safepoint_(ptls);
-        if (jl_mutex_trylock(&jl_uv_mutex)) {
+        if (jl_atomic_load(&jl_uv_n_waiters) == 0 && jl_mutex_trylock(&jl_uv_mutex)) {
             loop->stop_flag = 0;
             int r = uv_run(loop, UV_RUN_NOWAIT);
             JL_UV_UNLOCK();

src/julia_internal.h

@@ -91,6 +91,37 @@ void JL_UV_LOCK(void);
 extern "C" {
 #endif
 
+//--------------------------------------------------
+// timers
+// Returns time in nanosec
+JL_DLLEXPORT uint64_t jl_hrtime(void);
+
+// number of cycles since power-on
+static inline uint64_t cycleclock(void)
+{
+#if defined(_CPU_X86_64_)
+    uint64_t low, high;
+    __asm__ volatile("rdtsc" : "=a"(low), "=d"(high));
+    return (high << 32) | low;
+#elif defined(_CPU_X86_)
+    int64_t ret;
+    __asm__ volatile("rdtsc" : "=A"(ret));
+    return ret;
+#elif defined(_CPU_AARCH64_)
+    // System timer of ARMv8 runs at a different frequency than the CPU's.
+    // The frequency is fixed, typically in the range 1-50MHz.  It can be
+    // read at CNTFRQ special register.  We assume the OS has set up
+    // the virtual timer properly.
+    int64_t virtual_timer_value;
+    __asm__ volatile("mrs %0, cntvct_el0" : "=r"(virtual_timer_value));
+    return virtual_timer_value;
+#else
+    #warning No cycleclock() definition for your platform
+    // copy from https://github.com/google/benchmark/blob/v1.5.0/src/cycleclock.h
+    return 0;
+#endif
+}
+
 #include "timing.h"
 
 #ifdef _COMPILER_MICROSOFT_
@@ -829,11 +860,6 @@ void jl_push_excstack(jl_excstack_t **stack JL_REQUIRE_ROOTED_SLOT JL_ROOTING_AR
                       jl_bt_element_t *bt_data, size_t bt_size);
 void jl_copy_excstack(jl_excstack_t *dest, jl_excstack_t *src) JL_NOTSAFEPOINT;
 
-//--------------------------------------------------
-// timers
-// Returns time in nanosec
-JL_DLLEXPORT uint64_t jl_hrtime(void);
-
 // congruential random number generator
 // for a small amount of thread-local randomness
 // we could just use libc:`rand()`, but we want to ensure this is fast

src/julia_threads.h

@@ -7,9 +7,12 @@
 #include <atomics.h>
 // threading ------------------------------------------------------------------
 
-// WARNING: Threading support is incomplete and experimental
-// Nonetheless, we define JL_THREAD and use it to give advanced notice to
-// maintainers of what eventual threading support will change.
+// JULIA_ENABLE_THREADING may be controlled by altering JULIA_THREADS in Make.user
+
+// When running into scheduler issues, this may help provide information on the
+// sequence of events that led to the issue. Normally, it is empty.
+//#define JULIA_DEBUG_SLEEPWAKE(x) x
+#define JULIA_DEBUG_SLEEPWAKE(x)
 
 //  Options for task switching algorithm (in order of preference):
 // JL_HAVE_ASM -- mostly setjmp
@@ -221,6 +224,13 @@ struct _jl_tls_states_t {
     // Saved exception for previous external API call or NULL if cleared.
     // Access via jl_exception_occurred().
     struct _jl_value_t *previous_exception;
+
+    JULIA_DEBUG_SLEEPWAKE(
+        uint64_t uv_run_enter;
+        uint64_t uv_run_leave;
+        uint64_t sleep_enter;
+        uint64_t sleep_leave;
+    )
 };
 
 // Update codegen version in `ccall.cpp` after changing either `pause` or `wake`