[OpenMP] Make use of sched_yield optional in runtime

This patch cleans up the yielding code and makes it optional. An
environment variable, KMP_USE_YIELD, was added. Yielding is still
on by default (KMP_USE_YIELD=1), but can be turned off completely
(KMP_USE_YIELD=0), or turned on only when oversubscription is detected
(KMP_USE_YIELD=2). Note that oversubscription cannot always be detected
by the runtime (for example, when the runtime is initialized and the
process forks, oversubscription cannot be detected currently over
multiple instances of the runtime).

Because yielding can be controlled by user now, the library mode
settings (from KMP_LIBRARY) for throughput and turnaround have been
adjusted by altering blocktime, unless that was also explicitly set.

In the original code, there were a number of places where a double yield
might have been done under oversubscription. This version checks
oversubscription and if that's not going to yield, then it does
the spin check.

Patch by Terry Wilmarth

Differential Revision: https://reviews.llvm.org/D58148

llvm-svn: 355120

1 files changed, 4 insertions, 40 deletions

diff --git a/openmp/runtime/src/z_Linux_util.cpp b/openmp/runtime/src/z_Linux_util.cpp
index 08b9742980c..df1c47b57e1 100644
--- a/openmp/runtime/src/z_Linux_util.cpp
+++ b/openmp/runtime/src/z_Linux_util.cpp
@@ -437,7 +437,7 @@ void __kmp_terminate_thread(int gtid) {
                 __kmp_msg_null);
   }
 #endif
-  __kmp_yield(TRUE);
+  KMP_YIELD(TRUE);
 } //
 
 /* Set thread stack info according to values returned by pthread_getattr_np().
@@ -580,8 +580,6 @@ static void *__kmp_launch_monitor(void *thr) {
   sigset_t new_set;
 #endif /* KMP_BLOCK_SIGNALS */
   struct timespec interval;
-  int yield_count;
-  int yield_cycles = 0;
 
   KMP_MB(); /* Flush all pending memory write invalidates.  */
 
@@ -665,13 +663,6 @@ static void *__kmp_launch_monitor(void *thr) {
 
   KA_TRACE(10, ("__kmp_launch_monitor: #2 monitor\n"));
 
-  if (__kmp_yield_cycle) {
-    __kmp_yielding_on = 0; /* Start out with yielding shut off */
-    yield_count = __kmp_yield_off_count;
-  } else {
-    __kmp_yielding_on = 1; /* Yielding is on permanently */
-  }
-
   while (!TCR_4(__kmp_global.g.g_done)) {
     struct timespec now;
     struct timeval tval;
@@ -707,22 +698,6 @@ static void *__kmp_launch_monitor(void *thr) {
     status = pthread_mutex_unlock(&__kmp_wait_mx.m_mutex);
     KMP_CHECK_SYSFAIL("pthread_mutex_unlock", status);
 
-    if (__kmp_yield_cycle) {
-      yield_cycles++;
-      if ((yield_cycles % yield_count) == 0) {
-        if (__kmp_yielding_on) {
-          __kmp_yielding_on = 0; /* Turn it off now */
-          yield_count = __kmp_yield_off_count;
-        } else {
-          __kmp_yielding_on = 1; /* Turn it on now */
-          yield_count = __kmp_yield_on_count;
-        }
-        yield_cycles = 0;
-      }
-    } else {
-      __kmp_yielding_on = 1;
-    }
-
     TCW_4(__kmp_global.g.g_time.dt.t_value,
           TCR_4(__kmp_global.g.g_time.dt.t_value) + 1);
 
@@ -1011,8 +986,8 @@ retry:
   // Wait for the monitor thread is really started and set its *priority*.
   KMP_DEBUG_ASSERT(sizeof(kmp_uint32) ==
                    sizeof(__kmp_global.g.g_time.dt.t_value));
-  __kmp_wait_yield_4((kmp_uint32 volatile *)&__kmp_global.g.g_time.dt.t_value,
-                     -1, &__kmp_neq_4, NULL);
+  __kmp_wait_4((kmp_uint32 volatile *)&__kmp_global.g.g_time.dt.t_value, -1,
+               &__kmp_neq_4, NULL);
 #endif // KMP_REAL_TIME_FIX
 
 #ifdef KMP_THREAD_ATTR
@@ -1688,18 +1663,7 @@ void __kmp_resume_monitor() {
 }
 #endif // KMP_USE_MONITOR
 
-void __kmp_yield(int cond) {
-  if (!cond)
-    return;
-#if KMP_USE_MONITOR
-  if (!__kmp_yielding_on)
-    return;
-#else
-  if (__kmp_yield_cycle && !KMP_YIELD_NOW())
-    return;
-#endif
-  sched_yield();
-}
+void __kmp_yield() { sched_yield(); }
 
 void __kmp_gtid_set_specific(int gtid) {
   if (__kmp_init_gtid) {