[PATCH 4/5] ntdll: Implement NtYieldExecution() as usleep().

[Matteo Bruni]

Signed-off-by: Matteo Bruni <mbruni at codeweavers.com>
---
sched_yield() causes the thread to relinquish the CPU, but only to a
thread at the same priority level. If there is no other runnable
thread with the same priority, the caller gets rescheduled right away.
A number of games (Starcraft 2 and Shadow of the Tomb Raider are two
that I have explicitly mentioned in my notes) in particular call
Sleep(0) or NtYieldExecution() while on the main menu, with the idea
of sleeping for a while before getting rescheduled. sched_yield()
doesn't do what we want. usleep() does.
FWIW the man page for sched_yield() does mention that it's intended to
be used with realtime scheduling policies and calling it with
processes using the default SCHED_OTHER policy (like Wine) is a bad
idea.

Windows doesn't give any particular guarantee about minimal sleep
duration. MSDN just says that it yields the CPU for "the remainder of
its time slice". Also Sleep(0) changed a bit after Windows XP, going
from relinquishing the time slice to "other threads of equal priority"
to "any other thread", so I think this change makes the function
follow more closely current Windows behavior regardless.

 configure              | 1 -
 configure.ac           | 1 -
 dlls/ntdll/unix/sync.c | 6 +-----
 include/config.h.in    | 3 ---
 4 files changed, 1 insertion(+), 10 deletions(-)

diff --git a/configure b/configure
index db2e07679d4..096d750c9cc 100755
--- a/configure
+++ b/configure
@@ -17916,7 +17916,6 @@ for ac_func in \
 	prctl \
 	proc_pidinfo \
 	readlink \
-	sched_yield \
 	setproctitle \
 	setprogname \
 	sigprocmask \
diff --git a/configure.ac b/configure.ac
index 6321206b9db..ec65c6abc59 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2147,7 +2147,6 @@ AC_CHECK_FUNCS(\
 	prctl \
 	proc_pidinfo \
 	readlink \
-	sched_yield \
 	setproctitle \
 	setprogname \
 	sigprocmask \
diff --git a/dlls/ntdll/unix/sync.c b/dlls/ntdll/unix/sync.c
index 5a5d49bf4bd..1a8c386d3a8 100644
--- a/dlls/ntdll/unix/sync.c
+++ b/dlls/ntdll/unix/sync.c
@@ -1506,12 +1506,8 @@ NTSTATUS WINAPI NtSignalAndWaitForSingleObject( HANDLE signal, HANDLE wait,
  */
 NTSTATUS WINAPI NtYieldExecution(void)
 {
-#ifdef HAVE_SCHED_YIELD
-    sched_yield();
+    usleep(0);
     return STATUS_SUCCESS;
-#else
-    return STATUS_NO_YIELD_PERFORMED;
-#endif
 }
 
 
diff --git a/include/config.h.in b/include/config.h.in
index e27b23f658f..1cffe6ebac9 100644
--- a/include/config.h.in
+++ b/include/config.h.in
@@ -537,9 +537,6 @@
 /* Define to 1 if you have the `sched_setaffinity' function. */
 #undef HAVE_SCHED_SETAFFINITY
 
-/* Define to 1 if you have the `sched_yield' function. */
-#undef HAVE_SCHED_YIELD
-
 /* Define to 1 if `cmd' is a member of `scsireq_t'. */
 #undef HAVE_SCSIREQ_T_CMD
 
-- 
2.26.3