mutexes, sched: Introduce arch_mutex_cpu_relax()

The spinning mutex implementation uses cpu_relax() in busy loops as a
compiler barrier. Depending on the architecture, cpu_relax() may do more
than needed in this specific mutex spin loops. On System z we also give
up the time slice of the virtual cpu in cpu_relax(), which prevents
effective spinning on the mutex.

This patch replaces cpu_relax() in the spinning mutex code with
arch_mutex_cpu_relax(), which can be defined by each architecture that
selects HAVE_ARCH_MUTEX_CPU_RELAX. The default is still cpu_relax(), so
this patch should not affect other architectures than System z for now.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1290437256.7455.4.camel@thinkpad>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

3 files changed, 6 insertions, 0 deletions

diff --git a/arch/Kconfig b/arch/Kconfig
index 8bf0fa652eb6..f78c2be4242b 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -175,4 +175,7 @@ config HAVE_PERF_EVENTS_NMI
 config HAVE_ARCH_JUMP_LABEL
         bool
 
+config HAVE_ARCH_MUTEX_CPU_RELAX
+        bool
+
 source "kernel/gcov/Kconfig"
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index e0b98e71ff47..6c6d7b339aae 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -99,6 +99,7 @@ config S390
         select HAVE_KERNEL_LZMA
         select HAVE_KERNEL_LZO
         select HAVE_GET_USER_PAGES_FAST
+        select HAVE_ARCH_MUTEX_CPU_RELAX
         select ARCH_INLINE_SPIN_TRYLOCK
         select ARCH_INLINE_SPIN_TRYLOCK_BH
         select ARCH_INLINE_SPIN_LOCK
diff --git a/arch/s390/include/asm/mutex.h b/arch/s390/include/asm/mutex.h
index 458c1f7fbc18..688271f5f2e4 100644
--- a/arch/s390/include/asm/mutex.h
+++ b/arch/s390/include/asm/mutex.h
@@ -7,3 +7,5 @@
  */
 
 #include <asm-generic/mutex-dec.h>
+
+#define arch_mutex_cpu_relax()  barrier()