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>

6 files changed, 13 insertions, 2 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()
diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index f363bc8fdc74..94b48bd40dd7 100644
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -160,4 +160,8 @@ extern int mutex_trylock(struct mutex *lock);
 extern void mutex_unlock(struct mutex *lock);
 extern int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);
 
+#ifndef CONFIG_HAVE_ARCH_MUTEX_CPU_RELAX
+#define arch_mutex_cpu_relax()  cpu_relax()
+#endif
+
 #endif
diff --git a/kernel/mutex.c b/kernel/mutex.c
index 200407c1502f..a5889fb28ecf 100644
--- a/kernel/mutex.c
+++ b/kernel/mutex.c
@@ -199,7 +199,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
                  * memory barriers as we'll eventually observe the right
                  * values at the cost of a few extra spins.
                  */
-                cpu_relax();
+                arch_mutex_cpu_relax();
         }
 #endif
         spin_lock_mutex(&lock->wait_lock, flags);
diff --git a/kernel/sched.c b/kernel/sched.c
index 3e8a7db951a6..abe7aec55763 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -75,6 +75,7 @@
 
 #include <asm/tlb.h>
 #include <asm/irq_regs.h>
+#include <asm/mutex.h>
 
 #include "sched_cpupri.h"
 #include "workqueue_sched.h"
@@ -3888,7 +3889,7 @@ int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner)
                 if (task_thread_info(rq->curr) != owner || need_resched())
                         return 0;
 
-                cpu_relax();
+                arch_mutex_cpu_relax();
         }
 
         return 1;