mutex: have non-spinning mutexes on s390 by default

Impact: performance regression fix for s390

The adaptive spinning mutexes will not always do what one would expect on
virtualized architectures like s390. Especially the cpu_relax() loop in
mutex_spin_on_owner might hurt if the mutex holding cpu has been scheduled
away by the hypervisor.

We would end up in a cpu_relax() loop when there is no chance that the
state of the mutex changes until the target cpu has been scheduled again by
the hypervisor.

For that reason we should change the default behaviour to no-spin on s390.

We do have an instruction which allows to yield the current cpu in favour of
a different target cpu. Also we have an instruction which allows us to figure
out if the target cpu is physically backed.

However we need to do some performance tests until we can come up with
a solution that will do the right thing on s390.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
LKML-Reference: <20090409184834.7a0df7b2@osiris.boeblingen.de.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/arch/Kconfig b/arch/Kconfig
index dc81b34..78a35e9 100644 (file)
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -109,3 +109,6 @@ config HAVE_CLK
 
 config HAVE_DMA_API_DEBUG
        bool
+
+config HAVE_DEFAULT_NO_SPIN_MUTEXES
+       bool

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index dcb667c..2eca5fe 100644 (file)
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -82,6 +82,7 @@ config S390
        select USE_GENERIC_SMP_HELPERS if SMP
        select HAVE_SYSCALL_WRAPPERS
        select HAVE_FUNCTION_TRACER
+       select HAVE_DEFAULT_NO_SPIN_MUTEXES
        select HAVE_OPROFILE
        select HAVE_KPROBES
        select HAVE_KRETPROBES

diff --git a/kernel/mutex.c b/kernel/mutex.c
index 5d79781..507cf2b 100644 (file)
--- a/kernel/mutex.c
+++ b/kernel/mutex.c
@@ -148,7 +148,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 
        preempt_disable();
        mutex_acquire(&lock->dep_map, subclass, 0, ip);
-#if defined(CONFIG_SMP) && !defined(CONFIG_DEBUG_MUTEXES)
+#if defined(CONFIG_SMP) && !defined(CONFIG_DEBUG_MUTEXES) && \
+    !defined(CONFIG_HAVE_DEFAULT_NO_SPIN_MUTEXES)
        /*
         * Optimistic spinning.
         *