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 dc81b34c5d82e72c574217d4b63bfda319e14530..78a35e9dc10407e24eeeabe6f2331a9ff24b64b7 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 dcb667c4375af26cb873710fde1bd0508ee18b35..2eca5fe0e75b243a71b3f0450ac756d9dea2a1f9 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 5d79781394a306e75048668262a6c3f49f5321ef..507cf2b5e9f1e6328b2e335a3a05c0ead6a0ff3e 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.
         *