Commit 36cd3c9f authored Apr 09, 2009 by Heiko Carstens Committed by Ingo Molnar Apr 09, 2009

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>

parent 022624a7

arch/Kconfig

+3 −0

Original line number	Diff line number	Diff line
		@@ -109,3 +109,6 @@ config HAVE_CLK

		config HAVE_DMA_API_DEBUG
		bool

		config HAVE_DEFAULT_NO_SPIN_MUTEXES
		bool

arch/s390/Kconfig

+1 −0

Original line number	Diff line number	Diff line
		@@ -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

kernel/mutex.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -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.
		*

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