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    SubjectRe: [PATCH] mutex: have non-spinning mutexes on s390 by default
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    On Thu, 2009-04-09 at 17:47 +0200, Heiko Carstens wrote:
    > From: Heiko Carstens <heiko.carstens@de.ibm.com>
    >
    > 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.
    > Until then make the old behaviour default for us.
    >
    > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    > Cc: Ingo Molnar <mingo@elte.hu>
    > Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
    > ---
    > arch/Kconfig | 3 +++
    > arch/s390/Kconfig | 1 +
    > kernel/sched_features.h | 4 ++++
    > 3 files changed, 8 insertions(+)
    >
    > Index: linux-2.6/arch/Kconfig
    > ===================================================================
    > --- linux-2.6.orig/arch/Kconfig
    > +++ linux-2.6/arch/Kconfig
    > @@ -109,3 +109,6 @@ config HAVE_CLK
    >
    > config HAVE_DMA_API_DEBUG
    > bool
    > +
    > +config HAVE_DEFAULT_NO_SPIN_MUTEXES
    > + bool
    > Index: linux-2.6/arch/s390/Kconfig
    > ===================================================================
    > --- linux-2.6.orig/arch/s390/Kconfig
    > +++ linux-2.6/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
    > Index: linux-2.6/kernel/sched_features.h
    > ===================================================================
    > --- linux-2.6.orig/kernel/sched_features.h
    > +++ linux-2.6/kernel/sched_features.h
    > @@ -14,4 +14,8 @@ SCHED_FEAT(LB_WAKEUP_UPDATE, 1)
    > SCHED_FEAT(ASYM_EFF_LOAD, 1)
    > SCHED_FEAT(WAKEUP_OVERLAP, 0)
    > SCHED_FEAT(LAST_BUDDY, 1)
    > +#ifdef CONFIG_HAVE_DEFAULT_NO_SPIN_MUTEXES
    > +SCHED_FEAT(OWNER_SPIN, 0)
    > +#else
    > SCHED_FEAT(OWNER_SPIN, 1)
    > +#endif

    Hmm, I'd rather have you'd make the whole block in __mutex_lock_common
    go away on that CONFIG thingy.

    Would be nice though to get something working on s390, does it have a
    monitor wait like ins where it can properly sleep so that another
    virtual host can run?

    If so, we could possibly do a monitor wait on the lock owner field
    instead of spinning.


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