Re: [PATCH] mutex: have non-spinning mutexes on s390 by default

[David Miller <davem@davemloft.net>]

From: Heiko Carstens <heiko.carstens@de.ibm.com>
Date: Mon, 20 Apr 2009 14:01:03 +0200

> On Fri, 17 Apr 2009 23:42:12 +0200
> Folkert van Heusden <folkert@vanheusden.com> wrote:
> 
>> > 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.
>> 
>> Hmmm. Wouldn't this be a change that applies to ibm system p as wel?
>> E.g. with linux in an lpar.
> 
> This applies to every kernel that runs under a hypervisor.

I wouldn't say "every" because this change is definitely not
appropriate on sparc64 hypervisor based systems.