Re: [PATCH] mutex: Introduce mutex_cpu_relax()

[Gerald Schaefer <gerald.schaefer@de.ibm.com>]

On Thu, 2010-10-14 at 15:13 -0700, Andrew Morton wrote:
> On Thu, 14 Oct 2010 19:40:25 +0200
> Gerald Schaefer <gerald.schaefer@de.ibm.com> wrote:
> 
> > From: Gerald Schaefer <gerald.schaefer@de.ibm.com>
> > 
> > The spinning mutex implementation uses cpu_relax() in busy loops as a
> > compiler barrier. Depending on the architecture, cpu_relax() may do more
> > than needed in this specific mutex spin loops. On System z we also give
> > up the time slice of the virtual cpu in cpu_relax(), which prevents
> > effective spinning on the mutex.
> > 
> > This patch replaces cpu_relax() in the spinning mutex code with a new
> > function mutex_cpu_relax(), which can be defined by each architecture
> > that selects HAVE_MUTEX_CPU_RELAX. The default is still cpu_relax(), so
> > this patch should not affect other architectures than System z for now.
> > 
> > Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
> > ---
> >  arch/Kconfig                  |    3 +++
> >  arch/s390/Kconfig             |    1 +
> >  arch/s390/include/asm/mutex.h |    2 ++
> >  include/linux/mutex.h         |    4 ++++
> >  kernel/mutex.c                |    2 +-
> >  kernel/sched.c                |    2 +-
> >  6 files changed, 12 insertions(+), 2 deletions(-)
> > 
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -158,4 +158,7 @@ config HAVE_PERF_EVENTS_NMI
> >  	  subsystem.  Also has support for calculating CPU cycle events
> >  	  to determine how many clock cycles in a given period.
> >  
> > +config HAVE_MUTEX_CPU_RELAX
> > +	bool
> > +
> >  source "kernel/gcov/Kconfig"
> > --- a/arch/s390/Kconfig
> > +++ b/arch/s390/Kconfig
> > @@ -100,6 +100,7 @@ config S390
> >  	select HAVE_KERNEL_BZIP2
> >  	select HAVE_KERNEL_LZMA
> >  	select HAVE_KERNEL_LZO
> > +	select HAVE_MUTEX_CPU_RELAX
> >  	select ARCH_INLINE_SPIN_TRYLOCK
> >  	select ARCH_INLINE_SPIN_TRYLOCK_BH
> >  	select ARCH_INLINE_SPIN_LOCK
> 
> We could just omit the HAVE_MUTEX_CPU_RELAX
> 
> > --- a/arch/s390/include/asm/mutex.h
> > +++ b/arch/s390/include/asm/mutex.h
> > @@ -7,3 +7,5 @@
> >   */
> >  
> >  #include <asm-generic/mutex-dec.h>
> > +
> > +#define mutex_cpu_relax()	barrier()
> > --- a/include/linux/mutex.h
> > +++ b/include/linux/mutex.h
> > @@ -160,4 +160,8 @@ extern int mutex_trylock(struct mutex *l
> >  extern void mutex_unlock(struct mutex *lock);
> >  extern int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);
> >  
> > +#ifndef CONFIG_HAVE_MUTEX_CPU_RELAX
> > +#define mutex_cpu_relax()	cpu_relax()
> > +#endif
> 
> and do `#ifndef mutex_cpu_relax' here.  That's a pretty common trick. 
> It's best to add a comment telling people which arch header file should
> define mutex_cpu_relax, so everyone does it the same way.
> 
> It should perhaps be called arch_mutex_cpu_relax().

Nice trick, I'll send a new patch.