Re: [PATCH] mutex: Introduce arch_mutex_cpu_relax()

Peter Zijlstra
2010-11-23T14:19:57Z

On Tue, 2010-11-23 at 15:12 +0100, Gerald Schaefer wrote:
> On Mo, 2010-11-22 at 12:10 -0800, Andrew Morton wrote:
> > On Mon, 22 Nov 2010 15:47:36 +0100
> > 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
> > > arch_mutex_cpu_relax(), which can be defined by each architecture that
> > > selects HAVE_ARCH_MUTEX_CPU_RELAX. The default is still cpu_relax(), so
> > > this patch should not affect other architectures than System z for now.
> > >
> > > ...
> > >
> > > --- 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_ARCH_MUTEX_CPU_RELAX
> > > +#define arch_mutex_cpu_relax() cpu_relax()
> > > +#endif
> >
> > A simpler way of doing this is to remove the CONFIG_ variable
> > altogether and do
> >
> > #ifndef arch_mutex_cpu_relax
> > #define arch_mutex_cpu_relax() cpu_relax()
> > #endif
> >
> > When doing this, one should be clear about _which_ arch file has the
> > responsibility of defining arch_mutex_cpu_relax, and make sure that
> > this arch file is reliably included in the .c file.
>
> Well, I've tried that with my last approach, defining arch_mutex_cpu_relax()
> in <asm/mutex.h> and including that from <linux/mutex.h>. This didn't work
> well because of ugly header file dependencies, and Peter also commented
> that "including "asm/mutex.h" isn't advised". The problem is the following
> code in kernel/mutex.c (after including <linux/mutex.h>) when
> CONFIG_DEBUG_MUTEXES is set:
>
> #ifdef CONFIG_DEBUG_MUTEXES
> # include "mutex-debug.h"
> # include <asm-generic/mutex-null.h>
> #else
> # include "mutex.h"
> # include <asm/mutex.h>
> #endif
>
> So I can only include <asm/mutex.h> from <linux/mutex.h> with an ugly
> "#ifndef CONFIG_DEBUG_MUTEXES" around it, or use a completely different
> or new arch header file (but <asm/mutex.h> seems like the right place
> for this). The CONFIG_ approach avoids all this header file dependency
> mess, or did I miss something (or maybe it's just me and it is not ugly
> at all)?

Yeah, that all cause massive grief.. I've applied your patch as is,
assuming s390 already has the needed arch_mutex_cpu_relax()
implementation (otherwise I've just broken my s390 build).

Thanks