Re: [PATCH] mutex: Introduce mutex_cpu_relax()

Andrew Morton
2010-10-14T22:14:41Z

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().