[PATCH] ARM: implement optimized percpu variable access

[will.deacon@arm.com (Will Deacon)]

Hi Rob,

On Sun, Nov 11, 2012 at 03:20:40AM +0000, Rob Herring wrote:
> From: Rob Herring <rob.herring@calxeda.com>
> 
> Use the previously unused TPIDRPRW register to store percpu offsets.
> TPIDRPRW is only accessible in PL1, so it can only be used in the kernel.
> 
> This saves 2 loads for each percpu variable access which should yield
> improved performance, but the improvement has not been quantified.

The patch looks largely fine to me (one minor comment below), but we should
try and see what the performance difference is like on a few cores before
merging this. Have you tried something like hackbench to see if the
difference is measurable there? If not, I guess we'll need something more
targetted.

> diff --git a/arch/arm/include/asm/percpu.h b/arch/arm/include/asm/percpu.h
> new file mode 100644
> index 0000000..9eb7372
> --- /dev/null
> +++ b/arch/arm/include/asm/percpu.h
> @@ -0,0 +1,44 @@
> +/*
> + * Copyright 2012 Calxeda, Inc.
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms and conditions of the GNU General Public License,
> + * version 2, as published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope it will be useful, but WITHOUT
> + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
> + * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
> + * more details.
> + *
> + * You should have received a copy of the GNU General Public License along with
> + * this program.  If not, see <http://www.gnu.org/licenses/>.
> + */
> +#ifndef _ASM_ARM_PERCPU_H_
> +#define _ASM_ARM_PERCPU_H_
> +
> +/*
> + * Same as asm-generic/percpu.h, except that we store the per cpu offset
> + * in the TPIDRPRW.
> + */
> +#if defined(CONFIG_SMP) && (__LINUX_ARM_ARCH__ >= 6)
> +
> +static inline void set_my_cpu_offset(unsigned long off)
> +{
> +	asm volatile("mcr p15, 0, %0, c13, c0, 4	@ set TPIDRPRW" : : "r" (off) : "cc" );
> +}

You don't need the "cc" here.

Will