ARM: 7587/1: implement optimized percpu variable access

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 replaces 2 loads with a mrc instruction for each percpu variable
access. With hackbench, the performance improvement is 1.4% on Cortex-A9
(highbank). Taking an average of 30 runs of "hackbench -l 1000" yields:

Before: 6.2191
After: 6.1348

Will Deacon reported similar delta on v6 with 11MPCore.

The asm "memory clobber" are needed here to ensure the percpu offset
gets reloaded. Testing by Will found that this would not happen in
__schedule() which is a bit of a special case as preemption is disabled
but the execution can move cores.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

4 files changed, 54 insertions, 2 deletions

diff --git a/arch/arm/include/asm/Kbuild b/arch/arm/include/asm/Kbuild
index f70ae175a3d6..2ffdaacd461c 100644
--- a/arch/arm/include/asm/Kbuild
+++ b/arch/arm/include/asm/Kbuild
@@ -16,7 +16,6 @@ generic-y += local64.h
 generic-y += msgbuf.h
 generic-y += param.h
 generic-y += parport.h
-generic-y += percpu.h
 generic-y += poll.h
 generic-y += resource.h
 generic-y += sections.h
diff --git a/arch/arm/include/asm/percpu.h b/arch/arm/include/asm/percpu.h
new file mode 100644
index 000000000000..968c0a14e0a3
--- /dev/null
+++ b/arch/arm/include/asm/percpu.h
@@ -0,0 +1,45 @@
+/*
+ * 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. TPIDRPRW only exists on V6K and V7
+ */
+#if defined(CONFIG_SMP) && !defined(CONFIG_CPU_V6)
+static inline void set_my_cpu_offset(unsigned long off)
+{
+        /* Set TPIDRPRW */
+        asm volatile("mcr p15, 0, %0, c13, c0, 4" : : "r" (off) : "memory");
+}
+
+static inline unsigned long __my_cpu_offset(void)
+{
+        unsigned long off;
+        /* Read TPIDRPRW */
+        asm("mrc p15, 0, %0, c13, c0, 4" : "=r" (off) : : "memory");
+        return off;
+}
+#define __my_cpu_offset __my_cpu_offset()
+#else
+#define set_my_cpu_offset(x)    do {} while(0)
+
+#endif /* CONFIG_SMP */
+
+#include <asm-generic/percpu.h>
+
+#endif /* _ASM_ARM_PERCPU_H_ */
diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
index f739fb1d217a..9a89bf4aefe1 100644
--- a/arch/arm/kernel/setup.c
+++ b/arch/arm/kernel/setup.c
@@ -383,6 +383,12 @@ void cpu_init(void)
                 BUG();
         }
 
+        /*
+         * This only works on resume and secondary cores. For booting on the
+         * boot cpu, smp_prepare_boot_cpu is called after percpu area setup.
+         */
+        set_my_cpu_offset(per_cpu_offset(cpu));
+
         cpu_proc_init();
 
         /*
diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index 7eacd84cdc9c..f3a2be5837aa 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -314,9 +314,10 @@ asmlinkage void __cpuinit secondary_start_kernel(void)
         current->active_mm = mm;
         cpumask_set_cpu(cpu, mm_cpumask(mm));
 
+        cpu_init();
+
         printk("CPU%u: Booted secondary processor\n", cpu);
 
-        cpu_init();
         preempt_disable();
         trace_hardirqs_off();
 
@@ -372,6 +373,7 @@ void __init smp_cpus_done(unsigned int max_cpus)
 
 void __init smp_prepare_boot_cpu(void)
 {
+        set_my_cpu_offset(per_cpu_offset(smp_processor_id()));
 }
 
 void __init smp_prepare_cpus(unsigned int max_cpus)