Now we dynamically allocate the paca array, it takes an extra load
whenever we want to access another cpu's paca. One place we do that a lot
is per cpu variables. A simple example:
DEFINE_PER_CPU(unsigned long, vara);
unsigned long test4(int cpu)
{
return per_cpu(vara, cpu);
}
This takes 4 loads, 5 if you include the actual load of the per cpu variable:
ld r11,-32760(r30) # load address of paca pointer
ld r9,-32768(r30) # load link address of percpu variable
sldi r3,r29,9 # get offset into paca (each entry is 512 bytes)
ld r0,0(r11) # load paca pointer
add r3,r0,r3 # paca + offset
ld r11,64(r3) # load paca[cpu].data_offset
ldx r3,r9,r11 # load per cpu variable
If we remove the ppc64 specific per_cpu_offset(), we get the generic one
which indexes into a statically allocated array. This removes one load and
one add:
ld r11,-32760(r30) # load address of __per_cpu_offset
ld r9,-32768(r30) # load link address of percpu variable
sldi r3,r29,3 # get offset into __per_cpu_offset (each entry 8 bytes)
ldx r11,r11,r3 # load __per_cpu_offset[cpu]
ldx r3,r9,r11 # load per cpu variable
Having all the offsets in one array also helps when iterating over a per cpu
variable across a number of cpus, such as in the scheduler. Before we would
need to load one paca cacheline when calculating each per cpu offset. Now we
have 16 (128 / sizeof(long)) per cpu offsets in each cacheline.
Signed-off-by: Anton Blanchard <an...@samba.org>
---
Index: powerpc.git/arch/powerpc/include/asm/percpu.h
===================================================================
--- powerpc.git.orig/arch/powerpc/include/asm/percpu.h 2010-06-01
11:10:16.225954322 +1000
+++ powerpc.git/arch/powerpc/include/asm/percpu.h 2010-06-01
11:32:27.713476455 +1000
@@ -1,7 +1,6 @@
#ifndef _ASM_POWERPC_PERCPU_H_
#define _ASM_POWERPC_PERCPU_H_
#ifdef __powerpc64__
-#include <linux/compiler.h>
/*
* Same as asm-generic/percpu.h, except that we store the per cpu offset
@@ -12,9 +11,7 @@
#include <asm/paca.h>
-#define __per_cpu_offset(cpu) (paca[cpu].data_offset)
#define __my_cpu_offset local_paca->data_offset
-#define per_cpu_offset(x) (__per_cpu_offset(x))
#endif /* CONFIG_SMP */
#endif /* __powerpc64__ */
Index: powerpc.git/arch/powerpc/kernel/asm-offsets.c
===================================================================
--- powerpc.git.orig/arch/powerpc/kernel/asm-offsets.c 2010-06-01
11:10:16.195958268 +1000
+++ powerpc.git/arch/powerpc/kernel/asm-offsets.c 2010-06-01
11:32:27.713476455 +1000
@@ -194,7 +194,6 @@ int main(void)
DEFINE(PACA_STARTSPURR, offsetof(struct paca_struct, startspurr));
DEFINE(PACA_USER_TIME, offsetof(struct paca_struct, user_time));
DEFINE(PACA_SYSTEM_TIME, offsetof(struct paca_struct, system_time));
- DEFINE(PACA_DATA_OFFSET, offsetof(struct paca_struct, data_offset));
DEFINE(PACA_TRAP_SAVE, offsetof(struct paca_struct, trap_save));
#ifdef CONFIG_KVM_BOOK3S_64_HANDLER
DEFINE(PACA_KVM_SVCPU, offsetof(struct paca_struct, shadow_vcpu));
Index: powerpc.git/arch/powerpc/kernel/setup_64.c
===================================================================
--- powerpc.git.orig/arch/powerpc/kernel/setup_64.c 2010-06-01
11:10:16.205958158 +1000
+++ powerpc.git/arch/powerpc/kernel/setup_64.c 2010-06-01 11:32:27.713476455
+1000
@@ -604,6 +604,9 @@ static int pcpu_cpu_distance(unsigned in
return REMOTE_DISTANCE;
}
+unsigned long __per_cpu_offset[NR_CPUS] __read_mostly;
+EXPORT_SYMBOL(__per_cpu_offset);
+
void __init setup_per_cpu_areas(void)
{
const size_t dyn_size = PERCPU_MODULE_RESERVE + PERCPU_DYNAMIC_RESERVE;
@@ -628,8 +631,10 @@ void __init setup_per_cpu_areas(void)
panic("cannot initialize percpu area (err=%d)", rc);
delta = (unsigned long)pcpu_base_addr - (unsigned long)__per_cpu_start;
- for_each_possible_cpu(cpu)
- paca[cpu].data_offset = delta + pcpu_unit_offsets[cpu];
+ for_each_possible_cpu(cpu) {
+ __per_cpu_offset[cpu] = delta + pcpu_unit_offsets[cpu];
+ paca[cpu].data_offset = __per_cpu_offset[cpu];
+ }
}
#endif
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