On Tue, 2010-06-01 at 14:45 +1000, Anton Blanchard wrote:
> 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:
Can't we dedicate a GPR instead ? Or it isn't worth it ? Something we
almost never use in the kernel like r12 ?
Cheers,
Ben.
> 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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