Messages in this thread Patch in this message | | | From | Rusty Russell <> | Subject | Re: [PATCHSET x86/core/percpu] improve the first percpu chunk allocation | Date | Wed, 4 Mar 2009 10:33:08 +1030 |
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On Wednesday 25 February 2009 01:07:24 Tejun Heo wrote: > it always > saves a 2MB TLB entry for all the non-NUMA machines out there.
Note that everyone keeps talking about "a" TLB entry; I wanted to make sure (esp. for those of us reading from the sidelines), it's not: it's up to num_possible_cpus() TLB entries. Of course, many paths won't access other CPU's data, but it'd be interesting (and pretty easy) to actually instrument how rare this is...
Hmm, fairly rare, but not incredibly:
percpu: measure use
With the idea of using virtual mappings for percpu regions, we wonder how often we access other CPU's per-cpu variables.
32-bit 4-way SMP (under kvm), kernel make -j4: get_cpu_var() 52,358,618 raw_get_cpu_var() 287,191 per_cpu(): 17,371,648 per_cpu(same): 16,020,390
Total same-cpu calls: 68,666,199 Cross-per-cpu calls: 1,351,258
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> --- arch/x86/Makefile_32.cpu | 2 +- include/asm-generic/percpu.h | 10 +++++++--- kernel/module.c | 11 +++++++++++ kernel/smp.c | 21 +++++++++++++++++++++ 4 files changed, 40 insertions(+), 4 deletions(-)
diff --git a/arch/x86/Makefile_32.cpu b/arch/x86/Makefile_32.cpu --- a/arch/x86/Makefile_32.cpu +++ b/arch/x86/Makefile_32.cpu @@ -47,5 +47,5 @@ cflags-$(CONFIG_X86_GENERIC) += $(call # Bug fix for binutils: this option is required in order to keep # binutils from generating NOPL instructions against our will. ifneq ($(CONFIG_X86_P6_NOP),y) -cflags-y += $(call cc-option,-Wa$(comma)-mtune=generic32,) +#cflags-y += $(call cc-option,-Wa$(comma)-mtune=generic32,) endif diff --git a/include/asm-generic/percpu.h b/include/asm-generic/percpu.h --- a/include/asm-generic/percpu.h +++ b/include/asm-generic/percpu.h @@ -53,12 +53,16 @@ extern unsigned long __per_cpu_offset[NR * established ways to produce a usable pointer from the percpu variable * offset. */ +void count_per_cpu(unsigned int cpu); +void count_get_cpu_var(void); +void count_raw_get_cpu_var(void); + #define per_cpu(var, cpu) \ - (*SHIFT_PERCPU_PTR(&per_cpu_var(var), per_cpu_offset(cpu))) + (*(count_per_cpu(cpu), SHIFT_PERCPU_PTR(&per_cpu_var(var), per_cpu_offset(cpu)))) #define __get_cpu_var(var) \ - (*SHIFT_PERCPU_PTR(&per_cpu_var(var), my_cpu_offset)) + (*(count_get_cpu_var(), SHIFT_PERCPU_PTR(&per_cpu_var(var), my_cpu_offset))) #define __raw_get_cpu_var(var) \ - (*SHIFT_PERCPU_PTR(&per_cpu_var(var), __my_cpu_offset)) + (*(count_raw_get_cpu_var(), SHIFT_PERCPU_PTR(&per_cpu_var(var), __my_cpu_offset))) #ifdef CONFIG_HAVE_SETUP_PER_CPU_AREA diff --git a/kernel/module.c b/kernel/module.c --- a/kernel/module.c +++ b/kernel/module.c @@ -2705,6 +2705,17 @@ static const struct seq_operations modul static int modules_open(struct inode *inode, struct file *file) { + extern atomic_t get_cpu_var_count, raw_get_cpu_var_count, per_cpu_count[], unnecessary_count[]; + unsigned int i; + + printk("get_cpu_var_count: %i\n", atomic_xchg(&get_cpu_var_count, 0)); + printk("raw_get_cpu_var_count: %i\n", + atomic_xchg(&raw_get_cpu_var_count, 0)); + for_each_online_cpu(i) + printk("per_cpu %i: %u (%u self)\n", + i, atomic_xchg(&per_cpu_count[i], 0), + atomic_xchg(&unnecessary_count[i], 0)); + return seq_open(file, &modules_op); } diff --git a/kernel/smp.c b/kernel/smp.c --- a/kernel/smp.c +++ b/kernel/smp.c @@ -10,6 +10,27 @@ #include <linux/rcupdate.h> #include <linux/rculist.h> #include <linux/smp.h> + +atomic_t get_cpu_var_count, raw_get_cpu_var_count, per_cpu_count[CONFIG_NR_CPUS], unnecessary_count[CONFIG_NR_CPUS]; +void count_per_cpu(unsigned int cpu) +{ + if (cpu == raw_smp_processor_id()) + atomic_inc(&unnecessary_count[cpu]); + atomic_inc(&per_cpu_count[cpu]); +} +EXPORT_SYMBOL(count_per_cpu); + +void count_get_cpu_var(void) +{ + atomic_inc(&get_cpu_var_count); +} +EXPORT_SYMBOL(count_get_cpu_var); + +void count_raw_get_cpu_var(void) +{ + atomic_inc(&raw_get_cpu_var_count); +} +EXPORT_SYMBOL(count_raw_get_cpu_var); static DEFINE_PER_CPU(struct call_single_queue, call_single_queue); static LIST_HEAD(call_function_queue);
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