linux kernel synchronization 2

Per CPU Variables

• A CPU should not access the elements of the array corresponding to other CPU.
• 每个CPU拥有该变量的独立副本
• 无需加锁 - 由于每个CPU只操作自己的副本，因此读写自己的副本时不会产生竞争条件
• 缓存优化 - 每个数据结构在主存中对齐，确保每个数据结构落在硬件缓存的不同行，提高缓存命中率

点击查看代码

// 获取当前CPU ID并禁用抢占
int cpu = get_cpu(); 
// 在这里执行需要在当前CPU上运行的代码
// 重新启用抢占
put_cpu();#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/percpu.h>MODULE_LICENSE("GPL");//定义per cpu 变量
DEFINE_PER_CPU(int, counter);static int __init test_hello_init(void)
{int num_cpus = num_online_cpus();int i = 0;int val;pr_info("Number of cpus available:%d\n", num_cpus);for (i = 0; i < num_cpus; i++) {int value = per_cpu(counter, i); //获取per cpu counter变量pr_info("Value of counter is %d at Processor:%d\n", value, i);}get_cpu_var(counter) = 10; //获取per cpu counter变量pr_info("Printing counter value of all processor after updating current processor:%d\n",smp_processor_id());put_cpu_var(counter);for (i = 0; i < num_cpus; i++) {int value = per_cpu(counter, i);pr_info("Value of counter is %d at Processor:%d\n", value, i);}return -1;
}static void __exit test_hello_exit(void)
{pr_info("%s: In exit\n", __func__);
}module_init(test_hello_init);
module_exit(test_hello_exit);

Dynamically allocated per-CPU variables

点击查看代码

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/percpu.h>MODULE_LICENSE("GPL");static int *dynamic_counter;static int __init test_hello_init(void)
{int cpu = get_cpu();int i;dynamic_counter = alloc_percpu(int);pr_info("cpu:%d\n", cpu);*per_cpu_ptr(dynamic_counter, cpu) = 1000;put_cpu();for (i = 0; i < num_online_cpus(); i++)pr_info("cpu:%d\tcounter:%d\n",i, *per_cpu_ptr(dynamic_counter, i));free_percpu(dynamic_counter);return -1;
}static void __exit test_hello_exit(void)
{pr_info("%s: In exit\n", __func__);
}module_init(test_hello_init);
module_exit(test_hello_exit);

只能保护多个cpu并发