Linux RT(1)－硬实时Linux(RT-Preempt Patch)在PC上的编译、使用和测试

by @宋宝华Barry

Linux kernel在spinlock、irq上下文方面无法抢占，因此高优先级任务被唤醒到得以执行的时间并不能完全确定。同时，Linux kernel本身也不处理优先级反转。RT-Preempt Patch是在Linux社区kernel的基础上，加上相关的补丁，以使得Linux满足硬实时的需求。本文描述了该patch在PC上的实践。我们的测试环境为Ubuntu 10.10，默认情况下使用Ubuntu 10.10自带的kernel：

RT-Preempt Patch试用
运行同样的测试cyclictest benchmark工具，结果迥异：
#include <stdlib.h>#include <stdio.h>#include <time.h>#include <sched.h>#include <sys/mman.h>#include <string.h>#define MY_PRIORITY (49) /* we use 49 as the PRREMPT_RT use 50                            as the priority of kernel tasklets                            and interrupt handler by default */#define MAX_SAFE_STACK (8*1024) /* The maximum stack size which is                                   guaranteed safe to access without                                   faulting */#define NSEC_PER_SEC    (1000000000) /* The number of nsecs per sec. */void stack_prefault(void) {        unsigned char dummy[MAX_SAFE_STACK];        memset(dummy, 0, MAX_SAFE_STACK);        return;}int main(int argc, char* argv[]){        struct timespec t;        struct sched_param param;        int interval = 50000; /* 50us*/        /* Declare ourself as a real time task */        param.sched_priority = MY_PRIORITY;        if(sched_setscheduler(0, SCHED_FIFO, ?m) == -1) {                perror("sched_setscheduler failed");                exit(-1);        }        /* Lock memory */        if(mlockall(MCL_CURRENT|MCL_FUTURE) == -1) {                perror("mlockall failed");                exit(-2);        }        /* Pre-fault our stack */        stack_prefault();        clock_gettime(CLOCK_MONOTONIC ,&t);        /* start after one second */        t.tv_sec++;        while(1) {                /* wait until next shot */                clock_nanosleep(CLOCK_MONOTONIC, TIMER_ABSTIME, &t, NULL);                /* do the stuff */                /* calculate next shot */                t.tv_nsec += interval;                while (t.tv_nsec >= NSEC_PER_SEC) {                       t.tv_nsec -= NSEC_PER_SEC;                        t.tv_sec++;                }   }}
编译之：gcc -o test_rt test_rt.c -lrt。本节就到这里，后续我们会有一系列博文来描述RT-Preempt Patch对kernel的主要改动，以及其工作原理。