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

来源：互联网发布：2016我国旅游业数据编辑：程序博客网时间：2024/05/17 20:28

http://21cnbao.blog.51cto.com/109393/1011931

Vanilla kernel的问题

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：

barry@barry-VirtualBox:/lib/modules$ uname -a2.6.35-32-generic #67-Ubuntu SMP Mon Mar 5 19:35:26 UTC 2012 i686 GNU/Linux

在Ubuntu 10.10，apt-get install rt-tests安装rt测试工具集，运行其中的cyclictest测试工具，默认创建5个SCHED_FIFO策略的realtime线程，优先级 76-80，运行周期是1000,1500,2000,2500,3000微秒：

barry@barry-VirtualBox:~/development/panda/android$ sudo cyclictest -p 80 -t5 -n [sudo] password for barry: policy: fifo: loadavg: 9.22 8.57 6.75 11/374 21385          T: 0 (20606) P:80 I:1000 C:  18973 Min:     26 Act:   76 Avg:  428 Max:   12637T: 1 (20607) P:79 I:1500 C:  12648 Min:     31 Act:   68 Avg:  447 Max:   10320T: 2 (20608) P:78 I:2000 C:   9494 Min:     28 Act:  151 Avg:  383 Max:    9481T: 3 (20609) P:77 I:2500 C:   7589 Min:     29 Act:  889 Avg:  393 Max:   12670T: 4 (20610) P:76 I:3000 C:   6325 Min:     37 Act:  167 Avg:  553 Max:   13673

由此可见在标准Linux内，rt线程投入运行的jitter非常不稳定，最小值在26-37微秒，平均值为68-889微秒，而最大值则分布在9481-13673微秒之间。

我们还是运行这个测试，但是在运行这个测试的过程中引入更多干扰，如mount /dev/sdb1 ~/development，则结果变为：

barry@barry-VirtualBox:~$ sudo cyclictest -p 80 -t5 -n policy: fifo: loadavg: 0.14 0.29 0.13 2/308 1908          T: 0 ( 1874) P:80 I:1000 C:  28521 Min:      0 Act:  440 Avg: 2095 Max:  331482T: 1 ( 1875) P:79 I:1500 C:  19014 Min:      2 Act:  988 Avg: 2099 Max:  330503T: 2 ( 1876) P:78 I:2000 C:  14261 Min:      7 Act:  534 Avg: 2096 Max:  329989T: 3 ( 1877) P:77 I:2500 C:  11409 Min:      4 Act:  554 Avg: 2073 Max:  328490T: 4 ( 1878) P:76 I:3000 C:   9507 Min:     12 Act:  100 Avg: 2081 Max:  328991

mount过程中引入的irq、softirq和spinlock导致最大jitter明显地加大甚至达到了331482us，充分显示出了标准Linux内核中RT线程投入运行时间的不可预期性(硬实时要求意味着可预期)。

如果我们编译一份kernel，选择的是“Voluntary Kernel Preemption (Desktop)“，这类似于2.4不支持kernel抢占的情况，我们运行同样的case，时间的不确定性大地几乎让我们无法接受：

barry@barry-VirtualBox:~$ sudo /usr/local/bin/cyclictest -p 80 -t5 -n# /dev/cpu_dma_latency set to 0uspolicy: fifo: loadavg: 0.23 0.30 0.15 3/247 5086           T: 0 ( 5082) P:80 I:1000 C:   5637 Min:     60 Act:15108679 Avg:11195196 Max:15108679T: 1 ( 5083) P:80 I:1500 C:   5723 Min:     48 Act:12364955 Avg:6389691 Max:12364955T: 2 ( 5084) P:80 I:2000 C:   4821 Min:     32 Act:11119979 Avg:8061814 Max:11661123T: 3 ( 5085) P:80 I:2500 C:   3909 Min:     27 Act:11176854 Avg:4563549 Max:11176854T: 4 ( 5086) P:80 I:3000 C:   3598 Min:     37 Act:9951432 Avg:8761137 Max:116026155

RT-Preempt Patch使能

RT-Preempt Patch对Linux kernel的主要改造包括：

Making in-kernel locking-primitives (using spinlocks) preemptible though reimplementation with rtmutexes:
Critical sections protected by i.e. spinlock_t and rwlock_t are now preemptible. The creation of non-preemptible sections (in kernel) is still possible with raw_spinlock_t (same APIs like spinlock_t)
Implementing priority inheritance for in-kernel spinlocks and semaphores. For more information on priority inversion and priority inheritance please consultIntroduction to Priority Inversion
Converting interrupt handlers into preemptible kernel threads: The RT-Preempt patch treats soft interrupt handlers in kernel thread context, which is represented by a task_struct like a common userspace process. However it is also possible to register an IRQ in kernel context.
Converting the old Linux timer API into separate infrastructures for high resolution kernel timers plus one for timeouts, leading to userspace POSIX timers with high resolution.

在本试验中，我们取的带RT- Preempt Patch的kernel tree是git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-stable- rt.git，使用其v3.4-rt-rebase branch，编译kernel时选中了"Fully Preemptible Kernel"抢占模型:

───────────────────────── Preemption Model ─────────────────────────┐

│ │          ( ) No Forced Preemption (Server)
│ │          ( ) Voluntary Kernel Preemption (Desktop)
│ │          ( ) Preemptible Kernel (Low-Latency Desktop)
│ │          ( ) Preemptible Kernel (Basic RT)
│ │          (X) Fully Preemptible Kernel (RT)

另外，kernel中需支持tickless和高精度timer：

┌───────────────────Processor type and features ─────────────────────────┐
│ │ [*] Tickless System (Dynamic Ticks)
│ │ [*] High Resolution Timer Support

make modules_install、make install、mkintramfs后，我们得到一个可以在Ubuntu中启动的RT kernel。具体编译方法可详见http://www.linuxidc.com/Linux/2012-01/50749.htm，根据该文修改版本号等信息即可，我们运行的命令包括：

安装模块

barry@barry-VirtualBox:~/development/linux-2.6$ sudo make modules_install....  INSTALL /lib/firmware/whiteheat_loader.fw  INSTALL /lib/firmware/whiteheat.fw  INSTALL /lib/firmware/keyspan_pda/keyspan_pda.fw  INSTALL /lib/firmware/keyspan_pda/xircom_pgs.fw  INSTALL /lib/firmware/cpia2/stv0672_vp4.bin  INSTALL /lib/firmware/yam/1200.bin  INSTALL /lib/firmware/yam/9600.bin  DEPMOD  3.4.11-rt19

安装kernel

barry@barry-VirtualBox:~/development/linux-2.6$ sudo make install sh /home/barry/development/linux-2.6/arch/x86/boot/install.sh 3.4.11-rt19 arch/x86/boot/bzImage \ System.map "/boot"

制作initrd

barry@barry-VirtualBox:~/development/linux-2.6$ sudo mkinitramfs 3.4.11-rt19 -o /boot/initrd.img-3.4.11-rt19

修改grub配置

在grub.conf中增加新的启动entry，仿照现有的menuentry，增加一个新的，把其中的相关版本号都变更为3.4.11-rt19，我们的修改如下：

 menuentry 'Ubuntu, with Linux 3.4.11-rt19' --class ubuntu --class gnu-linux --class gnu --class os {    recordfail    insmod part_msdos    insmod ext2    set root='(hd0,msdos1)'    search --no-floppy --fs-uuid --set a0db5cf0-6ce3-404f-9808-88ce18f0177a    linux    /boot/vmlinuz-3.4.11-rt19 root=UUID=a0db5cf0-6ce3-404f-9808-88ce18f0177a ro   quiet splash    initrd    /boot/initrd.img-3.4.11-rt19}

开机时选择3.4.11-rt19启动：

RT-Preempt Patch试用

运行同样的测试cyclictest benchmark工具，结果迥异：

barry@barry-VirtualBox:~$ sudo cyclictest -p 80 -t5 -nWARNING: Most functions require kernel 2.6policy: fifo: loadavg: 0.71 0.42 0.17 1/289 1926          T: 0 ( 1921) P:80 I:1000 C:   7294 Min:      7 Act:   89 Avg:  197 Max:    3177T: 1 ( 1922) P:79 I:1500 C:   4863 Min:     10 Act:   85 Avg:  186 Max:    2681T: 2 ( 1923) P:78 I:2000 C:   3647 Min:     15 Act:   93 Avg:  160 Max:    2504T: 3 ( 1924) P:77 I:2500 C:   2918 Min:     23 Act:   67 Avg:  171 Max:    2114T: 4 ( 1925) P:76 I:3000 C:   2432 Min:     19 Act:  134 Avg:  339 Max:    3129

我们还是运行这个测试，但是在运行这个测试的过程中引入更多干扰，如mount /dev/sdb1 ~/development，则结果变为：

barry@barry-VirtualBox:~$ sudo cyclictest -p 80 -t5 -n# /dev/cpu_dma_latency set to 0uspolicy: fifo: loadavg: 0.11 0.12 0.13 1/263 2860          T: 0 ( 2843) P:80 I:1000 C:  28135 Min:      5 Act:  198 Avg:  200 Max:    7387T: 1 ( 2844) P:80 I:1500 C:  18756 Min:     22 Act:  169 Avg:  188 Max:    6875T: 2 ( 2845) P:80 I:2000 C:  14067 Min:      7 Act:   91 Avg:  149 Max:    7288T: 3 ( 2846) P:80 I:2500 C:  11254 Min:     19 Act:  131 Avg:  155 Max:    6287T: 4 ( 2847) P:80 I:3000 C:   9378 Min:     25 Act:   58 Avg:  172 Max:    6121

时间在可预期的范围内，没有出现标准kernel里面jitter达到331482的情况。需要说明的是，这个jitter大到超过了我们的预期，达到了10ms量级，相信是受到了我们的测试都是在Virtualbox虚拟机进行的影响。按照其他文档显示，这个jitter应该在数十us左右。

我们在这个kernel里面运行ps aux命令，可以看出线程化了的irq：

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMANDroot         1  0.8  0.1   2880  1788 ?        Ss   18:39   0:03 initroot         2  0.0  0.0      0     0 ?        S    18:39   0:00 kthreadd...root        45  0.0  0.0      0     0 ?        S    18:39   0:00 irq/14-ata_piixroot        46  0.0  0.0      0     0 ?        S    18:39   0:00 irq/15-ata_piixroot        50  0.0  0.0      0     0 ?        S    18:39   0:00 irq/19-ehci_hcdroot        51  0.0  0.0      0     0 ?        S    18:39   0:00 irq/22-ohci_hcdroot        55  0.0  0.0      0     0 ?        S    18:39   0:00 irq/12-i8042root        56  0.0  0.0      0     0 ?        S    18:39   0:00 irq/1-i8042root        57  0.0  0.0      0     0 ?        S    18:39   0:00 irq/8-rtc0root       863  0.0  0.0      0     0 ?        S    18:39   0:00 irq/19-eth0root       864  0.0  0.0      0     0 ?        S    18:39   0:00 irq/16-eth1root      1002  0.5  0.0      0     0 ?        S    18:39   0:01 irq/21-snd_inte...

在其中编写一个RT 线程的应用程序，通常需要如下步骤：

Setting a real time scheduling policy and priority.
Locking memory so that page faults caused by virtual memory will not undermine deterministic behavior
Pre-faulting the stack, so that a future stack fault will not undermine deterministic behavior

例子test_rt.c，其中的mlockall是为了防止进程的虚拟地址空间对应的物理页面被swap出去，而stack_prefault()则故意提前导致stack往下增长8KB，因此其后的函数调用和局部变量的使用将不再导致栈增长（依赖于page fault和内存申请）：

#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。

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