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  

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 -a
2.6.35-32-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:   12637
T: 1 (20607) P:79 I:1500 C:  12648 Min:     31 Act:   68 Avg:  447 Max:   10320
T: 2 (20608) P:78 I:2000 C:   9494 Min:     28 Act:  151 Avg:  383 Max:    9481
T: 3 (20609) P:77 I:2500 C:   7589 Min:     29 Act:  889 Avg:  393 Max:   12670
T: 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:  331482
T: 1 ( 1875) P:79 I:1500 C:  19014 Min:      2 Act:  988 Avg: 2099 Max:  330503
T: 2 ( 1876) P:78 I:2000 C:  14261 Min:      7 Act:  534 Avg: 2096 Max:  329989
T: 3 ( 1877) P:77 I:2500 C:  11409 Min:      4 Act:  554 Avg: 2073 Max:  328490
T: 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 0us
policy: 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:15108679
T: 1 ( 5083) P:80 I:1500 C:   5723 Min:     48 Act:12364955 Avg:6389691 Max:12364955
T: 2 ( 5084) P:80 I:2000 C:   4821 Min:     32 Act:11119979 Avg:8061814 Max:11661123
T: 3 ( 5085) P:80 I:2500 C:   3909 Min:     27 Act:11176854 Avg:4563549 Max:11176854
T: 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