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:116026155RT-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 ─────────────────────────┐