Vanilla kernel的問題
Linux kernel在spinlock、irq上下文方面無法搶占，因此高優先級任務被喚醒到得以執行的時間並不能完全確定。同時，Linux kernel本身也不處理優先級反轉。RT-Preempt Patch是在Linux社區kernel的基礎上，加上相關的補丁，以使得Linux滿足硬實時的需求。本文描述了該patch在PC上的實踐。我們的 測試環境為Ubuntu 10.10，默認情況下使用Ubuntu 10.10自帶的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安裝rt測試工具集，運行其中的cyclictest測試工具，默認創建5個SCHED_FIFO策略的realtime線程，優先級 76-80，運行周期是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
由此可見在標准Linux內，rt線程投入運行的jitter非常不穩定，最小值在26-37微秒，平均值為68-889微秒，而最大值則分布在9481-13673微秒之間。
我們還是運行這個測試，但是在運行這個測試的過程中引入更多干擾，如mount /dev/sdb1 ~/development，則結果變為：
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過程中引入的irq、softirq和spinlock導致最大jitter明顯地加大甚至達到了331482us，充分顯示出了標准Linux內核中RT線程投入運行時間的不可預期性(硬實時要求意味著可預期)。
如果我們編譯一份kernel，選擇的是“Voluntary Kernel Preemption (Desktop)“，這類似於2.4不支持kernel搶占的情況，我們運行同樣的case，時間的不確定性大地幾乎讓我們無法接受：
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對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.
在本試驗中，我們取的帶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，編譯kernel時選中了"Fully Preemptible Kernel"搶占模型:
───────────────────────── 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後，我們得到一個可以在Ubuntu中啟動的RT kernel。具體編譯方法可詳見http://www.linuxidc.com/Linux/2012-01/50749.htm，根據該文修改版本 號等信息即可，我們運行的命令包括：
安裝模塊
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
安裝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中增加新的啟動entry，仿照現有的menuentry，增加一個新的，把其中的相關版本號都變更為3.4.11-rt19，我們的修改如下：
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
}
開機時選擇3.4.11-rt19啟動：
