Linux RT(2)－硬实时Linux(RT-Preempt Patch)的中断线程化

特别声明：本系列文章LiAnLab.org著作权所有，转载请注明出处。by  @宋宝华Barry

底半部：线程化IRQ

线程化中断的支持在2009年已经进入Linux官方内核，详见Thomas Gleixner的patch:

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=3aa551c9b4c40018f0e261a178e3d25478dc04a9

该patch提供一个能力，驱动可以通过

int request_threaded_irq(unsigned int irq, irq_handler_t handler,
                         irq_handler_t thread_fn, unsigned long irqflags,
                         const char *devname, void *dev_id)

申请一个线程化的IRQ，kernel会为中断的底版本创建一个名字为irq/%d-%s的线程，%d对应着中断号。其中顶半部（硬中断）handler在做完必要的处理工作之后，会返回IRQ_WAKE_THREAD，之后kernel会唤醒irq/%d-%s线程，而该kernel线程会调用thread_fn函数，因此，该线程成为底半部。在后续维护的过程中，笔者曾参与进一步完善该功能的讨论，后续patch包括nested、oneshot等的支持，详见patch：

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=399b5da29b9f851eb7b96e2882097127f003e87c

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=70aedd24d20e75198f5a0b11750faabbb56924e2

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=b25c340c195447afb1860da580fe2a85a6b652c5

该机制目前在kernel中使用已经十分广泛，可以认为是继softirq（含tasklet）和workqueue之后的又一大中断底半部方式。

顶半部：强制线程化

在使能Linux RT-Preempt后，默认情况下会强制透过request_irq()申请的IRQ的顶半部函数在线程中执行，我们都知道request_irq的原型为：

static inline int __must_check
request_irq(unsigned int irq, irq_handler_t handler, unsigned long flags,
            const char *name, void *dev)
{                        
        return request_threaded_irq(irq, handler, NULL, flags, name, dev);
}      

这意味着通过request_irq()申请的IRQ，在没有Rt-Preepmt的情况下，kernel并不会为其创建irq线程，因为它在最终调用request_threaded_irq()的时候传递的thread_fn为NULL。

如果使能了RT-Preempt Patch的情况下，其中的genirq-force-threading.patch会强制ARM使用threaded irq：

Index: linux-stable/arch/arm/Kconfig
===================================================================
--- linux-stable.orig/arch/arm/Kconfig
+++ linux-stable/arch/arm/Kconfig
@@ -40,6 +40,7 @@ config ARM
        select GENERIC_IRQ_SHOW
        select ARCH_WANT_IPC_PARSE_VERSION
        select HARDIRQS_SW_RESEND
+       select IRQ_FORCED_THREADING
        select CPU_PM if (SUSPEND || CPU_IDLE)
        select GENERIC_PCI_IOMAP
        select HAVE_BPF_JIT

在RT-Preempt Patch中，会针对使能了IRQ_FORCED_THREADING的情况，对这一原先没有线程化IRQ的case进行强制线程化，代码见__setup_irq()：

 887 static int
 888 __setup_irq(unsigned int irq, struct irq_desc *desc, struct irqaction *new)
 889 {
 890         ...
 903 
 904         /*
 905          * Check whether the interrupt nests into another interrupt
 906          * thread.
 907          */
 908         nested = irq_settings_is_nested_thread(desc);
 909         if (nested) {
 910                 ...
 920         } else {
 921                 if (irq_settings_can_thread(desc))
 922                         irq_setup_forced_threading(new);
 923         }
 925         /*
 926          * Create a handler thread when a thread function is supplied
 927          * and the interrupt does not nest into another interrupt
 928          * thread.
 929          */
 930         if (new->thread_fn && !nested) {
 931                 struct task_struct *t;
 932 
 933                 t = kthread_create(irq_thread, new, "irq/%d-%s", irq,
 934                                    new->name);
 935                 ...
 939                 /*
 940                  * We keep the reference to the task struct even if
 941                  * the thread dies to avoid that the interrupt code
 942                  * references an already freed task_struct.
 943                  */
 944                 get_task_struct(t);
 945                 new->thread = t;
 946