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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会为中断的底半部创建一个名字为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 }
我们重点看一下其中的921行：
867 static void irq_setup_forced_threading(struct irqaction *new)
868 {
869 if (!force_irqthreads)
870 return;
871 if (new->flags & (IRQF_NO_THREAD | IRQF_PERCPU | IRQF_ONESHOT))
872 return;
873
874 new->flags |= IRQF_ONESHOT;
875
876 if (!new->thread_fn) {
877 set_bit(IRQTF_FORCED_THREAD, &new->thread_flags);
878 new->thread_fn = new->handler;
879 new->handler = irq_default_primary_handler;
880 }
881 }
第878行和879行，强制将原先的handler复制给thread_fn，而又强制把原来的handler变更为irq_default_primary_handler()，而这个函数，其实神马都不做，只是直接返回IRQ_WAKE_THREAD：
613 /*
614 * Default primary interrupt handler for threaded interrupts. Is
615 * assigned as primary handler when request_threaded_irq is called
616 * with handler == NULL. Useful for oneshot interrupts.
617 */
618 static irqreturn_t irq_default_primary_handler(int irq, void *dev_id)
619 {
620 return IRQ_WAKE_THREAD;
621 }
第874的IRQF_ONESHOT就用到了我们前面说的oneshot功能。
所以，RT-Preempt实际上是把原先的顶半部底半部化了，而现在伪造了一个假的顶半部，它只是直接返回一个IRQ_WAKE_THREAD标记而已。
我们来看一下一个中断发生后，Linux RT-Preempt处理的全过程，首先是会跳到
arch/arm/kernel/entry-armv.S
arch/arm/include/asm/entry-macro-multi.S
中的汇编入口，再进入arm/kernel/irq.c下的asm_do_IRQ 、handle_IRQ，之后generic的handle_irq_event_percpu()被调用：
133 handle_irq_event_percpu(struct irq_desc *desc, struct irqaction *action)
134 {
135 irqreturn_t retval = IRQ_NONE;
136 unsigned int flags = 0, irq = desc->irq_data.irq;
137
138 do {
139 irqreturn_t res;
140
141 trace_irq_handler_entry(irq, action);
142 res = action->handler(irq, action->dev_id);
143 trace_irq_handler_exit(irq, action, res);
144
145 if (WARN_ONCE(!irqs_disabled(),"irq %u handler %pF enabled interrupts\n",
146 irq, action->handler))
147 local_irq_disable();
148
149 switch (res) {
150 case IRQ_WAKE_THREAD:
151 /*
152 * Catch drivers which return WAKE_THREAD but
153 * did not set up a thread function
154 */
155 if (unlikely(!action->thread_fn)) {
156 warn_no_thread(irq, action);
157 break;
158 }
159
160 irq_wake_thread(desc, action);
161
162 /* Fall through to add to randomness */
163 case IRQ_HANDLED:
164 flags |= action->flags;
165 break;
166
167 default:
我们关注其中的第142行，本质上是调用irq_default_primary_handler()，接到150行，由于 irq_default_primary_handler()返回了IRQ_WAKE_THREAD，因此，generic的中断处理流程会执行 irq_wake_thread(desc, action);去唤醒前面的irq/%d-%s线程，该线程的代码是
789 static int irq_thread(void *data)
790 {
791 static const struct sched_param param = {
792 .sched_priority = MAX_USER_RT_PRIO/2,
793 };
794 struct irqaction *action = data;
795 struct irq_desc *desc = irq_to_desc(action->irq);
796 irqreturn_t (*handler_fn)(struct irq_desc *desc,
797 struct irqaction *action);
798
799 if (force_irqthreads && test_bit(IRQTF_FORCED_THREAD,
800 &action->thread_flags))
801 handler_fn = irq_forced_thread_fn;
802 else
803 handler_fn = irq_thread_fn;
804
805 sched_setscheduler(current, SCHED_FIFO, ¶m);
806 current->irq_thread = 1;
807
808 while (!irq_wait_for_interrupt(action)) {
809 irqreturn_t action_ret;
810
811 irq_thread_check_affinity(desc, action);
812
813 action_ret = handler_fn(desc, action);
814 if (!noirqdebug)
815 note_interrupt(action->irq, desc, action_ret);
816
817 wake_threads_waitq(desc);
818 }
819
820 /*
821 * This is the regular exit path. __fr
其中的813行会调用最终的被赋值给thread_fn的原来的handler，这样原来的中断顶半部就整个在irq_thread里面执行了，实现了所谓的顶半部的线程化。
绕开顶半部线程化
当然，在使能了RT-Preempt的情况之下，我们仍然可以绕开顶半部线程化的过程，避免前面的强势变更，只需要申请中断的时候设置IRQ_NOTHREAD标志，如其中的patch：
Subject: arm: Mark pmu interupt IRQF_NO_THREAD
From: Thomas Gleixner <tglx@linutronix.de>
Date: Wed, 16 Mar 2011 14:45:31 +0100
PMU interrupt must not be threaded. Remove IRQF_DISABLED while at it
as we run all handlers with interrupts disabled anyway.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
arch/arm/kernel/perf_event.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
Index: linux-stable/arch/arm/kernel/perf_event.c
===================================================================
--- linux-stable.orig/arch/arm/kernel/perf_event.c
+++ linux-stable/arch/arm/kernel/perf_event.c
@@ -430,7 +430,7 @@ armpmu_reserve_hardware(struct arm_pmu *
}
err = request_irq(irq, handle_irq,
- IRQF_DISABLED | IRQF_NOBALANCING,
+ IRQF_NOBALANCING | IRQF_NO_THREAD,
"arm-pmu", armpmu);
if (err) {
r_err("unable to request IRQ%d for ARM PMU counters\n",
http://blog.51cto.com/21cnbao/1031702
