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 interruptsn",

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

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

---

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 countersn",