最近在爲3.8版本的Linux內核打RT_PREEMPT補丁，並且優化系統實時性，這篇文章主要對RTlinux中中斷線程化部分進行分析。我們知道在RT_PREEMPT補丁中之所以要將中斷線程化就是因爲硬中斷的實時性太高，會影響實時進程的實時性，所以需要將中斷處理程序線程化並設置優先級，使中斷處理線程的優先級比實時進程優先級低，從而提高系統實時性。
網上看到一些網友說在2.6.25.8版本的內核，linux引入了中斷線程化，具體是不是2.6.25.8版本開始引入中斷線程化我沒有去求證，因爲版本比較老了改動很多，但據我的查證從2.6.30開始內核引入request_threaded_irq函數，從這個版本開始可以通過在申請中斷時爲request_irq設置不同的參數決定是否線程化該中斷。而在2.6.39版內核__setup_irq引入irq_setup_forced_threading函數，開始可以通過# define force_irqthreads(true)強制使中斷線程化，那麼從這個版本開始想實現中斷線程化就已經變得很簡單了，讓force_irqthreads爲真即可，所以在3.8版本的實時補丁中，正是這一段代碼實現了中斷的線程化：
#ifdef CONFIG_IRQ_FORCED_THREADING
-extern bool force_irqthreads;
+# ifndef CONFIG_PREEMPT_RT_BASE
+ extern bool force_irqthreads;
+# else
+# define force_irqthreads (true)
+# endif
#else
-#define force_irqthreads (0)
+#define force_irqthreads (false)
#endifLinux內核常見申請中斷的函數request_irq，在內核源碼include/linux/interrupt.h頭文件中可以看到request_irq僅包含return request_threaded_irq(irq, handler, NULL, flags, name, dev);調用，request_threaded_irq函數在源碼目錄kernel/irq/manage.c文件中，下面通過分析manage.c中各個相關函數解讀中斷線程化的實現過程。
根據request_irq的調用，首先分析request_threaded_irq
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)
{
struct irqaction *action;
struct irq_desc *desc;
int retval;
/*
* Sanity-check: shared interrupts must pass in a real dev-ID,
* otherwise we'll have trouble later trying to figure out
* which interrupt is which (messes up the interrupt freeing
* logic etc).
*/
if ((irqflags & IRQF_SHARED) && !dev_id) //共享中斷必須有唯一確定的設備號，不然中斷處理函數找不到發出中斷請求的設備，註釋寫的很清楚
return -EINVAL;
desc = irq_to_desc(irq);
if (!desc)
return -EINVAL;
if (!irq_settings_can_request(desc) ||
WARN_ON(irq_settings_is_per_cpu_devid(desc)))
return -EINVAL;
if (!handler) { //handler和thread_fn都沒有指針傳入肯定是出錯了，有thread_fn無handler則將irq_default_primary_handler給handler
if (!thread_fn)
return -EINVAL;
handler = irq_default_primary_handler;
}
action = kzalloc(sizeof(struct irqaction), GFP_KERNEL);
if (!action)
return -ENOMEM;
action->handler = handler;
action->thread_fn = thread_fn;
action->flags = irqflags;
action->name = devname;
action->dev_id = dev_id;
chip_bus_lock(desc);
retval = __setup_irq(irq, desc, action); //在__setup_irq中確定是否線程化並完成中斷處理函數綁定
chip_bus_sync_unlock(desc);
if (retval)
kfree(action);
#ifdef CONFIG_DEBUG_SHIRQ_FIXME
if (!retval && (irqflags & IRQF_SHARED)) {
/*
* It's a shared IRQ -- the driver ought to be prepared for it
* to happen immediately, so let's make sure....
* We disable the irq to make sure that a 'real' IRQ doesn't
* run in parallel with our fake.
*/
unsigned long flags;
disable_irq(irq);
local_irq_save(flags);
handler(irq, dev_id);
local_irq_restore(flags);
enable_irq(irq);
}
#endif
return retval;
}static int
__setup_irq(unsigned int irq, struct irq_desc *desc, struct irqaction *new)
{
struct irqaction *old, **old_ptr;
unsigned long flags, thread_mask = 0;
int ret, nested, shared = 0;
cpumask_var_t mask;
if (!desc)
return -EINVAL;
if (desc->irq_data.chip == &no_irq_chip)
return -ENOSYS;
if (!try_module_get(desc->owner))
return -ENODEV;
/*
* Check whether the interrupt nests into another interrupt
* thread.
*/
nested = irq_settings_is_nested_thread(desc);
if (nested) {
if (!new->thread_fn) {
ret = -EINVAL;
goto out_mput;
}
/*
* Replace the primary handler which was provided from
* the driver for non nested interrupt handling by the
* dummy function which warns when called.
*/
new->handler = irq_nested_primary_handler;
} else {
if (irq_settings_can_thread(desc)) //request_irq調用通過設置參數_IRQ_NOTHREAD=0線程化，
//沒有手動設置IRQ_NOTHREAD=1的中斷都被線程化。Linux內核從2.6.39版本開始對中斷線程化
irq_setup_forced_threading(new); //實時補丁使force_irqthreads=true，開啓強制線程化中斷
}
/*
* Create a handler thread when a thread function is supplied
* and the interrupt does not nest into another interrupt
* thread.
*/
if (new->thread_fn && !nested) {
struct task_struct *t;
static const struct sched_param param = {
.sched_priority = MAX_USER_RT_PRIO/2, //所有被線程化中斷優先級都爲50
};
t = kthread_create(irq_thread, new, "irq/%d-%s", irq, //爲中斷創建內核線程
new->name);
if (IS_ERR(t)) {
ret = PTR_ERR(t);
goto out_mput;
}
sched_setscheduler(t, SCHED_FIFO, ¶m);
/*
* We keep the reference to the task struct even if
* the thread dies to avoid that the interrupt code
* references an already freed task_struct.
*/
get_task_struct(t);
new->thread = t;
/*
* Tell the thread to set its affinity. This is
* important for shared interrupt handlers as we do
* not invoke setup_affinity() for the secondary
* handlers as everything is already set up. Even for
* interrupts marked with IRQF_NO_BALANCE this is
* correct as we want the thread to move to the cpu(s)
* on which the requesting code placed the interrupt.
*/
set_bit(IRQTF_AFFINITY, &new->thread_flags);
}
if (!alloc_cpumask_var(&mask, GFP_KERNEL)) {
ret = -ENOMEM;
goto out_thread;
}
/*
* Drivers are often written to work w/o knowledge about the
* underlying irq chip implementation, so a request for a
* threaded irq without a primary hard irq context handler
* requires the ONESHOT flag to be set. Some irq chips like
* MSI based interrupts are per se one shot safe. Check the
* chip flags, so we can avoid the unmask dance at the end of
* the threaded handler for those.
*/
if (desc->irq_data.chip->flags & IRQCHIP_ONESHOT_SAFE)
new->flags &= ~IRQF_ONESHOT;
/*
* The following block of code has to be executed atomically
*/
raw_spin_lock_irqsave(&desc->lock, flags);
old_ptr = &desc->action;
old = *old_ptr; //action和desc都是指針，用指向指針的指針獲取action的地址，再使old指向action
if (old) { //如果該中斷號的處理程序鏈表desc->action本身就是空，就無所謂共享了
/*
* Can't share interrupts unless both agree to and are
* the same type (level, edge, polarity). So both flag
* fields must have IRQF_SHARED set and the bits which
* set the trigger type must match. Also all must
* agree on ONESHOT.
*/
if (!((old->flags & new->flags) & IRQF_SHARED) ||
((old->flags ^ new->flags) & IRQF_TRIGGER_MASK) ||
((old->flags ^ new->flags) & IRQF_ONESHOT))
goto mismatch;
/* All handlers must agree on per-cpuness */
if ((old->flags & IRQF_PERCPU) !=
(new->flags & IRQF_PERCPU))
goto mismatch;
/* add new interrupt at end of irq queue */
do {
/*
* Or all existing action->thread_mask bits,
* so we can find the next zero bit for this
* new action.
*/
thread_mask |= old->thread_mask;
old_ptr = &old->next;
old = *old_ptr; //在desc->action鏈表中找到空指針，爲裏後面將new加進去
} while (old);
shared = 1;
}
/*
* Setup the thread mask for this irqaction for ONESHOT. For
* !ONESHOT irqs the thread mask is 0 so we can avoid a
* conditional in irq_wake_thread().
*/
if (new->flags & IRQF_ONESHOT) {
/*
* Unlikely to have 32 resp 64 irqs sharing one line,
* but who knows.
*/
if (thread_mask == ~0UL) {
ret = -EBUSY;
goto out_mask;
}
/*
* The thread_mask for the action is or'ed to
* desc->thread_active to indicate that the
* IRQF_ONESHOT thread handler has been woken, but not
* yet finished. The bit is cleared when a thread
* completes. When all threads of a shared interrupt
* line have completed desc->threads_active becomes
* zero and the interrupt line is unmasked. See
* handle.c:irq_wake_thread() for further information.
*
* If no thread is woken by primary (hard irq context)
* interrupt handlers, then desc->threads_active is
* also checked for zero to unmask the irq line in the
* affected hard irq flow handlers
* (handle_[fasteoi|level]_irq).
*
* The new action gets the first zero bit of
* thread_mask assigned. See the loop above which or's
* all existing action->thread_mask bits.
*/
new->thread_mask = 1 << ffz(thread_mask);
} else if (new->handler == irq_default_primary_handler &&
!(desc->irq_data.chip->flags & IRQCHIP_ONESHOT_SAFE)) {
/*
* The interrupt was requested with handler = NULL, so
* we use the default primary handler for it. But it
* does not have the oneshot flag set. In combination
* with level interrupts this is deadly, because the
* default primary handler just wakes the thread, then
* the irq lines is reenabled, but the device still
* has the level irq asserted. Rinse and repeat....
*
* While this works for edge type interrupts, we play
* it safe and reject unconditionally because we can't
* say for sure which type this interrupt really
* has. The type flags are unreliable as the
* underlying chip implementation can override them.
*/
pr_err("Threaded irq requested with handler=NULL and !ONESHOT for irq %d\n",
irq);
ret = -EINVAL;
goto out_mask;
}
if (!shared) { //中斷處理鏈表爲空，自己創建鏈表
init_waitqueue_head(&desc->wait_for_threads);
/* Setup the type (level, edge polarity) if configured: */
if (new->flags & IRQF_TRIGGER_MASK) {
ret = __irq_set_trigger(desc, irq,
new->flags & IRQF_TRIGGER_MASK);
if (ret)
goto out_mask;
}
desc->istate &= ~(IRQS_AUTODETECT | IRQS_SPURIOUS_DISABLED | \
IRQS_ONESHOT | IRQS_WAITING);
irqd_clear(&desc->irq_data, IRQD_IRQ_INPROGRESS);
if (new->flags & IRQF_PERCPU) {
irqd_set(&desc->irq_data, IRQD_PER_CPU);
irq_settings_set_per_cpu(desc);
}
if (new->flags & IRQF_ONESHOT)
desc->istate |= IRQS_ONESHOT;
if (irq_settings_can_autoenable(desc))
irq_startup(desc, true);
else
/* Undo nested disables: */
desc->depth = 1;
/* Exclude IRQ from balancing if requested */
if (new->flags & IRQF_NOBALANCING) {
irq_settings_set_no_balancing(desc);
irqd_set(&desc->irq_data, IRQD_NO_BALANCING);
}
if (new->flags & IRQF_NO_SOFTIRQ_CALL)
irq_settings_set_no_softirq_call(desc);
/* Set default affinity mask once everything is setup */
setup_affinity(irq, desc, mask);
} else if (new->flags & IRQF_TRIGGER_MASK) {
unsigned int nmsk = new->flags & IRQF_TRIGGER_MASK;
unsigned int omsk = irq_settings_get_trigger_mask(desc);
if (nmsk != omsk)
/* hope the handler works with current trigger mode */
pr_warning("irq %d uses trigger mode %u; requested %u\n",
irq, nmsk, omsk);
}
new->irq = irq;
*old_ptr = new; //添加到desc->action鏈表
/* Reset broken irq detection when installing new handler */
desc->irq_count = 0;
desc->irqs_unhandled = 0;
/*
* Check whether we disabled the irq via the spurious handler
* before. Reenable it and give it another chance.
*/
if (shared && (desc->istate & IRQS_SPURIOUS_DISABLED)) {
desc->istate &= ~IRQS_SPURIOUS_DISABLED;
__enable_irq(desc, irq, false);
}
raw_spin_unlock_irqrestore(&desc->lock, flags);
/*
* Strictly no need to wake it up, but hung_task complains
* when no hard interrupt wakes the thread up.
*/
if (new->thread)
wake_up_process(new->thread); //內核線程開始運行
register_irq_proc(irq, desc); //創建/proc/irq/目錄及文件（smp_affinity，smp_affinity_list 等 ）
new->dir = NULL;
register_handler_proc(irq, new);
free_cpumask_var(mask); //創建proc/irq/<irq>/handler/
return 0;
mismatch:
if (!(new->flags & IRQF_PROBE_SHARED)) {
pr_err("Flags mismatch irq %d. %08x (%s) vs. %08x (%s)\n",
irq, new->flags, new->name, old->flags, old->name);
#ifdef CONFIG_DEBUG_SHIRQ
dump_stack();
#endif
}
ret = -EBUSY;
out_mask:
raw_spin_unlock_irqrestore(&desc->lock, flags);
free_cpumask_var(mask);
out_thread:
if (new->thread) {
struct task_struct *t = new->thread;
new->thread = NULL;
kthread_stop(t);
put_task_struct(t);
}
out_mput:
module_put(desc->owner);
return ret;
}接下來因爲是首次在該中斷線創建處理函數，申請一個內核線程，設置線程調度策略（FIFO）和優先級（50），爲了讓使用該中斷號的其他進程共享這條中斷線，還必須建立一箇中斷處理進程action的單向鏈表，設置一些共享標識等。但是如果現在申請的這個中斷與其他已經建立中斷內核線程的中斷共享中斷線，那麼就不需要再次建立內核線程和隊列，只需在隊列中找到空指針（一般是末尾）並插入隊列即可。做完這些之後喚醒內核進程（kthread_create）創建的內核進程不能馬上執行，需要喚醒。
在Linux中申請中斷還可以通過request_any_context_irq、devm_request_threaded_irq等函數，他們最終都調用request_threaded_irq，request_threaded_irq函數的完整形式如下：
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)
