LInux中ThreadInfo中的preempt_count字段

最近看各种上下文，发现和ThreadInfo中的preemption字段密切，于是便调查了下。

看下Linux源码中的注释：

/*
* We put the hardirq and softirq counter into the preemption
* counter. The bitmask has the following meaning:
*
* - bits 0-7 are the preemption count (max preemption depth: 256)
* - bits 8-15 are the softirq count (max # of softirqs: 256)
*
* The hardirq count can in theory reach the same as NR_IRQS.
* In reality, the number of nested IRQS is limited to the stack
* size as well. For archs with over 1000 IRQS it is not practical
* to expect that they will all nest. We give a max of 10 bits for
* hardirq nesting. An arch may choose to give less than 10 bits.
* m68k expects it to be 8.
*
* - bits 16-25 are the hardirq count (max # of nested hardirqs: 1024)
* - bit 26 is the NMI_MASK
* - bit 27 is the PREEMPT_ACTIVE flag
*
* PREEMPT_MASK: 0x000000ff
* SOFTIRQ_MASK: 0x0000ff00
* HARDIRQ_MASK: 0x03ff0000
* NMI_MASK: 0x04000000
*/

这里其实把preempt_count划分成了四部分：抢占计数器、软中断计数、硬件中断计数、NMI计数。

抢占计数器：0-7位

软中断计数器：8-15位

硬中断计数器：16-25位

NMI标识：26位

基于上面的信息，看下Linux内核中判断各个上下文的宏：

#define hardirq_count()    (preempt_count() & HARDIRQ_MASK)
#define softirq_count()    (preempt_count() & SOFTIRQ_MASK)
#define irq_count()    (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_MASK   | NMI_MASK))

可以看到这里判断硬件中断/软中断/中断上下文的实质，就是根据thread_info结构中的preempt_count字段，额忘记列出preempt_count()的实现了：#define preempt_count() (current_thread_info()->preempt_count)，这时明白了吧！！！

不过后面几个掩码的计算有点绕，参考下源码：

#define __IRQ_MASK(x)    ((1UL << (x))-1)

#define PREEMPT_MASK    (__IRQ_MASK(PREEMPT_BITS) << PREEMPT_SHIFT)
#define SOFTIRQ_MASK    (__IRQ_MASK(SOFTIRQ_BITS) << SOFTIRQ_SHIFT)
#define HARDIRQ_MASK    (__IRQ_MASK(HARDIRQ_BITS) << HARDIRQ_SHIFT)
#define NMI_MASK    (__IRQ_MASK(NMI_BITS)     << NMI_SHIFT)

实质上就是取对应的位。

*
 * Are we doing bottom half or hardware interrupt processing?
 * Are we in a softirq context? Interrupt context?
 * in_softirq - Are we currently processing softirq or have bh disabled?
 * in_serving_softirq - Are we currently processing softirq?
 */
#define in_irq()        (hardirq_count())
#define in_softirq()        (softirq_count())
#define in_interrupt()        (irq_count())
#define in_serving_softirq()    (softirq_count() & SOFTIRQ_OFFSET)

而这里只是对上面各种计数的包装，只要对应的计数不为0，则表示在对应的上下文中。in_interrupt表示中断上下文，包括了硬件中断、软中断和NMI中断。in_serving_softirq判断是否当前正在处理软中断。

#define in_nmi()    (preempt_count() & NMI_MASK)

in_nmi判断当前是否在NMI中断上下文。

既然如此，咱们再结合内核抢占调度的情况，看下抢占调度函数

asmlinkage void __sched notrace preempt_schedule(void)
{
    struct thread_info *ti = current_thread_info();

    /*
     * If there is a non-zero preempt_count or interrupts are disabled,
     * we do not want to preempt the current task. Just return..
     */
    if (likely(ti->preempt_count || irqs_disabled()))
        return;

    do {
        add_preempt_count_notrace(PREEMPT_ACTIVE);
        __schedule();
        sub_preempt_count_notrace(PREEMPT_ACTIVE);

        /*
         * Check again in case we missed a preemption opportunity
         * between schedule and now.
         */
        barrier();
    } while (need_resched());
}

可以看到这里进入函数起始，便对当前线程的preempt_count进行了判断，根据上面的介绍，当内核处理任何一个上下文中时，preempt_count均不可能为0，所以我们也可以根据此发现在软中断、硬件中断、禁止内核抢占、NMI上下文中均不允许调度。当然，我们看到后面后又个irqs_disabled，意味着如果当前禁用了硬件中断，则同样不会发生抢占调度。

补充：

关闭中断的实质是设置EFLAGS寄存器的IF标志位，CLI指令可执行这一操作，当标志位被设置为0时，表示当前关闭中断状态，而STI指令开中断，二者匹配使用。这里的中断指的是可屏蔽的硬件中断。

参考资料：

LInux3.10.1内核源码