preempt_count分析

kernel通过preempt_count变量来判断是否处于中断(soft/hard/nmi)上下文,是否可以抢占,先看示例

#define in_irq()     (hardirq_count())
#define hardirq_count() (preempt_count() & HARDIRQ_MASK)
static __always_inline int preempt_count(void)
{return current_thread_info()->preempt_count;
}static inline struct thread_info *current_thread_info(void) __attribute_const__;struct thread_info {unsigned long      flags;      /* low level flags */int            preempt_count;  /* 0 => preemptable, <0 => bug */mm_segment_t        addr_limit; /* address limit */struct task_struct   *task;      /* main task structure */__u32          cpu;        /* cpu */__u32          cpu_domain; /* cpu domain */struct cpu_context_save cpu_context;    /* cpu context */__u32          syscall;    /* syscall number */__u8            used_cp[16];    /* thread used copro */unsigned long        tp_value[2];    /* TLS registers */
#ifdef CONFIG_CRUNCHstruct crunch_state crunchstate;
#endifunion fp_state        fpstate __attribute__((aligned(8)));union vfp_state     vfpstate;
#ifdef CONFIG_ARM_THUMBEEunsigned long      thumbee_state;  /* ThumbEE Handler Base register */
#endif
};

preempt_count具体定义

linux/preempt.h

/** 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 could in theory be the same as the number of* interrupts in the system, but we run all interrupt handlers with* interrupts disabled, so we cannot have nesting interrupts. Though* there are a few palaeontologic drivers which reenable interrupts in* the handler, so we need more than one bit here.**         PREEMPT_MASK:    0x000000ff*         SOFTIRQ_MASK:   0x0000ff00*         HARDIRQ_MASK:   0x000f0000*             NMI_MASK:   0x00100000* PREEMPT_NEED_RESCHED:   0x80000000*/
#define PREEMPT_BITS    8
#define SOFTIRQ_BITS    8
#define HARDIRQ_BITS    4
#define NMI_BITS    1#define PREEMPT_SHIFT  0
#define SOFTIRQ_SHIFT   (PREEMPT_SHIFT + PREEMPT_BITS)
#define HARDIRQ_SHIFT   (SOFTIRQ_SHIFT + SOFTIRQ_BITS)
#define NMI_SHIFT   (HARDIRQ_SHIFT + HARDIRQ_BITS)#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)#define PREEMPT_OFFSET (1UL << PREEMPT_SHIFT)
#define SOFTIRQ_OFFSET  (1UL << SOFTIRQ_SHIFT)
#define HARDIRQ_OFFSET  (1UL << HARDIRQ_SHIFT)
#define NMI_OFFSET  (1UL << NMI_SHIFT)#define SOFTIRQ_DISABLE_OFFSET  (2 * SOFTIRQ_OFFSET)/* We use the MSB mostly because its available */
#define PREEMPT_NEED_RESCHED    0x80000000/* preempt_count() and related functions, depends on PREEMPT_NEED_RESCHED */
#include <asm/preempt.h>#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))/** Are we doing bottom half or hardware interrupt processing?** in_irq()       - We're in (hard) IRQ context* in_softirq()   - We have BH disabled, or are processing softirqs* in_interrupt() - We're in NMI,IRQ,SoftIRQ context or have BH disabled* in_serving_softirq() - We're in softirq context* in_nmi()       - We're in NMI context* in_task()    - We're in task context** Note: due to the BH disabled confusion: in_softirq(),in_interrupt() really*       should not be used in new code.*/
#define in_irq()        (hardirq_count())
#define in_softirq()        (softirq_count())
#define in_interrupt()      (irq_count())
#define in_serving_softirq()    (softirq_count() & SOFTIRQ_OFFSET)
#define in_nmi()        (preempt_count() & NMI_MASK)
#define in_task()       (!(preempt_count() & \(NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))
NMI (Non Maskable Interrupt)

硬中断会有如下流程，如PPI和SPI

int __handle_domain_irq(struct irq_domain *domain, unsigned int hwirq,bool lookup, struct pt_regs *regs)
{irq_enter();irq = irq_find_mapping(domain, hwirq);generic_handle_irq(irq);irq_exit();}void irq_enter(void){preempt_count_add(HARDIRQ_OFFSET);
}void irq_exit(void)
{preempt_count_sub(HARDIRQ_OFFSET);
}

软中断处理函数

asmlinkage __visible void __softirq_entry __do_softirq(void)
{__local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET);
restart:........................................__local_bh_enable(SOFTIRQ_OFFSET);
}static __always_inline void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
{preempt_count_add(cnt);
}
static void __local_bh_enable(unsigned int cnt)
{preempt_count_sub(cnt);
}

#define preempt_disable() \
do { \preempt_count_inc(); \barrier(); \
} while (0)
#define preempt_enable() \
do { \barrier(); \if (unlikely(preempt_count_dec_and_test())) \__preempt_schedule(); \
} while (0)

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