當從內核態返回到用戶態的時候，要檢查是否進行調度，而調度要看兩個條件： 1.preempt_count是否為02.rescheduled是否置位ret_from_exception:
?preempt_stop(CLBR_ANY)
ret_from_intr:
?GET_THREAD_INFO(%ebp)
check_userspace:
?movl PT_EFLAGS(%esp), %eax?# mix EFLAGS and CS
?movb PT_CS(%esp), %al
?andl $(VM_MASK | SEGMENT_RPL_MASK), %eax
?cmpl $USER_RPL, %eax
?jb resume_kernel??# not returning to v8086 or userspaceENTRY(resume_userspace)
?LOCKDEP_SYS_EXIT
??DISABLE_INTERRUPTS(CLBR_ANY)?# make sure we don't miss an interrupt
?????# setting need_resched or sigpending
?????# between sampling and the iret
?movl TI_flags(%ebp), %ecx
?andl $_TIF_WORK_MASK, %ecx?# is there any work to be done on
?????# int/exception return?
?jne work_pending
?jmp restore_all
END(ret_from_exception)#ifdef CONFIG_PREEMPT
ENTRY(resume_kernel)
?DISABLE_INTERRUPTS(CLBR_ANY)
?cmpl $0,TI_preempt_count(%ebp)?# non-zero preempt_count ?
?jnz restore_nocheck
need_resched:
?movl TI_flags(%ebp), %ecx?# need_resched set ?
?testb $_TIF_NEED_RESCHED, %cl
?jz restore_all
?testl $IF_MASK,PT_EFLAGS(%esp)?# interrupts off (exception path) ?
?jz restore_all
?call preempt_schedule_irq
?jmp need_resched
END(resume_kernel)
#endif
?CFI_ENDPROC檢查preempt_count的時候，是統一檢查是否為0，也就是說，有4個條件限制，可能不能夠進行調度。1.preempt_disable()?#define preempt_disable() \
do { \
?inc_preempt_count(); \
?barrier(); \
} while (0)#define inc_preempt_count() add_preempt_count(1)會在preempt_enable中釋放2.add_preempt_count(HARDIRQ_OFFSET)是在irq_enter()中調用的，記錄進入硬件中斷處理的次數（計數），會在irq_exit()中釋放3.__local_bh_disable((unsigned long)__builtin_return_address(0));在do_softirq中調用的，static inline void __local_bh_disable(unsigned long ip)
{
?add_preempt_count(SOFTIRQ_OFFSET);
?barrier();
}4.總開關，第一位。

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