Linux调度-关于抢占的几个配置

config PREEMPT_NONE
	bool "No Forced Preemption (Server)"
	help
	  This is the traditional Linux preemption model, geared towards
 	  throughput. It will still provide good latencies most of the
 	  time, but there are no guarantees and occasional longer delays
 	  are possible.
 
 	  Select this option if you are building a kernel for a server or
 	  scientific/computation system, or if you want to maximize the
 	  raw processing power of the kernel, irrespective of scheduling
 	  latencies.
 
config PREEMPT_VOLUNTARY
	bool "Voluntary Kernel Preemption (Desktop)"
	help
	  This option reduces the latency of the kernel by adding more
	  "explicit preemption points" to the kernel code. These new
	  preemption points have been selected to reduce the maximum
	  latency of rescheduling, providing faster application reactions,
	  at the cost of slightly lower throughput.

	  This allows reaction to interactive events by allowing a
	  low priority process to voluntarily preempt itself even if it
	  is in kernel mode executing a system call. This allows
	  applications to run more 'smoothly' even when the system is
	  under load.

	  Select this if you are building a kernel for a desktop system.

config PREEMPT
	bool "Preemptible Kernel (Low-Latency Desktop)"
	select PREEMPT_COUNT
	select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
	help
	  This option reduces the latency of the kernel by making
	  all kernel code (that is not executing in a critical section)
	  preemptible.  This allows reaction to interactive events by
	  permitting a low priority process to be preempted involuntarily
	  even if it is in kernel mode executing a system call and would
	  otherwise not be about to reach a natural preemption point.
	  This allows applications to run more 'smoothly' even when the
	  system is under load, at the cost of slightly lower throughput
	  and a slight runtime overhead to kernel code.

	  Select this if you are building a kernel for a desktop or
	  embedded system with latency requirements in the milliseconds
	  range.

PREEMPT_NONE：就是禁止内核抢占，进程陷入内核态之后只能在返回用户态时才能被抢占

CONFIG_PREEMPT：启用内核抢占，进程从中断返回内核时，可以执行内核抢占

CONFIG_PREEMPT_VOLUNTARY：在不选CONFIG_PREEMPT没啥用，是空实现。如果选了通过会内核中的might_sleep会变成抢占点：

#ifndef CONFIG_PREEMPT
extern int _cond_resched(void);
#else
static inline int _cond_resched(void) { return 0; }
#endif

#ifdef CONFIG_PREEMPT_VOLUNTARY
extern int _cond_resched(void);
# define might_resched() _cond_resched()
#else
# define might_resched() do { } while (0)
#endif

#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
# define might_sleep() \
	do { __might_sleep(__FILE__, __LINE__, 0); might_resched(); } while (0)
#else
# define might_sleep() do { might_resched(); } while (0)
#endif

但是抢占点太多也不是什么好事，频繁上下文切换会导致系统的吞吐量下降。

RTLinux的配置添加了PREEMPT_RT_BASE和PREEMPT_RT_FULL两个选项，用于实现硬实时。