CONFIG_PREEMPT 和 CONFIG_PREEMPT_VOLUNTARY的区别

在kernel/kernel/Kconfig.preempt中定义了两种抢占策略，PREEMPT_VOLUNTARY和PREEMPT
其中PREEMPT_VOLUNTARY 适用于有桌面的环境，而PREEMPT 则可以用于桌面或者嵌入式，其调度延迟是毫秒级别的。详细说明如下：
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

从code中看主要影响下面这两个函数
static inline void crypto_yield(u32 flags)
{
#if !defined(CONFIG_PREEMPT) || defined(CONFIG_PREEMPT_VOLUNTARY)
    if (flags & CRYPTO_TFM_REQ_MAY_SLEEP)
        cond_resched();
#endif
}
在crypto_yield 中如果定义了CONFIG_PREEMPT_VOLUNTARY 就调度，而CONFIG_PREEMPT 则是不调度的，继续留在原来的task中
和
#ifdef CONFIG_PREEMPT_VOLUNTARY
extern int _cond_resched(void);
# define might_resched() _cond_resched()
#else
# define might_resched() do { } while (0)
#endif
如果code中调用might_resched的话，就发生调度。

从code中看CONFIG_PREEMPT_VOLUNTARY 就是会比 CONFIG_PREEMPT 的调度延迟少一点。