在kernel/kernel/Kconfig.PReempt中定義了兩種搶占策略，PREEMPT_VOLUNTARY和PREEMPT其中PREEMPT_VOLUNTARY 適用于有桌面的環境，而PREEMPT 則可以用于桌面或者嵌入式，其調度延遲是毫秒級別的。詳細說明如下：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從code中看主要影響下面這兩個函數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 中如果定義了CONFIG_PREEMPT_VOLUNTARY 就調度，而CONFIG_PREEMPT 則是不調度的，繼續留在原來的task中和#ifdef CONFIG_PREEMPT_VOLUNTARYextern int _cond_resched(void);# define might_resched() _cond_resched()#else# define might_resched() do { } while (0)#endif如果code中調用might_resched的話，就發生調度。從code中看CONFIG_PREEMPT_VOLUNTARY 就是會比 CONFIG_PREEMPT 的調度延遲少一點。
