Documentation_scheduler_sched-arch.txt

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以下为正文
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 CPU Scheduler implementation hints for architecture specific code

 CPU调度实现架构特定的代码提示
 
 Nick Piggin, 2005

Context switch

上下文切换

==============
1. Runqueue locking

1。运行队列锁

By default, the switch_to arch function is called with the runqueue
locked. This is usually not a problem unless switch_to may need to
take the runqueue lock. This is usually due to a wake up operation in
the context switch. See arch/ia64/include/asm/system.h for an example.

默认情况下，switch_to主要函数随着运行队列锁定被调用。这通常不是问题，除
非switch_to可能需要运行队列锁。这通常是由于上下文切换中的唤醒操作。见一
个例子arch/ia64/include/asm/system.h。

To request the scheduler call switch_to with the runqueue unlocked,
you must `#define __ARCH_WANT_UNLOCKED_CTXSW` in a header file
(typically the one where switch_to is defined).

要求调度运行随着队列解锁调用switch_to你必须定义__ ARCH_WANT_UNLOCKED_CTXSW`
在头文件（通常switch_to定义的一方）。

Unlocked context switches introduce only a very minor performance
penalty to the core scheduler implementation in the CONFIG_SMP case.

解锁上下文切换只介绍一个非常次要的表现CONFIG_SMP情况下的核心调度实施的罚款。

CPU idle

CPU空闲
========
Your cpu_idle routines need to obey the following rules:

你的cpu_idle程序需要遵守以下规则：

1. Preempt should now disabled over idle routines. Should only
   be enabled to call schedule() then disabled again.
  
1. 抢占现在应该禁用超过闲置例程。应该只启用调用schedule（），然后再禁用。

2. need_resched/TIF_NEED_RESCHED is only ever set, and will never
   be cleared until the running task has called schedule(). Idle
   threads need only ever query need_resched, and may never set or
   clear it.
  
2. need_resched/TIF_NEED_RESCHED只是设置也永远不会被清零，直到正在运行的
   任务调用schedule（）。空闲线程只需要查询need_resched，而且可能永远不会
   设置或清除它。

3. When cpu_idle finds (need_resched() == 'true'), it should call
   schedule(). It should not call schedule() otherwise.
  
   3。当cpu_idle发现（need_resched（）=='true'），它应该调用schedule（）。
   否则它不应该调用schedule（）。

4. The only time interrupts need to be disabled when checking
    is if we are about to sleep the processor until
   the next interrupt (this doesn't provide any protection of
   need_resched, it prevents losing an interrupt).
  
4. 唯一一次中断需要禁用当检查need_resched是否是我们停用的处理器，直到
   下一个中断，（不提供任何保护need_resched，它可以防止丢失中断）。
  
   need_resched

 4a. Common problem with this type of sleep appears to be:
         local_irq_disable();
         if (!need_resched()) {
                 local_irq_enable();
                 *** resched interrupt arrives here ***
                 __asm__("sleep until next interrupt");
         }

5. TIF_POLLING_NRFLAG can be set by idle routines that do not
   need an interrupt to wake them up when need_resched goes high.
   In other words, they must be periodically polling need_resched,
   although it may be reasonable to do some background work or enter
   a low CPU priority.
  
5. TIF_POLLING_NRFLAG可以由闲置例程设置而不需要当need_resched高电平时
   中断唤醒他们。换句话说，它们必须是定期轮询need_resched 虽然它可能是
   合理的做一些后台的工作，或进入低的CPU优先级。

    5a. If TIF_POLLING_NRFLAG is set, and we do decide to enter
     an interrupt sleep, it needs to be cleared then a memory
     barrier issued (followed by a test of need_resched with
     interrupts disabled, as explained in 3).
  
  如果TIF_POLLING_NRFLAG被设置，并且我们决定进入中断的睡眠，它需
  要被清除，然后发出一个内存屏障（need_resched禁用中断测试，如解
  释3）。

arch/x86/kernel/process.c has examples of both polling and
sleeping idle functions.

Possible arch/ problems
=======================

Possible arch problems I found (and either tried to fix or didn't):

h8300 - Is such sleeping racy vs interrupts? (See #4a).
        The H8/300 manual I found indicates yes, however disabling IRQs
        over the sleep mean only NMIs can wake it up, so can't fix easily
        without doing spin waiting.

ia64 - is safe_halt call racy vs interrupts? (does it sleep?) (See #4a)

sh64 - Is sleeping racy vs interrupts? (See #4a)

sparc - IRQs on at this point(?), change local_irq_save to _disable.
      - TODO: needs secondary CPUs to disable preempt (See #1)