CPU Scheduler implementation hints for architecture specific code
  
  	Nick Piggin, 2005
  
  Context switch
  ==============
  1. Runqueue locking
  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/switch_to.h for an example.

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

  CPU idle
  ========
  Your cpu_idle routines need to obey the following rules:
  
  1. Preempt should now disabled over idle routines. Should only
     be enabled to call schedule() then disabled again.
  
  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.
  
  3. When cpu_idle finds (need_resched() == 'true'), it should call
     schedule(). It should not call schedule() otherwise.
  
  4. The only time interrupts need to be disabled when checking
     need_resched 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).
  
  	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.
  
     	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).

  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):

  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)