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 include/asm-ia64/system.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.

2. Interrupt status
By default, the switch_to arch function is called with interrupts
disabled. Interrupts may be enabled over the call if it is likely to
introduce a significant interrupt latency by adding the line
`#define __ARCH_WANT_INTERRUPTS_ON_CTXSW` in the same place as for
unlocked context switches. This define also implies
`__ARCH_WANT_UNLOCKED_CTXSW`. See include/asm-arm/system.h for an
example.


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/i386/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)