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+        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)
+