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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 arch/ia64/include/asm/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 arch/arm/include/asm/system.h for an
-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):
-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)