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CPU Scheduler implementation hints for architecture specific code
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=================================================================
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Nick Piggin, 2005
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Context switch
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==============
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1. Runqueue locking
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By default, the switch_to arch function is called with the runqueue
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locked. This is usually not a problem unless switch_to may need to
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take the runqueue lock. This is usually due to a wake up operation in
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the context switch. See arch/ia64/include/asm/switch_to.h for an example.
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To request the scheduler call switch_to with the runqueue unlocked,
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you must `#define __ARCH_WANT_UNLOCKED_CTXSW` in a header file
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(typically the one where switch_to is defined).
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Unlocked context switches introduce only a very minor performance
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penalty to the core scheduler implementation in the CONFIG_SMP case.
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CPU idle
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========
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Your cpu_idle routines need to obey the following rules:
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1. Preempt should now disabled over idle routines. Should only
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be enabled to call schedule() then disabled again.
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2. need_resched/TIF_NEED_RESCHED is only ever set, and will never
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be cleared until the running task has called schedule(). Idle
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threads need only ever query need_resched, and may never set or
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clear it.
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3. When cpu_idle finds (need_resched() == 'true'), it should call
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schedule(). It should not call schedule() otherwise.
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4. The only time interrupts need to be disabled when checking
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need_resched is if we are about to sleep the processor until
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the next interrupt (this doesn't provide any protection of
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need_resched, it prevents losing an interrupt):
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4a. Common problem with this type of sleep appears to be::
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local_irq_disable();
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if (!need_resched()) {
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local_irq_enable();
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*** resched interrupt arrives here ***
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__asm__("sleep until next interrupt");
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}
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5. TIF_POLLING_NRFLAG can be set by idle routines that do not
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need an interrupt to wake them up when need_resched goes high.
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In other words, they must be periodically polling need_resched,
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although it may be reasonable to do some background work or enter
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a low CPU priority.
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- 5a. If TIF_POLLING_NRFLAG is set, and we do decide to enter
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an interrupt sleep, it needs to be cleared then a memory
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barrier issued (followed by a test of need_resched with
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interrupts disabled, as explained in 3).
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arch/x86/kernel/process.c has examples of both polling and
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sleeping idle functions.
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Possible arch/ problems
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=======================
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Possible arch problems I found (and either tried to fix or didn't):
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ia64 - is safe_halt call racy vs interrupts? (does it sleep?) (See #4a)
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sh64 - Is sleeping racy vs interrupts? (See #4a)
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sparc - IRQs on at this point(?), change local_irq_save to _disable.
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- TODO: needs secondary CPUs to disable preempt (See #1)
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