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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
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)
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