1	CPU Scheduler implementation hints for architecture specific code
2
3	Nick Piggin, 2005
4
5Context switch
6==============
71. Runqueue locking
8By default, the switch_to arch function is called with the runqueue
9locked. This is usually not a problem unless switch_to may need to
10take the runqueue lock. This is usually due to a wake up operation in
11the context switch. See arch/ia64/include/asm/switch_to.h for an example.
12
13To request the scheduler call switch_to with the runqueue unlocked,
14you must `#define __ARCH_WANT_UNLOCKED_CTXSW` in a header file
15(typically the one where switch_to is defined).
16
17Unlocked context switches introduce only a very minor performance
18penalty to the core scheduler implementation in the CONFIG_SMP case.
19
20CPU idle
21========
22Your cpu_idle routines need to obey the following rules:
23
241. Preempt should now disabled over idle routines. Should only
25   be enabled to call schedule() then disabled again.
26
272. need_resched/TIF_NEED_RESCHED is only ever set, and will never
28   be cleared until the running task has called schedule(). Idle
29   threads need only ever query need_resched, and may never set or
30   clear it.
31
323. When cpu_idle finds (need_resched() == 'true'), it should call
33   schedule(). It should not call schedule() otherwise.
34
354. The only time interrupts need to be disabled when checking
36   need_resched is if we are about to sleep the processor until
37   the next interrupt (this doesn't provide any protection of
38   need_resched, it prevents losing an interrupt).
39
40	4a. Common problem with this type of sleep appears to be:
41	        local_irq_disable();
42	        if (!need_resched()) {
43	                local_irq_enable();
44	                *** resched interrupt arrives here ***
45	                __asm__("sleep until next interrupt");
46	        }
47
485. TIF_POLLING_NRFLAG can be set by idle routines that do not
49   need an interrupt to wake them up when need_resched goes high.
50   In other words, they must be periodically polling need_resched,
51   although it may be reasonable to do some background work or enter
52   a low CPU priority.
53
54   	5a. If TIF_POLLING_NRFLAG is set, and we do decide to enter
55	    an interrupt sleep, it needs to be cleared then a memory
56	    barrier issued (followed by a test of need_resched with
57	    interrupts disabled, as explained in 3).
58
59arch/x86/kernel/process.c has examples of both polling and
60sleeping idle functions.
61
62
63Possible arch/ problems
64=======================
65
66Possible arch problems I found (and either tried to fix or didn't):
67
68ia64 - is safe_halt call racy vs interrupts? (does it sleep?) (See #4a)
69
70sh64 - Is sleeping racy vs interrupts? (See #4a)
71
72sparc - IRQs on at this point(?), change local_irq_save to _disable.
73      - TODO: needs secondary CPUs to disable preempt (See #1)
74
75