1=================================================================
2CPU Scheduler implementation hints for architecture specific code
3=================================================================
4
5	Nick Piggin, 2005
6
7Context switch
8==============
91. Runqueue locking
10By default, the switch_to arch function is called with the runqueue
11locked. This is usually not a problem unless switch_to may need to
12take the runqueue lock. This is usually due to a wake up operation in
13the context switch. See arch/ia64/include/asm/switch_to.h for an example.
14
15To request the scheduler call switch_to with the runqueue unlocked,
16you must `#define __ARCH_WANT_UNLOCKED_CTXSW` in a header file
17(typically the one where switch_to is defined).
18
19Unlocked context switches introduce only a very minor performance
20penalty to the core scheduler implementation in the CONFIG_SMP case.
21
22CPU idle
23========
24Your cpu_idle routines need to obey the following rules:
25
261. Preempt should now disabled over idle routines. Should only
27   be enabled to call schedule() then disabled again.
28
292. need_resched/TIF_NEED_RESCHED is only ever set, and will never
30   be cleared until the running task has called schedule(). Idle
31   threads need only ever query need_resched, and may never set or
32   clear it.
33
343. When cpu_idle finds (need_resched() == 'true'), it should call
35   schedule(). It should not call schedule() otherwise.
36
374. The only time interrupts need to be disabled when checking
38   need_resched is if we are about to sleep the processor until
39   the next interrupt (this doesn't provide any protection of
40   need_resched, it prevents losing an interrupt):
41
42	4a. Common problem with this type of sleep appears to be::
43
44	        local_irq_disable();
45	        if (!need_resched()) {
46	                local_irq_enable();
47	                *** resched interrupt arrives here ***
48	                __asm__("sleep until next interrupt");
49	        }
50
515. TIF_POLLING_NRFLAG can be set by idle routines that do not
52   need an interrupt to wake them up when need_resched goes high.
53   In other words, they must be periodically polling need_resched,
54   although it may be reasonable to do some background work or enter
55   a low CPU priority.
56
57      - 5a. If TIF_POLLING_NRFLAG is set, and we do decide to enter
58	an interrupt sleep, it needs to be cleared then a memory
59	barrier issued (followed by a test of need_resched with
60	interrupts disabled, as explained in 3).
61
62arch/x86/kernel/process.c has examples of both polling and
63sleeping idle functions.
64
65
66Possible arch/ problems
67=======================
68
69Possible arch problems I found (and either tried to fix or didn't):
70
71ia64 - is safe_halt call racy vs interrupts? (does it sleep?) (See #4a)
72
73sh64 - Is sleeping racy vs interrupts? (See #4a)
74
75sparc - IRQs on at this point(?), change local_irq_save to _disable.
76      - TODO: needs secondary CPUs to disable preempt (See #1)
77