1 =================================================================
2 CPU Scheduler implementation hints for architecture specific code
3 =================================================================
4
5 Nick Piggin, 2005
6
7 Context switch
8 ==============
9 1. Runqueue locking
10 By default, the switch_to arch function is called with the runqueue
11 locked. This is usually not a problem unless switch_to may need to
12 take the runqueue lock. This is usually due to a wake up operation in
13 the context switch.
14
15 To request the scheduler call switch_to with the runqueue unlocked,
16 you must `#define __ARCH_WANT_UNLOCKED_CTXSW` in a header file
17 (typically the one where switch_to is defined).
18
19 Unlocked context switches introduce only a very minor performance
20 penalty to the core scheduler implementation in the CONFIG_SMP case.
21
22 CPU idle
23 ========
24 Your cpu_idle routines need to obey the following rules:
25
26 1. Preempt should now disabled over idle routines. Should only
27 be enabled to call schedule() then disabled again.
28
29 2. 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
34 3. When cpu_idle finds (need_resched() == 'true'), it should call
35 schedule(). It should not call schedule() otherwise.
36
37 4. 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
51 5. 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
62 arch/x86/kernel/process.c has examples of both polling and
63 sleeping idle functions.
64
65
66 Possible arch/ problems
67 =======================
68
69 Possible arch problems I found (and either tried to fix or didn't):
70
71 sparc - IRQs on at this point(?), change local_irq_save to _disable.
72 - TODO: needs secondary CPUs to disable preempt (See #1)
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.