1 ==============================================
2 CPU Scheduler implementation hints for archite
3 ==============================================
4
5 Nick Piggin, 2005
6
7 Context switch
8 ==============
9 1. Runqueue locking
10 By default, the switch_to arch function is cal
11 locked. This is usually not a problem unless s
12 take the runqueue lock. This is usually due to
13 the context switch.
14
15 To request the scheduler call switch_to with t
16 you must `#define __ARCH_WANT_UNLOCKED_CTXSW`
17 (typically the one where switch_to is defined)
18
19 Unlocked context switches introduce only a ver
20 penalty to the core scheduler implementation i
21
22 CPU idle
23 ========
24 Your cpu_idle routines need to obey the follow
25
26 1. Preempt should now disabled over idle routi
27 be enabled to call schedule() then disabled
28
29 2. need_resched/TIF_NEED_RESCHED is only ever
30 be cleared until the running task has calle
31 threads need only ever query need_resched,
32 clear it.
33
34 3. When cpu_idle finds (need_resched() == 'tru
35 schedule(). It should not call schedule() o
36
37 4. The only time interrupts need to be disable
38 need_resched is if we are about to sleep th
39 the next interrupt (this doesn't provide an
40 need_resched, it prevents losing an interru
41
42 4a. Common problem with this type of s
43
44 local_irq_disable();
45 if (!need_resched()) {
46 local_irq_enable();
47 *** resched interrupt
48 __asm__("sleep until n
49 }
50
51 5. TIF_POLLING_NRFLAG can be set by idle routi
52 need an interrupt to wake them up when need
53 In other words, they must be periodically p
54 although it may be reasonable to do some ba
55 a low CPU priority.
56
57 - 5a. If TIF_POLLING_NRFLAG is set, and
58 an interrupt sleep, it needs to be cle
59 barrier issued (followed by a test of
60 interrupts disabled, as explained in 3
61
62 arch/x86/kernel/process.c has examples of both
63 sleeping idle functions.
64
65
66 Possible arch/ problems
67 =======================
68
69 Possible arch problems I found (and either tri
70
71 sparc - IRQs on at this point(?), change local
72 - TODO: needs secondary CPUs to disable
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.