Commit | Line | Data |
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64c7c8f8 NP |
1 | CPU Scheduler implementation hints for architecture specific code |
2 | ||
3 | Nick Piggin, 2005 | |
4 | ||
5 | Context switch | |
6 | ============== | |
7 | 1. Runqueue locking | |
8 | By default, the switch_to arch function is called with the runqueue | |
9 | locked. This is usually not a problem unless switch_to may need to | |
10 | take the runqueue lock. This is usually due to a wake up operation in | |
11 | the context switch. See include/asm-ia64/system.h for an example. | |
12 | ||
13 | To request the scheduler call switch_to with the runqueue unlocked, | |
14 | you must `#define __ARCH_WANT_UNLOCKED_CTXSW` in a header file | |
15 | (typically the one where switch_to is defined). | |
16 | ||
17 | Unlocked context switches introduce only a very minor performance | |
18 | penalty to the core scheduler implementation in the CONFIG_SMP case. | |
19 | ||
20 | 2. Interrupt status | |
21 | By default, the switch_to arch function is called with interrupts | |
22 | disabled. Interrupts may be enabled over the call if it is likely to | |
23 | introduce a significant interrupt latency by adding the line | |
24 | `#define __ARCH_WANT_INTERRUPTS_ON_CTXSW` in the same place as for | |
25 | unlocked context switches. This define also implies | |
26 | `__ARCH_WANT_UNLOCKED_CTXSW`. See include/asm-arm/system.h for an | |
27 | example. | |
28 | ||
29 | ||
30 | CPU idle | |
31 | ======== | |
32 | Your cpu_idle routines need to obey the following rules: | |
33 | ||
34 | 1. Preempt should now disabled over idle routines. Should only | |
35 | be enabled to call schedule() then disabled again. | |
36 | ||
37 | 2. need_resched/TIF_NEED_RESCHED is only ever set, and will never | |
38 | be cleared until the running task has called schedule(). Idle | |
39 | threads need only ever query need_resched, and may never set or | |
40 | clear it. | |
41 | ||
42 | 3. When cpu_idle finds (need_resched() == 'true'), it should call | |
43 | schedule(). It should not call schedule() otherwise. | |
44 | ||
45 | 4. The only time interrupts need to be disabled when checking | |
46 | need_resched is if we are about to sleep the processor until | |
47 | the next interrupt (this doesn't provide any protection of | |
48 | need_resched, it prevents losing an interrupt). | |
49 | ||
50 | 4a. Common problem with this type of sleep appears to be: | |
51 | local_irq_disable(); | |
52 | if (!need_resched()) { | |
53 | local_irq_enable(); | |
54 | *** resched interrupt arrives here *** | |
55 | __asm__("sleep until next interrupt"); | |
56 | } | |
57 | ||
58 | 5. TIF_POLLING_NRFLAG can be set by idle routines that do not | |
59 | need an interrupt to wake them up when need_resched goes high. | |
60 | In other words, they must be periodically polling need_resched, | |
61 | although it may be reasonable to do some background work or enter | |
62 | a low CPU priority. | |
63 | ||
64 | 5a. If TIF_POLLING_NRFLAG is set, and we do decide to enter | |
65 | an interrupt sleep, it needs to be cleared then a memory | |
66 | barrier issued (followed by a test of need_resched with | |
67 | interrupts disabled, as explained in 3). | |
68 | ||
69 | arch/i386/kernel/process.c has examples of both polling and | |
70 | sleeping idle functions. | |
71 | ||
72 | ||
73 | Possible arch/ problems | |
74 | ======================= | |
75 | ||
76 | Possible arch problems I found (and either tried to fix or didn't): | |
77 | ||
78 | h8300 - Is such sleeping racy vs interrupts? (See #4a). | |
79 | The H8/300 manual I found indicates yes, however disabling IRQs | |
80 | over the sleep mean only NMIs can wake it up, so can't fix easily | |
81 | without doing spin waiting. | |
82 | ||
83 | ia64 - is safe_halt call racy vs interrupts? (does it sleep?) (See #4a) | |
84 | ||
85 | sh64 - Is sleeping racy vs interrupts? (See #4a) | |
86 | ||
87 | sparc - IRQs on at this point(?), change local_irq_save to _disable. | |
88 | - TODO: needs secondary CPUs to disable preempt (See #1) | |
89 |