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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 arch/ia64/include/asm/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 arch/arm/include/asm/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/x86/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 | |
90 |
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