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d6a3b247 MCC |
1 | ================================================================= |
2 | CPU Scheduler implementation hints for architecture specific code | |
3 | ================================================================= | |
64c7c8f8 NP |
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 | |
95663285 | 13 | the context switch. See arch/ia64/include/asm/switch_to.h for an example. |
64c7c8f8 NP |
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 | ||
64c7c8f8 NP |
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 | |
d6a3b247 MCC |
40 | need_resched, it prevents losing an interrupt): |
41 | ||
42 | 4a. Common problem with this type of sleep appears to be:: | |
64c7c8f8 | 43 | |
64c7c8f8 NP |
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 | ||
d6a3b247 MCC |
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). | |
64c7c8f8 | 61 | |
25eb650a | 62 | arch/x86/kernel/process.c has examples of both polling and |
64c7c8f8 NP |
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 | ||
64c7c8f8 NP |
71 | ia64 - is safe_halt call racy vs interrupts? (does it sleep?) (See #4a) |
72 | ||
73 | sh64 - Is sleeping racy vs interrupts? (See #4a) | |
74 | ||
75 | sparc - IRQs on at this point(?), change local_irq_save to _disable. | |
76 | - TODO: needs secondary CPUs to disable preempt (See #1) |