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Documentation / scheduler / sched-arch.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.

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/switch_to.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	CPU idle
21	========
22	Your cpu_idle routines need to obey the following rules:
23	
24	1. Preempt should now disabled over idle routines. Should only
25	   be enabled to call schedule() then disabled again.
26	
27	2. need_resched/TIF_NEED_RESCHED is only ever set, and will never
28	   be cleared until the running task has called schedule(). Idle
29	   threads need only ever query need_resched, and may never set or
30	   clear it.
31	
32	3. When cpu_idle finds (need_resched() == 'true'), it should call
33	   schedule(). It should not call schedule() otherwise.
34	
35	4. The only time interrupts need to be disabled when checking
36	   need_resched is if we are about to sleep the processor until
37	   the next interrupt (this doesn't provide any protection of
38	   need_resched, it prevents losing an interrupt).
39	
40		4a. Common problem with this type of sleep appears to be:
41		        local_irq_disable();
42		        if (!need_resched()) {
43		                local_irq_enable();
44		                *** resched interrupt arrives here ***
45		                __asm__("sleep until next interrupt");
46		        }
47	
48	5. TIF_POLLING_NRFLAG can be set by idle routines that do not
49	   need an interrupt to wake them up when need_resched goes high.
50	   In other words, they must be periodically polling need_resched,
51	   although it may be reasonable to do some background work or enter
52	   a low CPU priority.
53	
54	   	5a. If TIF_POLLING_NRFLAG is set, and we do decide to enter
55		    an interrupt sleep, it needs to be cleared then a memory
56		    barrier issued (followed by a test of need_resched with
57		    interrupts disabled, as explained in 3).
58	
59	arch/x86/kernel/process.c has examples of both polling and
60	sleeping idle functions.
61	
62	
63	Possible arch/ problems
64	=======================
65	
66	Possible arch problems I found (and either tried to fix or didn't):
67	
68	ia64 - is safe_halt call racy vs interrupts? (does it sleep?) (See #4a)
69	
70	sh64 - Is sleeping racy vs interrupts? (See #4a)
71	
72	sparc - IRQs on at this point(?), change local_irq_save to _disable.
73	      - TODO: needs secondary CPUs to disable preempt (See #1)
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