view Documentation/sched-arch.txt @ 452:c7ed6fe5dca0

kexec: dont initialise regions in reserve_memory()

There is no need to initialise efi_memmap_res and boot_param_res in
reserve_memory() for the initial xen domain as it is done in
machine_kexec_setup_resources() using values from the kexec hypercall.

Signed-off-by: Simon Horman <horms@verge.net.au>
author Keir Fraser <keir.fraser@citrix.com>
date Thu Feb 28 10:55:18 2008 +0000 (2008-02-28)
parents 831230e53067
line source
1 CPU Scheduler implementation hints for architecture specific code
3 Nick Piggin, 2005
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.
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).
17 Unlocked context switches introduce only a very minor performance
18 penalty to the core scheduler implementation in the CONFIG_SMP case.
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.
30 CPU idle
31 ========
32 Your cpu_idle routines need to obey the following rules:
34 1. Preempt should now disabled over idle routines. Should only
35 be enabled to call schedule() then disabled again.
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.
42 3. When cpu_idle finds (need_resched() == 'true'), it should call
43 schedule(). It should not call schedule() otherwise.
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).
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 }
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.
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).
69 arch/i386/kernel/process.c has examples of both polling and
70 sleeping idle functions.
73 Possible arch/ problems
74 =======================
76 Possible arch problems I found (and either tried to fix or didn't):
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.
83 ia64 - is safe_halt call racy vs interrupts? (does it sleep?) (See #4a)
85 sh64 - Is sleeping racy vs interrupts? (See #4a)
87 sparc - IRQs on at this point(?), change local_irq_save to _disable.
88 - TODO: needs secondary CPUs to disable preempt (See #1)