Documentation/scheduler/sched-arch.rst

   1 =================================================================
   2 CPU Scheduler implementation hints for architecture specific code
   3 =================================================================
   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
  13 the context switch. See arch/ia64/include/asm/switch_to.h for an example.
  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
  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
  40    need_resched, it prevents losing an interrupt):
  41
  42         4a. Common problem with this type of sleep appears to be::
  43
  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
  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).
  61
  62 arch/x86/kernel/process.c has examples of both polling and
  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
  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)