Kernel-4.18.0-80.el8_sched-arch

CPU Scheduler implementation hints for architecture specific code

Nick Piggin, 2005

Context switch

  1. Runqueue locking
    By default, the switch_to arch function is called with the runqueue
    locked. This is usually not a problem unless switch_to may need to
    take the runqueue lock. This is usually due to a wake up operation in
    the context switch. See arch/ia64/include/asm/switch_to.h for an example.

To request the scheduler call switch_to with the runqueue unlocked,
you must #define __ARCH_WANT_UNLOCKED_CTXSW in a header file
(typically the one where switch_to is defined).

Unlocked context switches introduce only a very minor performance
penalty to the core scheduler implementation in the CONFIG_SMP case.

CPU idle

Your cpu_idle routines need to obey the following rules:

  1. Preempt should now disabled over idle routines. Should only
    be enabled to call schedule() then disabled again.

  2. need_resched/TIF_NEED_RESCHED is only ever set, and will never
    be cleared until the running task has called schedule(). Idle
    threads need only ever query need_resched, and may never set or
    clear it.

  3. When cpu_idle finds (need_resched() == ‘true’), it should call
    schedule(). It should not call schedule() otherwise.

  4. The only time interrupts need to be disabled when checking
    need_resched is if we are about to sleep the processor until
    the next interrupt (this doesn’t provide any protection of
    need_resched, it prevents losing an interrupt).

    4a. Common problem with this type of sleep appears to be:

         local_irq_disable();
         if (!need_resched()) {
                 local_irq_enable();
                 *** resched interrupt arrives here ***
                 __asm__("sleep until next interrupt");
         }
    
  5. TIF_POLLING_NRFLAG can be set by idle routines that do not
    need an interrupt to wake them up when need_resched goes high.
    In other words, they must be periodically polling need_resched,
    although it may be reasonable to do some background work or enter
    a low CPU priority.

    5a. If TIF_POLLING_NRFLAG is set, and we do decide to enter
     an interrupt sleep, it needs to be cleared then a memory
     barrier issued (followed by a test of need_resched with
     interrupts disabled, as explained in 3).
    

arch/x86/kernel/process.c has examples of both polling and
sleeping idle functions.

Possible arch/ problems

Possible arch problems I found (and either tried to fix or didn’t):

ia64 - is safe_halt call racy vs interrupts? (does it sleep?) (See #4a)

sh64 - Is sleeping racy vs interrupts? (See #4a)

sparc - IRQs on at this point(?), change local_irq_save to _disable.
- TODO: needs secondary CPUs to disable preempt (See #1)