mirror of https://github.com/Qortal/Brooklyn
You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
76 lines
2.8 KiB
76 lines
2.8 KiB
================================================================= |
|
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)
|
|
|