path: root/Documentation
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authorNick Piggin <>2005-11-08 21:39:04 -0800
committerLinus Torvalds <>2005-11-09 07:56:33 -0800
commit64c7c8f88559624abdbe12b5da6502e8879f8d28 (patch)
tree02f85a35ddd0f24dec70e5d6ecd61073578fd8d6 /Documentation
parent5bfb5d690f36d316a5f3b4f7775fda996faa6b12 (diff)
[PATCH] sched: resched and cpu_idle rework
Make some changes to the NEED_RESCHED and POLLING_NRFLAG to reduce confusion, and make their semantics rigid. Improves efficiency of resched_task and some cpu_idle routines. * In resched_task: - TIF_NEED_RESCHED is only cleared with the task's runqueue lock held, and as we hold it during resched_task, then there is no need for an atomic test and set there. The only other time this should be set is when the task's quantum expires, in the timer interrupt - this is protected against because the rq lock is irq-safe. - If TIF_NEED_RESCHED is set, then we don't need to do anything. It won't get unset until the task get's schedule()d off. - If we are running on the same CPU as the task we resched, then set TIF_NEED_RESCHED and no further action is required. - If we are running on another CPU, and TIF_POLLING_NRFLAG is *not* set after TIF_NEED_RESCHED has been set, then we need to send an IPI. Using these rules, we are able to remove the test and set operation in resched_task, and make clear the previously vague semantics of POLLING_NRFLAG. * In idle routines: - Enter cpu_idle with preempt disabled. When the need_resched() condition becomes true, explicitly call schedule(). This makes things a bit clearer (IMO), but haven't updated all architectures yet. - Many do a test and clear of TIF_NEED_RESCHED for some reason. According to the resched_task rules, this isn't needed (and actually breaks the assumption that TIF_NEED_RESCHED is only cleared with the runqueue lock held). So remove that. Generally one less locked memory op when switching to the idle thread. - Many idle routines clear TIF_POLLING_NRFLAG, and only set it in the inner most polling idle loops. The above resched_task semantics allow it to be set until before the last time need_resched() is checked before going into a halt requiring interrupt wakeup. Many idle routines simply never enter such a halt, and so POLLING_NRFLAG can be always left set, completely eliminating resched IPIs when rescheduling the idle task. POLLING_NRFLAG width can be increased, to reduce the chance of resched IPIs. Signed-off-by: Nick Piggin <> Cc: Ingo Molnar <> Cc: Con Kolivas <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'Documentation')
1 files changed, 89 insertions, 0 deletions
diff --git a/Documentation/sched-arch.txt b/Documentation/sched-arch.txt
new file mode 100644
index 000000000000..941615a9769b
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+ 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 include/asm-ia64/system.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.
+2. Interrupt status
+By default, the switch_to arch function is called with interrupts
+disabled. Interrupts may be enabled over the call if it is likely to
+introduce a significant interrupt latency by adding the line
+`#define __ARCH_WANT_INTERRUPTS_ON_CTXSW` in the same place as for
+unlocked context switches. This define also implies
+`__ARCH_WANT_UNLOCKED_CTXSW`. See include/asm-arm/system.h for an
+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/i386/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):
+h8300 - Is such sleeping racy vs interrupts? (See #4a).
+ The H8/300 manual I found indicates yes, however disabling IRQs
+ over the sleep mean only NMIs can wake it up, so can't fix easily
+ without doing spin waiting.
+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)