On 09/07/11 07:01, Russell King - ARM Linux wrote:
> On Wed, Sep 07, 2011 at 12:57:44PM +0200, Thomas Gleixner wrote:
>> The problem is that if you enable interrupts on the CPU _BEFORE_ it is
>> set online AND active, then you can end up waking up kernel threads
>> which are bound to that CPU and the scheduler will happily schedule
>> them on an online CPU. That makes them lose the cpu affinity to the
>> CPU as well and hell breaks lose.
>
> How can that happen?
>
> 1. The only interrupts we're likely to receive are the local timer
> interrupts - we have not routed any other interrupts to this CPU.
Yes, it is the local timer interrupt.
>
> 2. We will not schedule on this CPU except at explicit scheduling
> points (such as contended mutexes or explicit calls to schedule)
> as we have a call to preempt_disable().
It is not a schedule. It is wake_up_process():
wake_up_process()
try_to_wake_up()
select_task_rq()
if (... || !cpu_online(cpu))
select_fallback_rq(task_cpu(p), p)
...
/* No more Mr. Nice Guy. */
dest_cpu = cpuset_cpus_allowed_fallback(p)
do_set_cpus_allowed(p, cpu_possible_mask)
# Thus ksoftirqd can now run on any cpu...
>
>> Frank has observed this with softirq threads, but the same thing is
>> true for any other CPU bound thread like the worker stuff.
>
> So who is scheduling a workqueue from the local timer?
do_local_timer()
ipi_timer()
irq_exit()
invoke_softirq()
wakeup_softirqd()
wake_up_process()
>
>> So moving the online, active thing BEFORE enabling interrupt is the
>> only sensible solution.
>
> Yes, that'll be why even x86 enables interrupts before setting the CPU
> online for the delay calibration.
-Frank
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