On 2011-06-17 13:06, Gilles Chanteperdrix wrote:
> On 06/17/2011 01:03 PM, Jan Kiszka wrote:
>> On 2011-06-17 12:55, Gilles Chanteperdrix wrote:
>>> On 06/17/2011 11:26 AM, Jan Kiszka wrote:
>>>> Our current interrupt handlers assume that they leave over the same task
>>>> and CPU they entered. But CONFIG_XENO_HW_UNLOCKED_SWITCH and commit
>>>> f6af9b831c broke this assumption: xnpod_schedule invoked from the
>>>> handler tail can now actually trigger a domain migration
>>>
>>> What unlocked context swith introduce from my point of view is simply
>>> sections where interrupt happen which do not reschedule.
>>>
>>> f6af9b831c introduce a rescheduling point, but does not change what
>>> happens during the interrupt handler either.
>>>
>>> So, I do not really understand this commit message. Either we can assume
>>> that interrupt handlers migrate tasks or not, but this does not seem to
>>> have anything to do with unlocked context switches or commit f6af9b831c.
>>
>> It has: Task is about to relax, re-enables interrupts in
>> xnpod_resume_thread, IRQ hits, handler is entered over the relaxing RT
>> task, xnpod_schedule in its tail performs the switch to root, which then
>> continues to relaxed task, IRQ tail resumes over a different task, on
>> SMP potentially also on a different CPU. I can send you a the trace if
>> you want to have a closer look.
> 
> Ok. Got it. But what has this to do with unlocked context switches ?

Also before commit f6af9b831c, there was a window with enabled IRQs in
the relaxation path. So the above scenario should have been possible
even earlier.

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux

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