Philippe Gerum wrote:
> On Thu, 2009-05-14 at 12:20 +0200, Jan Kiszka wrote:
>> Philippe Gerum wrote:
>>> On Wed, 2009-05-13 at 18:10 +0200, Jan Kiszka wrote:
>>>> Philippe Gerum wrote:
>>>>> On Wed, 2009-05-13 at 17:28 +0200, Jan Kiszka wrote:
>>>>>> Philippe Gerum wrote:
>>>>>>> On Wed, 2009-05-13 at 15:18 +0200, Jan Kiszka wrote:
>>>>>>>> Gilles Chanteperdrix wrote:
>>>>>>>>> Jan Kiszka wrote:
>>>>>>>>>> Hi Gilles,
>>>>>>>>>> I'm currently facing a nasty effect with switchtest over latest git 
>>>>>>>>>> head
>>>>>>>>>> (only tested this so far): running it inside my test VM (ie. with
>>>>>>>>>> frequent excessive latencies) I get a stalled Linux timer IRQ quite
>>>>>>>>>> quickly. System is otherwise still responsive, Xenomai timers are 
>>>>>>>>>> still
>>>>>>>>>> being delivered, other Linux IRQs too. switchtest complained about
>>>>>>>>>>     "Warning: Linux is compiled to use FPU in kernel-space."
>>>>>>>>>> when it was started. Kernels are and
>>>>>>>>>> (LTTng patched in, but unused), both show 
>>>>>>>>>> the
>>>>>>>>>> same effect.
>>>>>>>>>> Seen this before?
>>>>>>>>> The warning about Linux being compiled to use FPU in kernel-space 
>>>>>>>>> means
>>>>>>>>> that you enabled soft RAID or compiled for K7, Geode, or any other
>>>>>>>> RAID is on (ordinary server config).
>>>>>>>>> configuration using 3DNow for such simple operations as memcpy. It is
>>>>>>>>> harmless, it simply means that switchtest can not use fpu in 
>>>>>>>>> kernel-space.
>>>>>>>>> The bug you have is probably the same as the one described here, 
>>>>>>>>> which I
>>>>>>>>> am able to reproduce on my atom:
>>>>>>>>> Unfortunately, I for one am working on ARM issues and am not available
>>>>>>>>> to debug x86 issues. I think Philippe is busy too...
>>>>>>>> OK, looks like I got the same flu here.
>>>>>>>> Philippe, did you find out any more details in the meantime? Then I'm
>>>>>>>> afraid I have to pick this up.
>>>>>>> No, I did not resume this task yet. Working from the powerpc side of the
>>>>>>> universe here.
>>>>>> Hoho, don't think this rain here over x86 would have never made it down
>>>>>> to ARM or PPC land! ;)
>>>>>> Martin, could you check if this helps you, too?
>>>>>> Jan
>>>>>> (as usual, ready to be pulled from 'for-upstream')
>>>>>> --------->
>>>>>> Host IRQs may not only be triggered from non-root domains.
>>>>> Are you sure of this? I can't find any spot where this assumption would
>>>>> be wrong. host_pend() is basically there to relay RT timer ticks and
>>>>> device IRQs, and this only happens on behalf of the pipeline head. At
>>>>> least, this is how rthal_irq_host_pend() should be used in any case. If
>>>>> you did find a spot where this interface is being called from the lower
>>>>> stage, then this is the root bug to fix.
>>>> I haven't studied the I-pipe trace /wrt this in details yet, but I could
>>>> imagine that some shadow task is interrupted in primary mode by the
>>>> timer IRQ and then leaves the handler in secondary mode due to whatever
>>>> events between schedule-out and in at the end of xnintr_clock_handler.
>>> You need a thread context to move to secondary, I just can't see how
>>> such scenario would be possible.
>> Here is the trace of events:
>> => Shadow task starts migration to secondary
>> => in xnpod_suspend_thread, nklock is briefly released before
>>    xnpod_schedule
> Which is the root bug. Blame on me; this recent change in -head breaks a
> basic rule a lot of code is based on: a self-suspending thread may not
> be preempted while scheduling out, i.e. suspension and rescheduling must
> be atomically performed. xnshadow_relax() counts on this too.

Oh, good that you insisted on this. Will you fix it soon? We are
currently packaging a delivery of 2.5.git, and I would like to see this
hole closed there already.

>> => timer IRQ intercepts
>> => as the current CPU is marked for reschedule, we enter xnpod_schedule
>>    before propagating the host tick
>> => once the migrating thread comes in again, it will run the
>>    xnintr_clock_handler tail, i.e. xnarch_relay_tick, already over the
>>    root domain
> Ok, makes sense now. However, this can't happen with 2.4 which has no
> such lock release in xnpod_suspend_thread(). So the question is: was the
> "lost tick" bug observed also on 2.4, or not?

I haven't tested on 2.4, but Martin anyway reported that his problem is
still unfixed for 2.5 even with my patch.


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