Heiko Carstens wrote:

quoted text
>> Does s390 start out in nohz mode?  The reason I ask is that it feels like
>> an off-by-one error for the dynticks_progress_counter.
> 
> Actually I forgot to add a few ifdefs to make the code do something :)
> That just reveals that we have a conflict with the dynticks implementation
> and s390's nohz that shows up in what rcu_irq_enter/exit assume.
> I didn't patch s390 and common code so it will work, but I think the
> patch you mentionened will fix the problem I reported.
> So I guess we should either convert s390 to use the generic dynticks
> implementation or disable preemptible rcu on s390 until we converted
> our code.
> 
> Thanks for helping debugging this!

Heiko, thanks for reporting this.

This patch still didn't make it into -rc1, and it really should. Because 
without this patch, PREEMPT_RCU and NO_HZ together is broken, on all boxes.

The patch is in Ingo's sched-devel git tree, as 
9460545f81ea48b07dbb20456a8ede776d8ebc1b (last I checked) and titled:

     rcu: add support for dynamic ticks and preempt rcu


-- Steve
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