Nick Piggin wrote:
> Ingo Molnar wrote:
> 
>> PREEMPT_RT on SMP systems triggered weird (very high) load average
>> values rather easily, which turned out to be a mainline kernel
>> ->nr_uninterruptible handling bug in try_to_wake_up().
>>
>> the following code:
>>
>>         if (old_state == TASK_UNINTERRUPTIBLE) {
>>                 old_rq->nr_uninterruptible--;
>>
>> potentially executes with old_rq potentially being != rq, and hence
>> updating ->nr_uninterruptible without the lock held. Given a
>> sufficiently concurrent preemption workload the count can get out of
>> whack and updates might get lost, permanently skewing the global 
>> count. Nothing except the load-average uses nr_uninterruptible() so this
>> condition can go unnoticed quite easily.
>>
> 
> Hi Ingo,
> Yes you're right.
> 
> I have another idea. Revert back to the old code, then just transfer
> the nr_uninterruptible count when migrating a task. That way, the
> rq's nr_uninterruptible field always is a measure of the number of
> uninterruptible tasks on it. What do you think?

Something like this: