Re: [patch, 2.6.10-rc2] sched: fix ->nr_uninterruptible handling bugs

[Peter Williams <pwil3058@bigpond.net.au>]

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

I presume that you mean adjust rather than transfer.

> rq's nr_uninterruptible field always is a measure of the number of
> uninterruptible tasks on it. What do you think?

To make this work you need to do the adjustment every where that a task 
changes CPU while in the UNINTERRUPTIBLE state.  Are both run queue 
locks always held in these circumstances?  I don't think that they are 
in try_to_wake_up() but it may be possible to work around that.

Peter
-- 
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce