Messages in this thread | | | Date | Wed, 17 Nov 2004 10:32:27 +1100 | From | Peter Williams <> | Subject | Re: [patch, 2.6.10-rc2] sched: fix ->nr_uninterruptible handling bugs |
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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
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