Messages in this thread |  | | | Subject | Re: [BUG] rebuild_sched_domains considered dangerous | | From | Peter Zijlstra <> | | Date | Wed, 09 Mar 2011 11:19:58 +0100 |
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On Wed, 2011-03-09 at 13:58 +1100, Benjamin Herrenschmidt wrote: > So I've been experiencing hangs shortly after boot with recent kernels > on a Power7 machine. I was testing with PREEMPT & HZ=1024 which might > increase the frequency of the problem but I don't think they are > necessary to expose it. > > From what I've figured out, when the machine hangs, it's essentially > looping forever in update_sd_lb_stats(), due to a corrupted sd->groups > list (in my cases, the list contains a loop that doesn't loop back > the the first element). > > It appears that this corresponds to one CPU deciding to rebuild the > sched domains. There's various reasons why that can happen, the typical > one in our case is the new VPNH feature where the hypervisor informs us > of a change in node affinity of our virtual processors. s390 has a > similar feature and should be affected as well.
Ahh, so that's triggering it :-), just curious, how often does the HV do that to you?
> I suspect the problem could be reproduced on x86 by hammering the sysfs > file that can be used to trigger a rebuild as well on a sufficently > large machine.
Should, yeah, regular hotplug is racy too.
> From what I can tell, there's some missing locking here between > rebuilding the domains and find_busiest_group.
init_sched_build_groups() races against pretty much all sched_group iterations, like the one in update_sd_lb_stats() which is the most common one and the one you're getting stuck in.
> I haven't quite got my > head around how that -should- be done, though, as I an really not very > familiar with that code.
:-)
> For example, I don't quite get when domains are > attached to an rq, and whether code like build_numa_sched_groups() which > allocates groups and attach them to sched domains sd->groups does it on > a "live" domain or not (in that case, there's a problem since it kmalloc > and attaches the uninitialized result immediately).
No, the domain stuff is good, we allocate new domains and have a synchronize_sched() between us installing the new ones and freeing the old ones.
But the sched_group list is as said rather icky.
> I don't believe I understand enough of the scheduler to fix that quickly > and I'm really bogged down with some other urgent stuff, so I would very > much appreciate if you could provide some assistance here, even if it's > just in the form of suggestions/hints.
Yeah, sched_group rebuild is racy as hell, I haven't really managed to come up with a sane fix yet, will poke at it.
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