Re: [BUG] rebuild_sched_domains considered dangerous

[Peter Zijlstra]

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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