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Subjectcgroup pids controller -- WARN_ON_ONCE triggering
The commit below attempts to fix up pid controller charging:

commit afcf6c8b75444382e0f9996157207ebae34a8848
Author: Tejun Heo <tj@kernel.org>
Date: Thu Oct 15 16:41:53 2015 -0400

cgroup: add cgroup_subsys->free() method and use it to fix pids controller

Since this change we are seeing system hangs in early boot on multiple
architecures. We have a console log on ppc64el [1] which fingers
pids_cancel(). Manual debugging on amd64 VMs seems to indicate that we
are now tripping the WARN_ON_ONCE() below:

static void pids_cancel(struct pids_cgroup *pids, int num)
{
/*
* A negative count (or overflow for that matter) is invalid,
* and indicates a bug in the `pids` controller proper.
*/
WARN_ON_ONCE(atomic64_add_negative(-num, &pids->counter));
}

Converting this to a printk I was able to obtain confirmation that we are
indeed seeing this go negative in some cases.

Reverting the above commit seems to resolve the early boot issues in
my testing.

-apw

[1] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1523586


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