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cgroup pids controller -- WARN_ON_ONCE triggering

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  cgroup pids controller -- WARN_ON_ONCE triggering Andy Whitcroft <apw@canonical.com> - 2015-12-08 16:00 +0100
    Re: cgroup pids controller -- WARN_ON_ONCE triggering Tejun Heo <tj@kernel.org> - 2015-12-08 16:30 +0100
      Re: cgroup pids controller -- WARN_ON_ONCE triggering Andy Whitcroft <apw@canonical.com> - 2015-12-08 18:10 +0100

#1286560 — cgroup 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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#1286589

Hello, Andy.

On Tue, Dec 08, 2015 at 02:58:51PM +0000, Andy Whitcroft wrote:
> 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.

Fix already queued in the following git branch.  Pushing it out as we
speak.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git for-4.4-fixes

Thanks.

-- 
tejun
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#1286704

On Tue, Dec 08, 2015 at 10:24:28AM -0500, Tejun Heo wrote:
> Hello, Andy.
> 
> On Tue, Dec 08, 2015 at 02:58:51PM +0000, Andy Whitcroft wrote:
> > 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.
> 
> Fix already queued in the following git branch.  Pushing it out as we
> speak.
> 
>  git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git for-4.4-fixes
> 
> Thanks.

Cool, thanks.  That seems to fix me up here.

-apw
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