Re: Oops on Power8 (was Re: [PATCH v2 1/7] workqueue: make workqueue available early during boot)

From	Tejun Heo <tj@kernel.org>
Newsgroups	linux.kernel
Subject	Re: Oops on Power8 (was Re: [PATCH v2 1/7] workqueue: make workqueue available early during boot)
Date	2016-10-14 17:10 +0200
Message-ID	<sschr-2Xb-3@gated-at.bofh.it> (permalink)
References	(1 earlier) <shKFQ-2IM-33@gated-at.bofh.it> <sirB7-5py-13@gated-at.bofh.it> <sqGMF-zG-9@gated-at.bofh.it> <sqIv7-1FC-5@gated-at.bofh.it> <sr3pT-666-15@gated-at.bofh.it>
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Hello, Michael.

On Tue, Oct 11, 2016 at 10:22:13PM +1100, Michael Ellerman wrote:
> The oops happens because we're in enqueue_task_fair() and p->se->cfs_rq
> is NULL.
> 
> The cfs_rq is NULL because we did set_task_rq(p, 2048), where 2048 is
> NR_CPUS. That causes us to index past the end of the tg->cfs_rq array in
> set_task_rq() and happen to get NULL.
> 
> We never should have done set_task_rq(p, 2048), because 2048 is >=
> nr_cpu_ids, which means it's not a valid CPU number, and set_task_rq()
> doesn't cope with that.

Hmm... it doesn't reproduce it here and can't see how the commit would
affect this given that it doesn't really change when the kworker
kthreads are being created.

> Presumably we shouldn't be ending up with tsk_cpus_allowed() being
> empty, but I haven't had time to track down why that's happening.

Can you please add WARN_ON_ONCE(!tsk_nr_cpus_allowed(p)) to
select_task_rq() and post what that says?

Thanks.

-- 
tejun

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Re: Oops on Power8 (was Re: [PATCH v2 1/7] workqueue: make workqueue  available early during boot) Tejun Heo <tj@kernel.org> - 2016-10-14 17:10 +0200
  Re: Oops on Power8 (was Re: [PATCH v2 1/7] workqueue: make workqueue  available early during boot) Balbir Singh <bsingharora@gmail.com> - 2016-10-15 03:30 +0200
  Re: Oops on Power8 (was Re: [PATCH v2 1/7] workqueue: make workqueue available early during boot) Michael Ellerman <mpe@ellerman.id.au> - 2016-10-15 11:50 +0200
    Re: Oops on Power8 (was Re: [PATCH v2 1/7] workqueue: make workqueue  available early during boot) Tejun Heo <tj@kernel.org> - 2016-10-17 20:20 +0200
  Re: Oops on Power8 (was Re: [PATCH v2 1/7] workqueue: make workqueue available early during boot) Michael Ellerman <mpe@ellerman.id.au> - 2016-10-17 14:30 +0200
    Re: Oops on Power8 (was Re: [PATCH v2 1/7] workqueue: make workqueue  available early during boot) Balbir Singh <bsingharora@gmail.com> - 2016-10-17 15:00 +0200
      Re: Oops on Power8 (was Re: [PATCH v2 1/7] workqueue: make workqueue available early during boot) Michael Ellerman <mpe@ellerman.id.au> - 2016-10-18 04:40 +0200
    Re: Oops on Power8 (was Re: [PATCH v2 1/7] workqueue: make workqueue  available early during boot) Tejun Heo <tj@kernel.org> - 2016-10-17 20:20 +0200
      Re: Oops on Power8 (was Re: [PATCH v2 1/7] workqueue: make workqueue  available early during boot) Tejun Heo <tj@kernel.org> - 2016-10-17 21:40 +0200
        Re: Oops on Power8 (was Re: [PATCH v2 1/7] workqueue: make workqueue  available early during boot) Tejun Heo <tj@kernel.org> - 2016-10-18 21:10 +0200
          Re: Oops on Power8 (was Re: [PATCH v2 1/7] workqueue: make workqueue available early during boot) Michael Ellerman <mpe@ellerman.id.au> - 2016-10-19 17:00 +0200
            [PATCH wq/for-4.10] workqueue: move wq_numa_init() to  workqueue_init() Tejun Heo <tj@kernel.org> - 2016-10-19 18:20 +0200

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