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From:   Schspa Shi <schspa@gmail.com>
To:     Lai Jiangshan <jiangshanlai@gmail.com>
Cc:     Peter Zijlstra <peterz@infradead.org>, Tejun Heo <tj@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] workqueue: Use active mask for new worker when pool is
 DISASSOCIATED
Date:   Mon, 01 Aug 2022 12:42:33 +0800
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Lai Jiangshan <jiangshanlai@gmail.com> writes:

> On Sat, Jul 30, 2022 at 12:19 PM Schspa Shi <schspa@gmail.com> wrote:
>>
>>
>> Peter Zijlstra <peterz@infradead.org> writes:
>>
>> > On Wed, Jul 13, 2022 at 05:52:58PM +0800, Lai Jiangshan wrote:
>> >>
>> >>
>> >> CC Peter.
>> >> Peter has changed the CPU binding code in workqueue.c.
>> >
>> > [ 1622.829091] WARNING: CPU: 3 PID: 31 at kernel/sched/core.c:7756 sched_cpu_dying+0x74/0x204
>> > [ 1622.829374] CPU: 3 PID: 31 Comm: migration/3 Tainted: P           O      5.10.59-rt52 #2
>> >                                                                       ^^^^^^^^^^^^^^^^^^^^^
>> >
>> > I think we can ignore this as being some ancient kernel. Please try
>> > something recent.
>>
>> Hi peter:
>>
>> I spent a few days writing a test case and reproduced the problem on
>> kernel 5.19. I think it's time for us to review the V3 patch for a fix.
>>
>> The V3 patch is at
>> https://lore.kernel.org/all/20220714031645.28004-1-schspa@gmail.com/
>> Please help to review it.
>
> Because of the implementation of is_cpu_allowed(),
> I am still suspicious about how the newly created worker can be woken
> up in the dying cpu since it has no KTHREAD_IS_PER_CPU set.
>

I think the comments of the V3 patch have explained the problem, the
newly created worker runs on the dying CPU because we bind it to the
dying CPU via kthread_bind_mask or set_cpus_allowed_ptr.

Please refer to the following scenarios.

           CPU0                                  CPU1
------------------------------------------------------------------
sched_cpu_deactivate(cpu_active_mask clear)
workqueue_offline_cpu(work pool POOL_DISASSOCIATED)
  -- all worker will migrate to another cpu --
                                    worker_thread
                                    -- will create new worker if
                                       pool->worklist is not empty
                                       create_worker()
                                     -- new kworker will bound to CPU0
                               (pool->attrs->cpumask will be mask of CPU0).
      kworker/0:x will running on rq

sched_cpu_dying
  if (rq->nr_running != 1 || rq_has_pinned_tasks(rq))
    WARN(true, "Dying CPU not properly vacated!");
      ---------OOPS-------------

When CPUHP_HRTIMERS_PREPARE < CPU0's state < CPUHP_AP_WORKQUEUE_ONLINE,
the CPU0 still can schedule a new work, which will add a new work to
pool->worklist of a dying CPU's work pool.

> Is it a dying cpu when onlining a CPU fails?  I think balance_push
> is not set in this case when it fails at some point during online.
> But I haven't looked into the details and linked the clues.
>

Not this case, there is on onlining CPU fails log in my test log.

>
>>
>> Test branch as:
>> https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/tag/?h=v5.19-rc8-rt8
>>
>
> Can it only be triggered in the rt kernel?
>

I only test on rt kernel, because of I work on RT kernel now, But I
think non-rt kernel have the issues too.

>> I think this code is new enough to demonstrate that the problem persists.
>>
>> The log as fellowing:
>>
>> [ 3103.198684] ------------[ cut here ]------------
>> [ 3103.198684] Dying CPU not properly vacated!
>> [ 3103.198684] WARNING: CPU: 1 PID: 23 at kernel/sched/core.c:9575 sched_cpu_dying.cold+0xc/0xc3
>> [ 3103.198684] Modules linked in: work_test(O)
>
> Could you give me the code of this module?

The module's source code is at
https://github.com/schspa/code_snippet/blob/master/kernel_module/workqueue_test/work-test.c
Kernel defconfig is at
https://github.com/schspa/ktest/blob/main/configs/linux_rt_defconfig

I add a hook to the entry of worker_thread, which add extra 50ms delay
to make it reproduced faster.

-- 
BRs
Schspa Shi