linux-kernel - Re: [PATCH] workqueue: fix rebind bound workers warning

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Message-ID: <CANRm+CwHVRko4BhFXZCvR6JR2BGowJGyfYUzd9Ab8otBrkit9g@mail.gmail.com>
Date:	Tue, 10 May 2016 06:14:33 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Wanpeng Li <wanpeng.li@...mail.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH] workqueue: fix rebind bound workers warning

Cc Peterz, Frederic,
2016-05-10 5:50 GMT+08:00 Wanpeng Li <kernellwp@...il.com>:
> Cc Thomas, the new state machine author,
> 2016-05-10 1:00 GMT+08:00 Tejun Heo <tj@...nel.org>:
>> Hello,
>>
>> On Thu, May 05, 2016 at 09:41:31AM +0800, Wanpeng Li wrote:
>>> The boot CPU handles housekeeping duty(unbound timers, workqueues,
>>> timekeeping, ...) on behalf of full dynticks CPUs. It must remain
>>> online when nohz full is enabled. There is a priority set to every
>>> notifier_blocks:
>>>
>>> workqueue_cpu_up > tick_nohz_cpu_down > workqueue_cpu_down
>>>
>>> So tick_nohz_cpu_down callback failed when down prepare cpu 0, and
>>> notifier_blocks behind tick_nohz_cpu_down will not be called any
>>> more, which leads to workers are actually not unbound. Then hotplug
>>> state machine will fallback to undo and online cpu 0 again. Workers
>>> will be rebound unconditionally even if they are not unbound and
>>> trigger the warning in this progress.
>>
>> I'm a bit confused.  Are you saying that the hotplug statemachine may
>> invoke CPU_DOWN_FAILED w/o preceding CPU_DOWN on the same callback?
>
> I think so. CPU_DOWN_FAILED is detected in the process of CPU_DOWN_PREPARE
>
> Regards,
> Wanpeng Li