linux-kernel - Re: [PATCH] workqueue: clear workers of a pool after the CPU is offline

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Message-ID: <51F1DB45.90905@cn.fujitsu.com>
Date:	Fri, 26 Jul 2013 10:13:25 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] workqueue: clear workers of a pool after the CPU is offline

On 07/25/2013 11:31 PM, Tejun Heo wrote:
> Hello, Lai.
> 
> On Thu, Jul 25, 2013 at 06:52:02PM +0800, Lai Jiangshan wrote:
>> The unbound pools and their workers can be destroyed/cleared
>> when their refcnt become zero. But the cpu pool can't be destroyed
>> due to they are always referenced, their refcnt are always > 0.
>>
>> We don't want to destroy the cpu pools, but we want to destroy
>> the workers of the pool when the pool is full idle after the cpu
>> is offline. This is the default behavior in old days until
>> we removed the trustee_thread().
>>
>> We need to find a new way to restore this behavior,
>> We add offline_pool() and POOL_OFFLINE flag to do so.
> 
> Hmmm... if I'm not confused, now the cpu pools just behave like a
> normal unbound pool when the cpu goes down,

cpu pools are always referenced, they don't behave like unbound pool.

> which means that the idle
> cpu workers will exit once idle timeout is reached, right? 

No, no code to force the cpu workers quit currently.
you can just offline a cpu to see what happened to the workers.

> I really
> don't think it'd be worthwhile to add extra logic to accelerate the
> process.
> 
> Note that there actually are benefits to doing it asynchronously as
> CPUs go up and down very frequently on mobile platforms and destroying
> idle workers as soon as possible would just mean that we'd be doing a
> lot of work which isn't necessary.  I mean, we even grew an explicit
> mechanism to park kthreads to avoid repeatedly creating and destroying
> per-cpu kthreads as cpus go up and down.  I don't see any point in
> adding code to go the other direction.
> 
> Thanks.
> 

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