linux-kernel - 回复: 回复: [PATCH] debugobjects: install cpu hotplug callback

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Message-ID: <BYAPR11MB2632ABDD5A8334F74B4F539AFF540@BYAPR11MB2632.namprd11.prod.outlook.com>
Date:   Wed, 26 Aug 2020 08:34:51 +0000
From:   "Zhang, Qiang" <Qiang.Zhang@...driver.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Waiman Long <longman@...hat.com>,
        "elver@...gle.com" <elver@...gle.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: 回复: 回复: [PATCH] debugobjects: install cpu hotplug callback



________________________________________
发件人: linux-kernel-owner@...r.kernel.org <linux-kernel-owner@...r.kernel.org> 代表 Thomas Gleixner <tglx@...utronix.de>
发送时间: 2020年8月26日 7:53
收件人: Waiman Long; Zhang, Qiang; elver@...gle.com
抄送: linux-kernel@...r.kernel.org; akpm@...ux-foundation.org
主题: Re: 回复: [PATCH] debugobjects: install cpu hotplug callback

On Tue, Aug 25 2020 at 18:26, Waiman Long wrote:
> On 8/25/20 12:53 AM, Zhang, Qiang wrote:
>>
>> When a cpu going offline, we should free objects in "percpu_obj_pool"
>> free_objs list which corresponding to this cpu.
>
> The percpu free object pool is supposed to be accessed only by that
> particular cpu without any lock. Trying to access it from another cpu
> can cause a race condition unless one can make sure that the offline cpu
> won't become online in the mean time.

>It is actually safe because CPU hotplug is globally serialized and there
>is no way that an offline CPU will come back from death valley
>magically. If such a zombie ever surfaces then we have surely more
>serious problems than accessing that pool :)

> There shouldn't be too many free objects in the percpu pool. Is it
> worth the effort to free them?

>That's a really good question nevertheless. The only case where this
>ever matters is physical hotplug. All other CPU hotplug stuff is
>temporarily or in case of a late (post boottime) SMT disable it's going
>to be a handful of free objects on that pool. As debugobjects is as the
>name says a debug facility the benefit is questionable unless there is a
>good reason to do so.

 I don't know  there may not be too many objects  in the percpu pool,
 but that doesn't mean they no need to be free, a CPU may never be online after it is offline. some objects in percpu pool is never free.



>Thanks,

  >      tglx