linux-kernel - Re: [PATCH 2/2] mm, memory_hotplug: remove timeout from __offline

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Date:   Tue, 5 Sep 2017 14:24:26 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     Xishi Qiu <qiuxishi@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
        Reza Arbab <arbab@...ux.vnet.ibm.com>,
        Yasuaki Ishimatsu <yasu.isimatu@...il.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm, memory_hotplug: remove timeout from
 __offline_memory

On 09/05/2017 12:53 PM, Michal Hocko wrote:
> On Tue 05-09-17 11:16:57, Anshuman Khandual wrote:
>> On 09/04/2017 02:45 PM, Michal Hocko wrote:
>>> On Mon 04-09-17 17:05:15, Xishi Qiu wrote:
>>>> On 2017/9/4 17:01, Michal Hocko wrote:
>>>>
>>>>> On Mon 04-09-17 16:58:30, Xishi Qiu wrote:
>>>>>> On 2017/9/4 16:21, Michal Hocko wrote:
>>>>>>
>>>>>>> From: Michal Hocko <mhocko@...e.com>
>>>>>>>
>>>>>>> We have a hardcoded 120s timeout after which the memory offline fails
>>>>>>> basically since the hot remove has been introduced. This is essentially
>>>>>>> a policy implemented in the kernel. Moreover there is no way to adjust
>>>>>>> the timeout and so we are sometimes facing memory offline failures if
>>>>>>> the system is under a heavy memory pressure or very intensive CPU
>>>>>>> workload on large machines.
>>>>>>>
>>>>>>> It is not very clear what purpose the timeout actually serves. The
>>>>>>> offline operation is interruptible by a signal so if userspace wants
>>>>>> Hi Michal,
>>>>>>
>>>>>> If the user know what he should do if migration for a long time,
>>>>>> it is OK, but I don't think all the users know this operation
>>>>>> (e.g. ctrl + c) and the affect.
>>>>> How is this operation any different from other potentially long
>>>>> interruptible syscalls?
>>>>>
>>>> Hi Michal,
>>>>
>>>> I means the user should stop it by himself if migration always retry in endless.
>>> If the memory is migrateable then the migration should finish
>>> eventually. It can take some time but it shouldn't be an endless loop.
>>
>> But what if some how the temporary condition (page removed from the PCP
>> LRU list and has not been freed yet to the buddy) happens again and again.
> 
> How would that happen? We have all pages in the range MIGRATE_ISOLATE so
> no pages will get reallocated and we know that there are no unmigratable
> pages in the range. So we only should have temporary failures for
> migration. If that is not the case then we have a bug somewhere.

Right.

> 
>> I understand we have schedule() and yield() to make sure that the context
>> does not hold the CPU for ever but it can take theoretically very long
>> time if not endless to finish. In that case sending signal to the user
> 
> I guess you meant to say signal from the user space...

Yes.

> 
>> space process who initiated the offline request is the only way to stop
>> this retry loop. I think this is still a better approach than the 120
>> second timeout which was kind of arbitrary.
> 
> Yeah the context is interruptible so if the operation takes unbearably
> too long then a watchdog can be setup trivially and to the user defined
> value. There is a good reason we do not add hardocded timeouts to the
> kernel.
> 

Right.