linux-kernel - Re: [PATCH 2/5] softlockup: make detector be aware of task switch of processes hogging cpu

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Message-ID: <53F54D40.5090707@cn.fujitsu.com>
Date:	Thu, 21 Aug 2014 09:37:04 +0800
From:	Chai Wen <chaiw.fnst@...fujitsu.com>
To:	Don Zickus <dzickus@...hat.com>, Ingo Molnar <mingo@...nel.org>
CC:	<akpm@...ux-foundation.org>, <kvm@...r.kernel.org>,
	<pbonzini@...hat.com>, <mingo@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] softlockup: make detector be aware of task switch
 of processes hogging cpu

On 08/19/2014 09:36 AM, Chai Wen wrote:

> On 08/19/2014 04:38 AM, Don Zickus wrote:
> 
>> On Mon, Aug 18, 2014 at 09:02:00PM +0200, Ingo Molnar wrote:
>>>
>>> * Don Zickus <dzickus@...hat.com> wrote:
>>>
>>>>>>> So I agree with the motivation of this improvement, but 
>>>>>>> is this implementation namespace-safe?
>>>>>>
>>>>>> What namespace are you worried about colliding with?  I 
>>>>>> thought softlockup_ would provide the safety??  Maybe I 
>>>>>> am missing something obvious. :-(
>>>>>
>>>>> I meant PID namespaces - a PID in itself isn't guaranteed 
>>>>> to be unique across the system.
>>>>
>>>> Ah, I don't think we thought about that.  Is there a better 
>>>> way to do this?  Is there a domain id or something that can 
>>>> be OR'd with the pid?
>>>
>>> What is always unique is the task pointer itself. We use pids 
>>> when we interface with user-space - but we don't really do that 
>>> here, right?
>>
>> No, I don't believe so.  Ok, so saving 'current' and comparing that should
>> be enough, correct?
>>
> 
> 
> I am not sure of the safety about using pid here with namespace.
> But as to the pointer of process, is there a chance that we got a 'historical'
> address saved in the 'softlockup_warn_pid(or address)_saved' and the current
> hogging process happened to get the same task pointer address?
> If it never happens, I think the comparing of address is ok.
> 


Hi Ingo

what do you think of Don's solution- 'comparing of task pointer' ?
Anyway this is just an additional check about some very special cases,
so I think the issue that I am concerned above is not a problem at all.
And after learning some concepts about PID namespace, I think comparing
of task pointer is reliable dealing with PID namespace here.

And Don, If you want me to re-post this patch, please let me know that.

thanks
chai wen

> thanks
> chai wen
> 
>> Cheers,
>> Don
>> .
>>
> 
> 
> 



-- 
Regards

Chai Wen
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