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

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Date:	Wed, 20 Aug 2014 22:30:51 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Chai Wen <chaiw.fnst@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...nel.org>, 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 Thu, Aug 21, 2014 at 09:37:04AM +0800, Chai Wen wrote:
> 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.

Sure, just quickly test with the task pointer to make sure it still works
and then re-post.

Cheers,
Don
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