Date: Fri, 4 May 2012 13:49:08 -0700
From: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>, mingo@kernel.org,
        pjt@google.com, paul@paulmenage.org, akpm@linux-foundation.org,
        rjw@sisk.pl, nacc@us.ibm.com, paulmck@linux.vnet.ibm.com,
        tglx@linutronix.de, seto.hidetoshi@jp.fujitsu.com, rob@landley.net,
        tj@kernel.org, mschmidt@redhat.com, berrange@redhat.com,
        nikunj@linux.vnet.ibm.com, vatsa@linux.vnet.ibm.com,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-pm@vger.kernel.org
Subject: Re: [PATCH v2 0/7] CPU hotplug, cpusets: Fix issues with cpusets
 handling upon CPU hotplug
Message-ID: <20120504204908.GC18177@linux.vnet.ibm.com>
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On 04.05.2012 [22:28:01 +0200], Peter Zijlstra wrote:
> On Fri, 2012-05-04 at 22:14 +0200, Peter Zijlstra wrote:
> > > Also, we need to fix this problem at the CPU Hotplug level itself, and
> > > not just for the suspend/resume case. Because, we have had numerous bug
> > > reports and people complaining about this issue, in various scenarios,
> > > including those that didn't involve suspend/resume.
> > 
> > NO, absolutely not and I will NAK any and all such nonsense. WTF is a
> > cpuset worth if you can run on random other cpus? 
> 
> Sorting your cpuset 'problem' isn't nowhere near enough to make hotplug
> 'safe'. unplug also destroys task_struct::cpus_allowed.
> 
> Try it:
> 
>  # schedtool -a 2 $$
>  # grep Cpus_allowed /proc/self/status
>  Cpus_allowed:   000004
>  # echo 0 > /sys/devices/system/cpu/cpu2/online
>  # grep Cpus_allowed /proc/self/status
>  Cpus_allowed:   ffffff
> 
> 
> See, hotplug is destructive, it has to be, there's no saying the cpu
> will every come back.

I think it's ok for hotplug to be destructive. But I guess I'm not
entirely sure why cpusets can't retain user-inputted
configuration/policy information even while destroying things currently?
And re-instating that policy if possible in the future?

> So mucking about trying to make cpusets non-destructive is pointless.
> 
> The real bug is people using hotplug (for all kinds of stupid stuff) and
> expecting anything different.

Probably true :)

-Nish

-- 
Nishanth Aravamudan <nacc@us.ibm.com>
IBM Linux Technology Center

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