Message-ID: <1336163281.6509.69.camel@twins>
Subject: Re: [PATCH v2 0/7] CPU hotplug, cpusets: Fix issues with cpusets
 handling upon CPU hotplug
From: Peter Zijlstra <peterz@infradead.org>
To: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Cc: 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
Date: Fri, 04 May 2012 22:28:01 +0200
In-Reply-To: <1336162456.6509.63.camel@twins>
References: <20120504191535.4603.83236.stgit@srivatsabhat>
	 <1336159496.6509.51.camel@twins> <4FA434E9.6000305@linux.vnet.ibm.com>
	 <1336162456.6509.63.camel@twins>
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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.

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.
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