linux-kernel - Re: [PATCH v2 0/7] CPU hotplug, cpusets: Fix issues with cpusets handling upon CPU hotplug

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Date:	Fri, 04 May 2012 23:34:25 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Nishanth Aravamudan <nacc@...ux.vnet.ibm.com>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	mingo@...nel.org, pjt@...gle.com, paul@...lmenage.org,
	akpm@...ux-foundation.org, rjw@...k.pl, nacc@...ibm.com,
	paulmck@...ux.vnet.ibm.com, tglx@...utronix.de,
	seto.hidetoshi@...fujitsu.com, rob@...dley.net, tj@...nel.org,
	mschmidt@...hat.com, berrange@...hat.com,
	nikunj@...ux.vnet.ibm.com, vatsa@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: [PATCH v2 0/7] CPU hotplug, cpusets: Fix issues with cpusets
 handling upon CPU hotplug

On Fri, 2012-05-04 at 14:27 -0700, Nishanth Aravamudan wrote:
> >  - if you retain it for cpuset but not others that's confusing (too);
> 
> That's a good point.
> 
> Related, possibly counter-example, and perhaps I'm wrong about it.  When
> we hot-unplug a CPU, and a task's scheduler affinity (via
> sched_setaffinity) refers to that CPU only, do we kill that task? Can
> you sched_setaffinity a task to a CPU that is offline (alone or in a
> group of possible CPUs)? Or is it allowed to run anywhere? Do we destroy
> its affinity policy when that situation is run across?

See a few emails back, we destroy the affinity. Current cpuset behaviour
can be said to match that.

>  Or do we restore the task to the CPU again when we re-plug it?

Nope that information is lost forever from the kernels pov.

Keeping this information around for the off-chance of needing it is
rather expensive (512 bytes per task for your regular distro kernel that
has NR_CPUS=4096).

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