linux-kernel - Re: CPU Hotplug rework

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Message-ID: <877gya99uj.fsf@rustcorp.com.au>
Date:	Sat, 24 Mar 2012 09:57:32 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	"akpm\@linux-foundation.org" <akpm@...ux-foundation.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Milton Miller <miltonm@....com>,
	"mingo\@elte.hu" <mingo@...e.hu>, Tejun Heo <tj@...nel.org>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Linux PM mailing list <linux-pm@...r.kernel.org>
Subject: Re: CPU Hotplug rework

On Thu, 22 Mar 2012 15:49:20 -0700, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> On Thu, Mar 22, 2012 at 02:55:04PM +1030, Rusty Russell wrote:
> > On Wed, 21 Mar 2012 10:01:59 +0100, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> > > Thing is, if its really too much for some people, they can orchestrate
> > > it such that its not. Just move everybody in a cpuset, clear the to be
> > > offlined cpu from the cpuset's mask -- this will migrate everybody away.
> > > Then hotplug will find an empty runqueue and its fast, no?
> > 
> > I like this solution better.
> 
> As long as we have some way to handle kthreads that are algorithmically
> tied to a given CPU.  There are coding conventions to handle this, for
> example, do everything with preemption disabled and just after each
> preempt_disable() verify that you are in fact running on the correct
> CPU, but it is easy to imagine improvements.

I don't think we should move per-cpu kthreads at all.  Let's stop trying
to save a few bytes of memory, and just leave them frozen.  They'll run
again if/when the CPU returns.

Cheers,
Rusty.
-- 
  How could I marry someone with more hair than me?  http://baldalex.org
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