linux-kernel - Re: CPU Hotplug rework

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Message-ID: <1332769098.16159.111.camel@twins>
Date:	Mon, 26 Mar 2012 15:38:18 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Rusty Russell <rusty@...tcorp.com.au>, paulmck@...ux.vnet.ibm.com,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Arjan van de Ven <arjan@...radead.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Milton Miller <miltonm@....com>,
	"mingo@...e.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 Mon, 2012-03-26 at 09:09 -0400, Steven Rostedt wrote:
> On Mon, 2012-03-26 at 10:02 +0200, Peter Zijlstra wrote:
> > On Mon, 2012-03-26 at 11:11 +1030, Rusty Russell wrote:
> > > Obviously, having callbacks hanging around until the CPU comes back is
> > > not viable, nor is blocking preempt during the callbacks.  Calling
> > > get_online_cpus() is too heavy. 
> > 
> > -rt has patches (albeit somewhat ugly) to make get_online_cpus() a br
> > style rw-lock. 
> 
> Unfortunately, -rt still has broken cpu hotplug.

This is still the issue where we take the hotplug lock for write, but
then rely on other threads to complete -- say CPU_DOWN_PREPARE notifiers
doing flush_workqueue(), and the worker kthread needing to acquire the
hotplug lock for read, right?

Yes, that is bothersome, but doesn't hinder the work required to get
get_online_cpus() usably fast, right?
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