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Subject: Re: CPU Hotplug rework
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>, paulmck@linux.vnet.ibm.com,
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Date: Mon, 26 Mar 2012 15:38:18 +0200
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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?