linux-kernel - Re: Fw: Re: [mm PATCH 4/6] RCU: (now) CPU hotplug

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Message-Id: <20070126121739.be3e072a.akpm@osdl.org>
Date:	Fri, 26 Jan 2007 12:17:39 -0800
From:	Andrew Morton <akpm@...l.org>
To:	dipankar@...ibm.com
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Gautham Shenoy <ego@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: Fw: Re: [mm PATCH 4/6] RCU: (now) CPU hotplug

On Sat, 27 Jan 2007 01:16:22 +0530
Dipankar Sarma <dipankar@...ibm.com> wrote:

> > The plan is, I hope, to rip it all out and do freeze_processes() on the
> > hotplug side, so nobody else needs to worry about cpu hotplug any more. 
> > But at present everyone seems to be in hiding.
> 
> This would be ideal. However, we don't seem to have any momentum
> on this.

There's no point in expending effort on a fancy new lock until this option
has been eliminated, so yeah, things are stuck.

> The other thing we would need to do in this case is to
> check if all the users of cpu hotplug can tolerate a very slow
> hotplug step if there are 10s of thousands of processes and threads.

Yes, that needs evaluation.  If it's a problem then we might need to
introduce a more sophisticated user interface which enables userspace to
take multiple CPUs online and offline in a single step, rather than
one-at-a-time.  I expect that'd be fairly straightforward.

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