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Subject: Re: [PATCH 2/2] cpuhotplug: make get_online_cpus() scalability by
 using percpu counter
From: Peter Zijlstra <peterz@infradead.org>
To: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Oleg Nesterov <oleg@redhat.com>, Gautham R Shenoy <ego@in.ibm.com>,
       Rusty Russell <rusty@rustcorp.com.au>,
       Benjamin Herrenschmidt <benh@kernel.crashing.org>,
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Date: Mon, 12 Apr 2010 11:28:59 +0200
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On Mon, 2010-04-12 at 17:24 +0800, Lai Jiangshan wrote:
> Oleg Nesterov wrote:
> > On 04/07, Oleg Nesterov wrote:
> >> On 04/07, Lai Jiangshan wrote:
> >>> Old get_online_cpus() is read-preference, I think the goal of this ability
> >>> is allow get_online_cpus()/put_online_cpus() to be called nested.
> >> Sure, I understand why you added task_struct->get_online_cpus_nest.
> >>
> >>> and use per-task counter for allowing get_online_cpus()/put_online_cpus()
> >>> to be called nested, I think this deal is absolutely worth.
> >> As I said, I am not going to argue. I can't justify this tradeoff.
> > 
> > But, I must admit, I'd like to avoid adding the new member to task_struct.
> > 
> > What do you think about the code below?
> > 
> > I didn't even try to compile it, just to explain what I mean.
> > 
> > In short: we have the per-cpu fast counters, plus the slow counter
> > which is only used when cpu_hotplug_begin() is in progress.
> > 
> > Oleg.
> > 
> 
> get_online_cpus() in your code is still read-preference.
> I wish we quit this ability of get_online_cpus().

Why?