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Date: Mon, 12 Apr 2010 17:24:30 +0800
From: Lai Jiangshan <laijs@cn.fujitsu.com>
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To: Oleg Nesterov <oleg@redhat.com>
CC: Gautham R Shenoy <ego@in.ibm.com>, Rusty Russell <rusty@rustcorp.com.au>,
       Benjamin Herrenschmidt <benh@kernel.crashing.org>,
       Hugh Dickins <hugh.dickins@tiscali.co.uk>, Ingo Molnar <mingo@elte.hu>,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Nathan Fontenot <nfont@austin.ibm.com>,
       Peter Zijlstra <peterz@infradead.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Thomas Gleixner <tglx@linutronix.de>, Sachin Sant <sachinp@in.ibm.com>,
       "H. Peter Anvin" <hpa@zytor.com>, Shane Wang <shane.wang@intel.com>,
       Roland McGrath <roland@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] cpuhotplug: make get_online_cpus() scalability by
 using percpu counter
References: <4BB9BD8A.9040209@cn.fujitsu.com> <20100405162901.GA3567@redhat.com> <20100406120039.GC5680@redhat.com> <4BBC8A11.3040501@cn.fujitsu.com> <20100407135456.GA12029@redhat.com> <20100409121235.GA5784@redhat.com>
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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().

Lai.