Re: [PATCH 2/2] cpuhotplug: make get_online_cpus() scalability byusing percpu counter

[Lai Jiangshan]

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.
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