linux-kernel - Re: [PATCH 2/2] cpuhotplug: make get_online_cpus() scalability by using percpu counter

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Date:	Wed, 07 Apr 2010 21:35:13 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Oleg Nesterov <oleg@...hat.com>, Gautham R Shenoy <ego@...ibm.com>
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Ingo Molnar <mingo@...e.hu>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Nathan Fontenot <nfont@...tin.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sachin Sant <sachinp@...ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Shane Wang <shane.wang@...el.com>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] cpuhotplug: make get_online_cpus() scalability by
 using percpu counter

Oleg Nesterov wrote:
> On 04/05, Oleg Nesterov wrote:
>> On 04/05, Lai Jiangshan wrote:
>>> 1) get_online_cpus() must be allowed to be called recursively, so I added
>>>    get_online_cpus_nest for every task for new code.
>> Well, iirc one of the goals of
>>
>> 	cpu-hotplug: replace lock_cpu_hotplug() with get_online_cpus()
>> 	86ef5c9a8edd78e6bf92879f32329d89b2d55b5a
>>
>> was avoiding the new members in task_struct. I leave this up to you
>> and Gautham.

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.

But read-preference RWL may cause write side starvation, so I abandon this ability,
and use per-task counter for allowing get_online_cpus()/put_online_cpus()
to be called nested, I think this deal is absolutely worth.

>>
>>
>> Lai, I didn't read this patch carefully yet (and I can't apply it to
>> Linus's tree). But at first glance,
> 
> because I tried to apply it without 1/2 ;)
> 
>>>  void put_online_cpus(void)
>>>  {
>>> ...
>>> +	if (!--current->get_online_cpus_nest) {
>>> +		preempt_disable();
>>> +		__get_cpu_var(refcount)--;
>>> +		if (cpu_hotplug_task)
>>> +			wake_up_process(cpu_hotplug_task);
>> This looks unsafe. In theory nothing protects cpu_hotplug_task from
>> exiting if refcount_sum() becomes zero, this means wake_up_process()
>> can hit the freed/reused/unmapped task_struct. Probably cpu_hotplug_done()
>> needs another synhronize_sched() before return.
> 
> Yes, I think this is true, at least in theory.

preempt_disable() prevent cpu_hotplug_task from exiting.


Thanks, Lai
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