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

[Lai Jiangshan]

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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