Re: [RFC PATCH 01/10] CPU hotplug: Introduce "stable" cpu onlinemask, for atomic hotplug readers

[Michael Wang]

On 12/05/2012 10:56 AM, Michael Wang wrote:
[...]
>>
>> I wonder about the cpu-online case.  A typical caller might want to do:
>>
>>
>> /*
>>  * Set each online CPU's "foo" to "bar"
>>  */
>>
>> int global_bar;
>>
>> void set_cpu_foo(int bar)
>> {
>> 	get_online_cpus_stable_atomic();
>> 	global_bar = bar;
>> 	for_each_online_cpu_stable()
>> 		cpu->foo = bar;
>> 	put_online_cpus_stable_atomic()
>> }
>>
>> void_cpu_online_notifier_handler(void)
>> {
>> 	cpu->foo = global_bar;
>> }

Oh, forgive me for misunderstanding your question :(

In this case, we have to prevent hotplug happen, not just ensure the
online mask is correct.

Hmm..., we need more consideration.

Regards,
Michael Wang

>>
>> And I think that set_cpu_foo() would be buggy, because a CPU could come
>> online before global_bar was altered, and that newly-online CPU would
>> pick up the old value of `bar'.
>>
>> So what's the rule here?  global_bar must be written before we run
>> get_online_cpus_stable_atomic()?
>>
>> Anyway, please have a think and spell all this out?
> 
> That's right, actually this related to one question, should the hotplug
> happen during get_online and put_online?
> 
> Answer will be YES according to old API which using mutex, the hotplug
> won't happen in critical section, but the cost is get_online() will
> block, which will kill the performance.
> 
> So we designed this mechanism to do acceleration, but as you pointed
> out, although the result will never be wrong, but the 'stable' mask is
> not stable since it could be changed in critical section.
> 
> And we have two solution.
> 
> One is from Srivatsa, using 'read_lock' and 'write_lock', it will
> prevent hotplug happen just like the old rule, the cost is we need a
> global 'rw_lock' which perform bad on NUMA system, and no doubt,
> get_online() will block for short time when doing hotplug.
> 
> Another is to maintain a per-cpu cache mask, this mask will only be
> updated in get_online(), and be used in critical section, then we will
> get a real stable mask, but one flaw is, on different cpu in critical
> section, online mask will be different.
> 
> We will be appreciate if we could collect some comments on which one to
> be used in next version.
> 
> Regards,
> Michael Wang
> 
>>
>>>  struct take_cpu_down_param {
>>>  	unsigned long mod;
>>>  	void *hcpu;
>>> @@ -246,7 +351,9 @@ struct take_cpu_down_param {
>>>  static int __ref take_cpu_down(void *_param)
>>>  {
>>>  	struct take_cpu_down_param *param = _param;
>>> -	int err;
>>> +	int err, cpu = (long)(param->hcpu);
>>> +
>>
>> Like this please:
>>
>> 	int err;
>> 	int cpu = (long)(param->hcpu);
>>
>>> +	prepare_cpu_take_down(cpu);
>>>  
>>>  	/* Ensure this CPU doesn't handle any more interrupts. */
>>>  	err = __cpu_disable();
>>>
>>> ...
>>>
>>
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> 

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