Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 12/12/2012 11:32 PM, Oleg Nesterov wrote:
> On 12/12, Srivatsa S. Bhat wrote:
>>
>> On 12/12/2012 10:47 PM, Oleg Nesterov wrote:
>>>
>>> Why it needs to be per-cpu? It can be global and __read_mostly to avoid
>>> the false-sharing. OK, perhaps to put reader_percpu_refcnt/writer_signal
>>> into a single cacheline...
>>
>> Even I realized this (that we could use a global) after posting out the
>> series.. But do you think that it would be better to retain the per-cpu
>> variant itself, due to the cache effects?
> 
> I don't really know, up to you. This was the question ;)

OK :-)

> 
>>> Do we really need local_irq_save/restore in put_ ?
>>>
>>
>> Hmm.. good point! I don't think we need it.
> 
> And _perhaps_ get_ can avoid it too?
> 
> I didn't really try to think, probably this is not right, but can't
> something like this work?
> 
> 	#define XXXX	(1 << 16)
> 	#define MASK	(XXXX -1)
> 
> 	void get_online_cpus_atomic(void)
> 	{
> 		preempt_disable();
> 
> 		// only for writer
> 		__this_cpu_add(reader_percpu_refcnt, XXXX);
> 
> 		if (__this_cpu_read(reader_percpu_refcnt) & MASK) {
> 			__this_cpu_inc(reader_percpu_refcnt);
> 		} else {
> 			smp_wmb();
> 			if (writer_active()) {
> 				...
> 			}
> 		}
> 
> 		__this_cpu_dec(reader_percpu_refcnt, XXXX);
> 	}
> 

Sorry, may be I'm too blind to see, but I didn't understand the logic
of how the mask helps us avoid disabling interrupts.. Can you kindly
elaborate?

> 	void put_online_cpus_atomic(void)
> 	{
> 		if (__this_cpu_read(reader_percpu_refcnt) & MASK)
> 			__this_cpu_dec(reader_percpu_refcnt);
> 		else
> 			read_unlock(&hotplug_rwlock);
> 		preempt_enable();
> 	}
> 

Regards,
Srivatsa S. Bhat