Re: [PATCH] locking/osq: Drop the overload of osq lock

[panxinhui]

> 在 2016年6月26日，03:20，Peter Zijlstra <peterz@xxxxxxxxxxxxx> 写道：
> 
> On Sun, Jun 26, 2016 at 01:27:56AM +0800, panxinhui wrote:
>>>> Would that not have issues where the owner cpu is kept running but the
>>>> spinner (ie. _this_ vcpu) gets preempted? I would think that in that
>>>> case we too want to stop spinning.
>>>> 
>>> 
>> do  you mean that the spinner detect itself had yield out during the
>> big spin loop?
>> 
>> It is very possible to happen.  BUT if spinner(on this vcpu) yield
>> out, the next spinner would break the spin loop.  AND if spinner
>> detect itself yield out once, it’s very possible to get the osq lock
>> soon as long as the ower vcpu is running.
>> 
>> SO I think we need just check the owner vcpu’s yield_count.
> 
> I had a quick look at KVM and it looks like it only has
> kvm_cpu::preempted, which would suggest the interface boqun proposed.
> 
> We'll have to look at many of the other virt platforms as well to see
> what they can do.
> 
> We could also provide _both_ interfaces and a platform can implement
> whichever variant (or both) it can.
> 
the kvm code on ppc has implemented  yield_count.
It let me feel a little relaxed. :)

looks like we could  introduce the interface like below.

bool vcpu_is_preempted(int cpu)
{
	return arch_vcpu_is_preempted(cpu);
}

#ifdef arch_vcpu_has_yield_count 
bool vcpu_has_preemtped_once(int cpu, unsigned int yield_count)
 {
	return arch_get_vcpu_yield_count() != yield_count;
}

#else
bool vcpu_has_preemtped_once(int cpu, unsigned int yield_count)
{
	/*just let called know it is preepmpted*/
	return vcpu_is_preempted(cpu);
}
#endif