Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler

[Avi Kivity <avi@redhat.com>]

On 07/11/2012 02:18 PM, Christian Borntraeger wrote:
> On 11/07/12 13:04, Avi Kivity wrote:
>> On 07/11/2012 01:17 PM, Christian Borntraeger wrote:
>>> On 11/07/12 11:06, Avi Kivity wrote:
>>> [...]
>>>>> Almost all s390 kernels use diag9c (directed yield to a given guest cpu) for spinlocks, though.
>>>>
>>>> Perhaps x86 should copy this.
>>>
>>> See arch/s390/lib/spinlock.c
>>> The basic idea is using several heuristics:
>>> - loop for a given amount of loops
>>> - check if the lock holder is currently scheduled by the hypervisor
>>>   (smp_vcpu_scheduled, which uses the sigp sense running instruction)
>>>   Dont know if such thing is available for x86. It must be a lot cheaper
>>>   than a guest exit to be useful
>> 
>> We could make it available via shared memory, updated using preempt
>> notifiers.  Of course piling on more pv makes this less attractive.
>> 
>>> - if lock holder is not running and we looped for a while do a directed
>>>   yield to that cpu.
>>>
>>>>
>>>>> So there is no win here, but there are other cases were diag44 is used, e.g. cpu_relax.
>>>>> I have to double check with others, if these cases are critical, but for now, it seems 
>>>>> that your dummy implementation  for s390 is just fine. After all it is a no-op until 
>>>>> we implement something.
>>>>
>>>> Does the data structure make sense for you?  If so we can move it to
>>>> common code (and manage it in kvm_vcpu_on_spin()).  We can guard it with
>>>> CONFIG_KVM_HAVE_CPU_RELAX_INTERCEPT or something, so other archs don't
>>>> have to pay anything.
>>>
>>> Ignoring the name,
>> 
>> What name would you suggest?
> 
> maybe vcpu_no_progress instead of pause_loop_exited

Ah, I thouht you objected to the CONFIG var.  Maybe call it
cpu_relax_intercepted since that's the linuxy name for the instruction.

-- 
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