Re: [PATCH v7 04/11] KVM: MMU: zap pages in batch

[Xiao Guangrong]

On 05/28/2013 08:18 AM, Marcelo Tosatti wrote:
> On Mon, May 27, 2013 at 10:20:12AM +0800, Xiao Guangrong wrote:
>> On 05/25/2013 04:34 AM, Marcelo Tosatti wrote:
>>> On Thu, May 23, 2013 at 03:55:53AM +0800, Xiao Guangrong wrote:
>>>> Zap at lease 10 pages before releasing mmu-lock to reduce the overload
>>>> caused by requiring lock
>>>>
>>>> After the patch, kvm_zap_obsolete_pages can forward progress anyway,
>>>> so update the comments
>>>>
>>>> [ It improves kernel building 0.6% ~ 1% ]
>>>
>>> Can you please describe the overload in more detail? Under what scenario
>>> is kernel building improved?
>>
>> Yes.
>>
>> The scenario is we do kernel building, meanwhile, repeatedly read PCI rom
>> every one second.
>>
>> [
>>    echo 1 > /sys/bus/pci/devices/0000\:00\:03.0/rom
>>    cat /sys/bus/pci/devices/0000\:00\:03.0/rom > /dev/null
>> ]
> 
> Can't see why it reflects real world scenario (or a real world
> scenario with same characteristics regarding kvm_mmu_zap_all vs faults)?
> 
> Point is, it would be good to understand why this change 
> is improving performance? What are these cases where breaking out of
> kvm_mmu_zap_all due to either (need_resched || spin_needbreak) on zapped
> < 10 ?

When guest read ROM, qemu will set the memory to map the device's firmware,
that is why kvm_mmu_zap_all can be called in the scenario.

The reasons why it heart the performance are:
1): Qemu use a global io-lock to sync all vcpu, so that the io-lock is held
    when we do kvm_mmu_zap_all(). If kvm_mmu_zap_all() is not efficient, all
    other vcpus need wait a long time to do I/O.

2): kvm_mmu_zap_all() is triggered in vcpu context. so it can block the IPI
    request from other vcpus.

Is it enough?



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