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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, May 28, 2013 at 11:02:09PM +0800, Xiao Guangrong wrote:
> 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?

That is no problem. The problem is why you chose "10" as the minimum number of
pages to zap before considering reschedule. I would expect the need to
reschedule to be rare enough that one kvm_mmu_zap_all instance (between
schedule in and schedule out) to be able to release no less than a
thousand pages.

So i'd like to understand better what is the drive for this change (this
was the original question).