linux-kernel - Re: [RFC][PATCH 4/9] create aggregate kvm_total_used_mmu

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Date:	Wed, 16 Jun 2010 09:55:00 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC][PATCH 4/9] create aggregate kvm_total_used_mmu_pages
 value

On Wed, 2010-06-16 at 11:48 +0300, Avi Kivity wrote:
> > +static inline void kvm_mod_used_mmu_pages(struct kvm *kvm, int nr)
> > +{
> > +     kvm->arch.n_used_mmu_pages += nr;
> > +     kvm_total_used_mmu_pages += nr;
> >    
> 
> Needs an atomic operation, since there's no global lock here.  To avoid 
> bouncing this cacheline around, make the variable percpu and make 
> readers take a sum across all cpus.  Side benefit is that you no longer 
> need an atomic but a local_t, which is considerably cheaper.

We do have the stuff in:

	include/linux/percpu_counter.h

the downside being that they're not precise and they're *HUGE* according
to the comment. :)

It's actually fairly difficult to do a counter which is precise,
scalable, and works well for small CPU counts when NR_CPUS is large.  Do
you mind if we just stick with a plain atomic_t for now?

-- Dave

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