From: Dave Hansen <dave@linux.vnet.ibm.com>
Subject: Re: [RFC][PATCH 4/9] create aggregate kvm_total_used_mmu_pages
 value
Date: Wed, 16 Jun 2010 09:55:00 -0700
Message-ID: <1276707300.6437.17429.camel@nimitz>
References: <20100615135518.BC244431@kernel.beaverton.ibm.com>
	 <20100615135523.25D24A73@kernel.beaverton.ibm.com>
	 <4C188FC0.3050306@redhat.com>
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Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org
To: Avi Kivity <avi@redhat.com>
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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