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

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Message-Id: <1276700761.6437.16878.camel@nimitz>
Date:	Wed, 16 Jun 2010 08:06:01 -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:
> On 06/15/2010 04:55 PM, Dave Hansen wrote:
> > +/*
> > + * This value is the sum of all of the kvm instances's
> > + * kvm->arch.n_used_mmu_pages values.  We need a global,
> > + * aggregate version in order to make the slab shrinker
> > + * faster
> > + */
> > +static unsigned int kvm_total_used_mmu_pages;
> >    
> 
> The variable needs to be at the top of the file.

Gotcha, will do.

> > +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.

That's a good point.  All of the modifications are done under locks, but
the fast path isn't any more.  I'll fix it up.

-- Dave

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