Re: [RFC][PATCH 9/9] make kvm mmu shrinker more aggressive

Avi Kivity
2010-06-16T09:24:00Z

On 06/15/2010 04:55 PM, Dave Hansen wrote:
> In a previous patch, we removed the 'nr_to_scan' tracking.
> It was not being used to track the number of objects
> scanned, so we stopped using it entirely. Here, we
> strart using it again.
>
> The theory here is simple; if we already have the refcount
> and the kvm->mmu_lock, then we should do as much work as
> possible under the lock. The downside is that we're less
> fair about the KVM instances from which we reclaim. Each
> call to mmu_shrink() will tend to "pick on" one instance,
> after which it gets moved to the end of the list and left
> alone for a while.
>

That also increases the latency hit, as well as a potential fault storm,
on that instance. Spreading out is less efficient, but smoother.

> If mmu_shrink() has already done a significant amount of
> scanning, the use of 'nr_to_scan' inside shrink_kvm_mmu()
> will also ensure that we do not over-reclaim when we have
> already done a lot of work in this call.
>
> In the end, this patch defines a "scan" as:
> 1. An attempt to acquire a refcount on a 'struct kvm'
> 2. freeing a kvm mmu page
>
> This would probably be most ideal if we can expose some
> of the work done by kvm_mmu_remove_some_alloc_mmu_pages()
> as also counting as scanning, but I think we have churned
> enough for the moment.
>

It usually removes one page.

> Signed-off-by: Dave Hansen<dave@linux.vnet.ibm.com>
> ---
>
> linux-2.6.git-dave/arch/x86/kvm/mmu.c | 11 ++++++-----
> 1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff -puN arch/x86/kvm/mmu.c~make-shrinker-more-aggressive arch/x86/kvm/mmu.c
> --- linux-2.6.git/arch/x86/kvm/mmu.c~make-shrinker-more-aggressive 2010-06-14 11:30:44.000000000 -0700
> +++ linux-2.6.git-dave/arch/x86/kvm/mmu.c 2010-06-14 11:38:04.000000000 -0700
> @@ -2935,8 +2935,10 @@ static int shrink_kvm_mmu(struct kvm *kv
>
> idx = srcu_read_lock(&kvm->srcu);
> spin_lock(&kvm->mmu_lock);
> - if (kvm->arch.n_used_mmu_pages> 0)
> - freed_pages = kvm_mmu_remove_some_alloc_mmu_pages(kvm);
> + while (nr_to_scan> 0&& kvm->arch.n_used_mmu_pages> 0) {
> + freed_pages += kvm_mmu_remove_some_alloc_mmu_pages(kvm);
> + nr_to_scan--;
> + }
>

What tree are you patching?

--
error compiling committee.c: too many arguments to function