Re: [PATCH] kvm: don't call mmu_shrinker w/o used_mmu_pages

[Hillf Danton <dhillf@gmail.com>]

On Sat, Apr 14, 2012 at 6:38 AM, Ying Han <yinghan@google.com> wrote:
> The mmu_shrink() is heavy by itself by iterating all kvms and holding
> the kvm_lock. spotted the code w/ Rik during LSF, and it turns out we
> don't need to call the shrinker if nothing to shrink.
>
> Signed-off-by: Ying Han <yinghan@google.com>
> ---
>  arch/x86/kvm/mmu.c |   10 +++++++++-
>  1 files changed, 9 insertions(+), 1 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 4cb1642..7025736 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -188,6 +188,11 @@ static u64 __read_mostly shadow_mmio_mask;
>
>  static void mmu_spte_set(u64 *sptep, u64 spte);
>
> +static inline int get_kvm_total_used_mmu_pages()
> +{
> +       return percpu_counter_read_positive(&kvm_total_used_mmu_pages);
> +}
> +
>  void kvm_mmu_set_mmio_spte_mask(u64 mmio_mask)
>  {
>        shadow_mmio_mask = mmio_mask;
> @@ -3900,6 +3905,9 @@ static int mmu_shrink(struct shrinker *shrink, struct shrink_control *sc)
>        if (nr_to_scan == 0)
>                goto out;
>
> +       if (!get_kvm_total_used_mmu_pages())
> +               return 0;
> +
>        raw_spin_lock(&kvm_lock);
>
>        list_for_each_entry(kvm, &vm_list, vm_list) {
> @@ -3926,7 +3934,7 @@ static int mmu_shrink(struct shrinker *shrink, struct shrink_control *sc)
>        raw_spin_unlock(&kvm_lock);
>
>  out:
> -       return percpu_counter_read_positive(&kvm_total_used_mmu_pages);
> +       return get_kvm_total_used_mmu_pages();
>  }
>
Just nitpick.
If new helper not created, there is only one hunk needed.

btw, make sense to check nr_to_scan while scanning vm_list, and bail out
if it hits zero?

Good Weekend
-hd

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