On Thu, Sep 05, 2013 at 06:29:15PM +0800, Xiao Guangrong wrote:
> It is easy if the handler is in the vcpu context, in that case we can use
> walk_shadow_page_lockless_begin() and walk_shadow_page_lockless_end() that
> disable interrupt to stop shadow page being freed. But we are on the ioctl
> context and the paths we are optimizing for have heavy workload, disabling
> interrupt is not good for the system performance
>
> We add a indicator into kvm struct (kvm->arch.rcu_free_shadow_page), then
> use call_rcu() to free the shadow page if that indicator is set. Set/Clear the
> indicator are protected by slot-lock, so it need not be atomic and does not
> hurt the performance and the scalability
>
> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
> ---
> arch/x86/include/asm/kvm_host.h | 6 +++++-
> arch/x86/kvm/mmu.c | 32 ++++++++++++++++++++++++++++++++
> arch/x86/kvm/mmu.h | 22 ++++++++++++++++++++++
> 3 files changed, 59 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index c76ff74..8e4ca0d 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -226,7 +226,10 @@ struct kvm_mmu_page {
> /* The page is obsolete if mmu_valid_gen != kvm->arch.mmu_valid_gen. */
> unsigned long mmu_valid_gen;
>
> - DECLARE_BITMAP(unsync_child_bitmap, 512);
> + union {
> + DECLARE_BITMAP(unsync_child_bitmap, 512);
> + struct rcu_head rcu;
> + };
>
> #ifdef CONFIG_X86_32
> /*
> @@ -554,6 +557,7 @@ struct kvm_arch {
> */
> struct list_head active_mmu_pages;
> struct list_head zapped_obsolete_pages;
> + bool rcu_free_shadow_page;
>
> struct list_head assigned_dev_head;
> struct iommu_domain *iommu_domain;
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 2bf450a..f551fc7 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2355,6 +2355,30 @@ static int kvm_mmu_prepare_zap_page(struct kvm *kvm, struct kvm_mmu_page *sp,
> return ret;
> }
>
> +static void kvm_mmu_isolate_pages(struct list_head *invalid_list)
> +{
> + struct kvm_mmu_page *sp;
> +
> + list_for_each_entry(sp, invalid_list, link)
> + kvm_mmu_isolate_page(sp);
> +}
> +
> +static void free_pages_rcu(struct rcu_head *head)
> +{
> + struct kvm_mmu_page *next, *sp;
> +
> + sp = container_of(head, struct kvm_mmu_page, rcu);
> + while (sp) {
> + if (!list_empty(&sp->link))
> + next = list_first_entry(&sp->link,
> + struct kvm_mmu_page, link);
> + else
> + next = NULL;
> + kvm_mmu_free_page(sp);
> + sp = next;
> + }
> +}
> +
> static void kvm_mmu_commit_zap_page(struct kvm *kvm,
> struct list_head *invalid_list)
> {
> @@ -2375,6 +2399,14 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
> */
> kvm_flush_remote_tlbs(kvm);
>
> + if (kvm->arch.rcu_free_shadow_page) {
> + kvm_mmu_isolate_pages(invalid_list);
> + sp = list_first_entry(invalid_list, struct kvm_mmu_page, link);
> + list_del_init(invalid_list);
> + call_rcu(&sp->rcu, free_pages_rcu);
> + return;
> + }
This is unbounded (there was a similar problem with early fast page fault
implementations):
>From RCU/checklist.txt:
" An especially important property of the synchronize_rcu()
primitive is that it automatically self-limits: if grace periods
are delayed for whatever reason, then the synchronize_rcu()
primitive will correspondingly delay updates. In contrast,
code using call_rcu() should explicitly limit update rate in
cases where grace periods are delayed, as failing to do so can
result in excessive realtime latencies or even OOM conditions.
"
Moreover, freeing pages differently depending on some state should
be avoided.
Alternatives:
- Disable interrupts at write protect sites.
- Rate limit the number of pages freed via call_rcu
per grace period.
- Some better alternative.
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