Re: [PATCH] KVM: MMU: Introduce single thread to zap collapsible sptes

[Wanpeng Li]

ping,
On Thu, 6 Dec 2018 at 15:58, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
>
> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
>
> Last year guys from huawei reported that the call of memory_global_dirty_log_start/stop()
> takes 13s for 4T memory and cause guest freeze too long which increases the unacceptable
> migration downtime. [1] [2]
>
> Guangrong pointed out:
>
> | collapsible_sptes zaps 4k mappings to make memory-read happy, it is not
> | required by the semanteme of KVM_SET_USER_MEMORY_REGION and it is not
> | urgent for vCPU's running, it could be done in a separate thread and use
> | lock-break technology.
>
> [1] https://lists.gnu.org/archive/html/qemu-devel/2017-04/msg05249.html
> [2] https://www.mail-archive.com/qemu-devel@xxxxxxxxxx/msg449994.html
>
> Several TB memory guest is common now after NVDIMM is deployed in cloud environment.
> This patch utilizes worker thread to zap collapsible sptes in order to lazy collapse
> small sptes into large sptes during roll-back after live migration fails.
>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |  3 +++
>  arch/x86/kvm/mmu.c              | 37 ++++++++++++++++++++++++++++++++-----
>  arch/x86/kvm/x86.c              |  4 ++++
>  3 files changed, 39 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index fbda5a9..dde32f9 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -892,6 +892,8 @@ struct kvm_arch {
>         u64 master_cycle_now;
>         struct delayed_work kvmclock_update_work;
>         struct delayed_work kvmclock_sync_work;
> +       struct delayed_work kvm_mmu_zap_collapsible_sptes_work;
> +       bool zap_in_progress;
>
>         struct kvm_xen_hvm_config xen_hvm_config;
>
> @@ -1247,6 +1249,7 @@ void kvm_mmu_zap_all(struct kvm *kvm);
>  void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm, struct kvm_memslots *slots);
>  unsigned int kvm_mmu_calculate_mmu_pages(struct kvm *kvm);
>  void kvm_mmu_change_mmu_pages(struct kvm *kvm, unsigned int kvm_nr_mmu_pages);
> +void zap_collapsible_sptes_fn(struct work_struct *work);
>
>  int load_pdptrs(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu, unsigned long cr3);
>  bool pdptrs_changed(struct kvm_vcpu *vcpu);
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 7c03c0f..fe87dd3 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -5679,14 +5679,41 @@ static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm,
>         return need_tlb_flush;
>  }
>
> +void zap_collapsible_sptes_fn(struct work_struct *work)
> +{
> +       struct kvm_memory_slot *memslot;
> +       struct kvm_memslots *slots;
> +       struct delayed_work *dwork = to_delayed_work(work);
> +       struct kvm_arch *ka = container_of(dwork, struct kvm_arch,
> +                                          kvm_mmu_zap_collapsible_sptes_work);
> +       struct kvm *kvm = container_of(ka, struct kvm, arch);
> +       int i;
> +
> +       mutex_lock(&kvm->slots_lock);
> +       for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
> +               spin_lock(&kvm->mmu_lock);
> +               slots = __kvm_memslots(kvm, i);
> +               kvm_for_each_memslot(memslot, slots) {
> +                       slot_handle_leaf(kvm, (struct kvm_memory_slot *)memslot,
> +                               kvm_mmu_zap_collapsible_spte, true);
> +                       if (need_resched() || spin_needbreak(&kvm->mmu_lock))
> +                               cond_resched_lock(&kvm->mmu_lock);
> +               }
> +               spin_unlock(&kvm->mmu_lock);
> +       }
> +       kvm->arch.zap_in_progress = false;
> +       mutex_unlock(&kvm->slots_lock);
> +}
> +
> +#define KVM_MMU_ZAP_DELAYED (60 * HZ)
>  void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
>                                    const struct kvm_memory_slot *memslot)
>  {
> -       /* FIXME: const-ify all uses of struct kvm_memory_slot.  */
> -       spin_lock(&kvm->mmu_lock);
> -       slot_handle_leaf(kvm, (struct kvm_memory_slot *)memslot,
> -                        kvm_mmu_zap_collapsible_spte, true);
> -       spin_unlock(&kvm->mmu_lock);
> +       if (!kvm->arch.zap_in_progress) {
> +               kvm->arch.zap_in_progress = true;
> +               schedule_delayed_work(&kvm->arch.kvm_mmu_zap_collapsible_sptes_work,
> +                       KVM_MMU_ZAP_DELAYED);
> +       }
>  }
>
>  void kvm_mmu_slot_leaf_clear_dirty(struct kvm *kvm,
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d029377..c2af289 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9019,6 +9019,9 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>
>         INIT_DELAYED_WORK(&kvm->arch.kvmclock_update_work, kvmclock_update_fn);
>         INIT_DELAYED_WORK(&kvm->arch.kvmclock_sync_work, kvmclock_sync_fn);
> +       INIT_DELAYED_WORK(&kvm->arch.kvm_mmu_zap_collapsible_sptes_work,
> +                       zap_collapsible_sptes_fn);
> +       kvm->arch.zap_in_progress = false;
>
>         kvm_hv_init_vm(kvm);
>         kvm_page_track_init(kvm);
> @@ -9064,6 +9067,7 @@ void kvm_arch_sync_events(struct kvm *kvm)
>  {
>         cancel_delayed_work_sync(&kvm->arch.kvmclock_sync_work);
>         cancel_delayed_work_sync(&kvm->arch.kvmclock_update_work);
> +       cancel_delayed_work_sync(&kvm->arch.kvm_mmu_zap_collapsible_sptes_work);
>         kvm_free_pit(kvm);
>  }
>
> --
> 2.7.4
>