Messages in this thread | | | From | "Maciej S. Szmigiero" <> | Subject | Re: [PATCH v5 01/13] KVM: x86: Cache total page count to avoid traversing the memslot array | Date | Wed, 20 Oct 2021 20:40:19 +0200 |
| |
On 20.10.2021 00:31, Sean Christopherson wrote: > On Tue, Oct 19, 2021, Sean Christopherson wrote: >> On Mon, Sep 20, 2021, Maciej S. Szmigiero wrote: >>> From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com> >>> >>> There is no point in recalculating from scratch the total number of pages >>> in all memslots each time a memslot is created or deleted. >>> >>> Just cache the value and update it accordingly on each such operation so >>> the code doesn't need to traverse the whole memslot array each time. >>> >>> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com> >>> --- >>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >>> index 28ef14155726..65fdf27b9423 100644 >>> --- a/arch/x86/kvm/x86.c >>> +++ b/arch/x86/kvm/x86.c >>> @@ -11609,9 +11609,23 @@ void kvm_arch_commit_memory_region(struct kvm *kvm, >>> const struct kvm_memory_slot *new, >>> enum kvm_mr_change change) >>> { >>> - if (!kvm->arch.n_requested_mmu_pages) >>> - kvm_mmu_change_mmu_pages(kvm, >>> - kvm_mmu_calculate_default_mmu_pages(kvm)); >>> + if (change == KVM_MR_CREATE) >>> + kvm->arch.n_memslots_pages += new->npages; >>> + else if (change == KVM_MR_DELETE) { >>> + WARN_ON(kvm->arch.n_memslots_pages < old->npages); >>> + kvm->arch.n_memslots_pages -= old->npages; >>> + } >>> + >>> + if (!kvm->arch.n_requested_mmu_pages) { >> >> Hmm, once n_requested_mmu_pages is set it can't be unset. That means this can be >> further optimized to skip avoid taking mmu_lock on flags-only changes (and >> memslot movement). E.g. >> >> if (!kvm->arch.n_requested_mmu_pages && >> (change == KVM_MR_CREATE || change == KVM_MR_DELETE)) { >> >> } >> >> It's a little risky, but kvm_vm_ioctl_set_nr_mmu_pages() would need to be modified >> to allow clearing n_requested_mmu_pages and it already takes slots_lock, so IMO >> it's ok to force kvm_vm_ioctl_set_nr_mmu_pages() to recalculate pages if it wants >> to allow reverting back to the default. > > Doh, and then I read patch 2... > > I would swap the order of patch 2 and patch 1, that way the optimization patch is > super simple, and you don't end up reworking a bunch of code that was added in the > immediately preceding patch. E.g. as a first patch > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 28ef14155726..f3b1aed08566 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -11609,7 +11609,8 @@ void kvm_arch_commit_memory_region(struct kvm *kvm, > const struct kvm_memory_slot *new, > enum kvm_mr_change change) > { > - if (!kvm->arch.n_requested_mmu_pages) > + if (!kvm->arch.n_requested_mmu_pages && > + (change == KVM_MR_CREATE || change == KVM_MR_DELETE)) > kvm_mmu_change_mmu_pages(kvm, > kvm_mmu_calculate_default_mmu_pages(kvm)); > > >
Will do.
Thanks, Maciej
| |