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    SubjectRe: [PATCH v2 19/22] KVM: MMU: lockless walking shadow page table
    On 06/29/2011 07:18 PM, Avi Kivity wrote:
    > On 06/29/2011 02:16 PM, Xiao Guangrong wrote:
    >> >> @@ -1767,6 +1874,14 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
    >> >>
    >> >> kvm_flush_remote_tlbs(kvm);
    >> >>
    >> >> + if (atomic_read(&kvm->arch.reader_counter)) {
    >> >> + 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;
    >> >> + }
    >> >> +
    >> >
    >> > I think we should do this unconditionally. The cost of ping-ponging the shared cache line containing reader_counter will increase with large smp counts. On the other hand, zap_page is very rare, so it can be a little slower. Also, less code paths = easier to understand.
    >> >
    >>
    >> On soft mmu, zap_page is very frequently, it can cause performance regression in my test.
    >
    > Any idea what the cause of the regression is? It seems to me that simply deferring freeing shouldn't have a large impact.
    >

    I guess it is because the page is freed too frequently, i have done the test, it shows
    about 3219 pages is freed per second

    Kernbench performance comparing:

    the origin way: 3m27.723
    free all shadow page in rcu context: 3m30.519


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