On Wed, May 29, 2013 at 09:09:09PM +0800, Xiao Guangrong wrote:
> This information is I replied Gleb in his mail where he raced a question that
> why "collapse tlb flush is needed":
>
> ======
> It seems no.
> Since we have reloaded mmu before zapping the obsolete pages, the mmu-lock
> is easily contended. I did the simple track:
>
> + int num = 0;
> restart:
> list_for_each_entry_safe_reverse(sp, node,
> &kvm->arch.active_mmu_pages, link) {
> @@ -4265,6 +4265,7 @@ restart:
> if (batch >= BATCH_ZAP_PAGES &&
> cond_resched_lock(&kvm->mmu_lock)) {
> batch = 0;
> + num++;
> goto restart;
> }
>
> @@ -4277,6 +4278,7 @@ restart:
> * may use the pages.
> */
> kvm_mmu_commit_zap_page(kvm, &invalid_list);
> + printk("lock-break: %d.\n", num);
> }
>
> I do read pci rom when doing kernel building in the guest which
> has 1G memory and 4vcpus with ept enabled, this is the normal
> workload and normal configuration.
>
> # dmesg
> [ 2338.759099] lock-break: 8.
> [ 2339.732442] lock-break: 5.
> [ 2340.904446] lock-break: 3.
> [ 2342.513514] lock-break: 3.
> [ 2343.452229] lock-break: 3.
> [ 2344.981599] lock-break: 4.
>
> Basically, we need to break many times.
Should measure kvm_mmu_zap_all latency.
> ======
>
> You can see we should break 3 times to zap all pages even if we have zapoed
> 10 pages in batch. It is obviously that it need break more times without
> batch-zapping.
Again, breaking should be no problem, what matters is latency. Please
measure kvm_mmu_zap_all latency after all optimizations to justify
this minimum batching.
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