linux-kernel - Re: [PATCH v7 04/11] KVM: MMU: zap pages in batch

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Date:	Wed, 29 May 2013 10:32:43 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	gleb@...hat.com, avi.kivity@...il.com, pbonzini@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v7 04/11] KVM: MMU: zap pages in batch

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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