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

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Message-ID: <51A60A64.2080509@linux.vnet.ibm.com>
Date:	Wed, 29 May 2013 22:02:12 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Marcelo Tosatti <mtosatti@...hat.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 05/29/2013 09:32 PM, Marcelo Tosatti wrote:
> 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.

Okay, okay. I will benchmark the latency.


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