linux-kernel - Re: [RFC PATCH v2 2/2] KVM: x86: Not wr-protect huge page with init_all

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Message-ID: <d2dd806f-7df4-1ba4-9da5-073aece6da1c@huawei.com>
Date:   Thu, 29 Apr 2021 11:30:52 +0800
From:   Keqian Zhu <zhukeqian1@...wei.com>
To:     Ben Gardon <bgardon@...gle.com>
CC:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        "Wanghaibin (D)" <wanghaibin.wang@...wei.com>
Subject: Re: [RFC PATCH v2 2/2] KVM: x86: Not wr-protect huge page with
 init_all_set dirty log

Hi Ben,

On 2021/4/29 0:22, Ben Gardon wrote:
> On Wed, Apr 28, 2021 at 4:34 AM zhukeqian <zhukeqian1@...wei.com> wrote:
>>
>> Oh, I have to correct myself.
>>
>> without this opt:
>> first round dirtying: write fault and split large mapping
>> second round: write fault
>>
>> with this opt:
>> first round dirtying: no write fault
>> second round: write fault and split large mapping
>>
>> the total test time is expected to be reduced.
> 
> Oh yeah, good point. So we should really see the savings in the first
> round dirty memory time. Good catch.
> 
[...]

>>> It would probably also serve us well to have some kind of "hot" subset
>>> of memory for each vCPU, since some of the benefit of lazy large page
>>> splitting depend on that access pattern.
>>>
>>> 3. Lockstep dirtying and dirty log collection
>>> While this test is currently great for timing dirty logging
>>> operations, it's not great for trickier analysis, especially
>>> reductions to guest degradation. In order to measure that we'd need to
>>> change the test to collect the dirty log as quickly as possible,
>>> independent of what the guest is doing and then also record how much
>>> "progress" the guest is able to make while all that is happening.
>> Yes, make sense.
>>
>> Does the "dirty log collection" contains "dirty log clear"? As I understand, the dirty log
>> collection is very fast, just some memory copy. But for "dirty log clear", we should modify mappings
>> and perform TLBI, the time is much longer.
> 
> Yeah, sorry. By dirty log collection I meant get + clear since the
> test does both before it waits for the guest to dirty all memory
> again.
I see.

> 
>>
>>>
>>> I'd be happy to help review any improvements to the test which you
>>> feel like making.
>> Thanks, Ben. emm... I feel very sorry that perhaps I don't have enough time to do this, many works are queued...
>> On the other hand, I think the "Dirtying memory time" of first round can show us the optimization.
> 
> No worries, I think this is a good patch either way. No need to block
> on test improvements, from my perspective.
OK, thanks.


BRs,
Keqian