Re: [PATCH v2] arm64: enable THP_SWAP for arm64

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 5/30/22 15:23, Barry Song wrote:
> On Mon, May 30, 2022 at 7:07 PM Anshuman Khandual
> <anshuman.khandual@arm.com> wrote:
>>
>> Hello Barry,
> 
> Hi Anshuman,
> thanks!
> 
>>
>> On 5/27/22 15:36, Barry Song wrote:
>>> From: Barry Song <v-songbaohua@oppo.com>
>>>
>>> THP_SWAP has been proved to improve the swap throughput significantly
>>> on x86_64 according to commit bd4c82c22c367e ("mm, THP, swap: delay
>>> splitting THP after swapped out").
>> It will be useful to run similar experiments on arm64 platform to
>> demonstrate tangible benefit, else we might be just enabling this
>> feature just because x86 has it. Do you have some data points ?
>>
>>> As long as arm64 uses 4K page size, it is quite similar with x86_64
>>> by having 2MB PMD THP. So we are going to get similar improvement.
>>
>> This is an assumption without any data points (until now). Please
>> do provide some results.
> 
> Fair enough though I believe THP_SWP is arch-independent. Our testing
> will post data. Plus, we do need it for real use cases with some possible
> out-of-tree code for this moment. so this patch does not originate only
> because x86 has it :-)

I understand, but as you mentioned some data here will be helpful.

> 
>>
>>> For other page sizes such as 16KB and 64KB, PMD might be too large.
>>> Negative side effects such as IO latency might be a problem. Thus,
>>> we can only safely enable the counterpart of X86_64.
>>
>> Incorrect reasoning. Although sometimes it might be okay to enable
>> a feature on platforms with possible assumptions about its benefits,
>> but to claim 'similar improvement, safely, .. etc' while comparing
>> against x86 4K page config without data points, is not very helpful.
>>
>>> A corner case is that MTE has an assumption that only base pages
>>> can be swapped. We won't enable THP_SWP for ARM64 hardware with
>>> MTE support until MTE is re-arched.
>>
>> re-arched ?? Did you imply that MTE is reworked to support THP ?
> 
> I think at least MTE should be able to coexist with THP_SWP though
> I am not quite sure if MTE can be re-worked to fully support THP.

Understood but I just wanted the wording above in the commit message
to be changed to literally anything other than 're-arched'.