Re: [RFC 0/6] migrate_pages(): batch TLB flushing

[Huang, Ying]

haoxin <xhao@xxxxxxxxxxxxxxxxx> writes:

> Hi Huang,
>
> This is an exciting change, but on ARM64 machine the TLB
> flushing are not through IPI, it depends on 'vale1is'
>
> instructionso I'm wondering if there's also a benefit on arm64,
> and I'm going to test it on an ARM64 machine.

We have no arm64 machine to test and I know very little about arm64.
Thanks for information and testing.

Best Regards,
Huang, Ying

>
> ( 2022/9/21 H11:47, Zi Yan S:
>> On 21 Sep 2022, at 2:06, Huang Ying wrote:
>>
>>> From: "Huang, Ying" <ying.huang@xxxxxxxxx>
>>>
>>> Now, migrate_pages() migrate pages one by one, like the fake code as
>>> follows,
>>>
>>>    for each page
>>>      unmap
>>>      flush TLB
>>>      copy
>>>      restore map
>>>
>>> If multiple pages are passed to migrate_pages(), there are
>>> opportunities to batch the TLB flushing and copying.  That is, we can
>>> change the code to something as follows,
>>>
>>>    for each page
>>>      unmap
>>>    for each page
>>>      flush TLB
>>>    for each page
>>>      copy
>>>    for each page
>>>      restore map
>>>
>>> The total number of TLB flushing IPI can be reduced considerably.  And
>>> we may use some hardware accelerator such as DSA to accelerate the
>>> page copying.
>>>
>>> So in this patch, we refactor the migrate_pages() implementation and
>>> implement the TLB flushing batching.  Base on this, hardware
>>> accelerated page copying can be implemented.
>>>
>>> If too many pages are passed to migrate_pages(), in the naive batched
>>> implementation, we may unmap too many pages at the same time.  The
>>> possibility for a task to wait for the migrated pages to be mapped
>>> again increases.  So the latency may be hurt.  To deal with this
>>> issue, the max number of pages be unmapped in batch is restricted to
>>> no more than HPAGE_PMD_NR.  That is, the influence is at the same
>>> level of THP migration.
>>>
>>> We use the following test to measure the performance impact of the
>>> patchset,
>>>
>>> On a 2-socket Intel server,
>>>
>>>   - Run pmbench memory accessing benchmark
>>>
>>>   - Run `migratepages` to migrate pages of pmbench between node 0 and
>>>     node 1 back and forth.
>>>
>>> With the patch, the TLB flushing IPI reduces 99.1% during the test and
>>> the number of pages migrated successfully per second increases 291.7%.
>> Thank you for the patchset. Batching page migration will definitely
>> improve its throughput from my past experiments[1] and starting with
>> TLB flushing is a good first step.
>>
>> BTW, what is the rationality behind the increased page migration
>> success rate per second?
>>
>>> This patchset is based on v6.0-rc5 and the following patchset,
>>>
>>> [PATCH -V3 0/8] migrate_pages(): fix several bugs in error path
>>> https://lore.kernel.org/lkml/20220817081408.513338-1-ying.huang@xxxxxxxxx/
>>>
>>> The migrate_pages() related code is converting to folio now. So this
>>> patchset cannot apply recent akpm/mm-unstable branch.  This patchset
>>> is used to check the basic idea.  If it is OK, I will rebase the
>>> patchset on top of folio changes.
>>>
>>> Best Regards,
>>> Huang, Ying
>>
>> [1] https://lwn.net/Articles/784925/
>>
>> --
>> Best Regards,
>> Yan, Zi