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

[haoxin <xhao@linux.alibaba.com>]

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.


在 2022/9/21 下午11:47, Zi Yan 写道:
> On 21 Sep 2022, at 2:06, Huang Ying wrote:
>
>> From: "Huang, Ying" <ying.huang@intel.com>
>>
>> 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@intel.com/
>>
>> 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