Re: [PATCH -v4 0/9] migrate_pages(): batch TLB flushing

[Huang, Ying]

haoxin <xhao@xxxxxxxxxxxxxxxxx> writes:

> On my arm64 server with 128 cores, 2 numa nodes.
>
> I used memhog as benchmark :
>
>   numactl -m -C 5 memhog -r100000 1G
>
> The test result as below:
>
> With this patch:
>
>   #time migratepages 8490 0 1
>
>   real 0m1.161s
>
>   user 0m0.000s
>
>   sys 0m1.161s
>
> without this patch:
>
>   #time migratepages 8460 0 1
>
>   real 0m2.068s
>
>   user 0m0.001s
>
>   sys 0m2.068s
>
> So you can see the migration performance improvement about *+78%*
>
> This is the perf record info.
>
> w/o
> +  51.07%   0.09% migratepages
> [kernel.kallsyms] [k] migrate_folio_extra
> +  42.43%   0.04% migratepages [kernel.kallsyms] [k] folio_copy
> +  42.34%  42.34% migratepages [kernel.kallsyms] [k] __pi_copy_page
> +  33.99%   0.09% migratepages [kernel.kallsyms] [k] rmap_walk_anon
> +  32.35%   0.04% migratepages [kernel.kallsyms] [k] try_to_migrate
> *+  27.78%  27.78% migratepages
>  [kernel.kallsyms] [k] ptep_clear_flush *
> +  8.19%   6.64% migratepages
> [kernel.kallsyms] [k] folio_migrate_flagsmigrati_tlb_flush
>
> w/ this patch
> +  18.57%   0.13%
> migratepages   [kernel.kallsyms]  [k]
> migrate_pages
> +  18.23%   0.07%
> migratepages   [kernel.kallsyms]  [k]
> migrate_pages_batch
> +  16.29%   0.13%
> migratepages   [kernel.kallsyms]  [k]
> migrate_folio_move
> +  12.73%   0.10%
> migratepages   [kernel.kallsyms]  [k]
> move_to_new_folio
> +  12.52%   0.06%
> migratepages   [kernel.kallsyms]  [k]
> migrate_folio_extra
>
> Therefore, this patch helps improve performance in page migration
>
>
> So, you can add Tested-by: Xin Hao <xhao@xxxxxxxxxxxxxxxxx>

Thank you very much!

Best Regards,
Huang, Ying

>
> ( 2023/2/6 H2:33, Huang Ying S:
>> From: "Huang, Ying"<ying.huang@xxxxxxxxx>
>>
>> Now, migrate_pages() migrate folios one by one, like the fake code as
>> follows,
>>
>>    for each folio
>>      unmap
>>      flush TLB
>>      copy
>>      restore map
>>
>> If multiple folios 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 folio
>>      unmap
>>    for each folio
>>      flush TLB
>>    for each folio
>>      copy
>>    for each folio
>>      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
>> folio copying.
>>
>> So in this patch, we refactor the migrate_pages() implementation and
>> implement the TLB flushing batching.  Base on this, hardware
>> accelerated folio copying can be implemented.
>>
>> If too many folios are passed to migrate_pages(), in the naive batched
>> implementation, we may unmap too many folios at the same time.  The
>> possibility for a task to wait for the migrated folios to be mapped
>> again increases.  So the latency may be hurt.  To deal with this
>> issue, the max number of folios be unmapped in batch is restricted to
>> no more than HPAGE_PMD_NR in the unit of page.  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%.
>>
>> This patchset is based on v6.2-rc4.
>>
>> Changes:
>>
>> v4:
>>
>> - Fixed another bug about non-LRU folio migration.  Thanks Hyeonggon!
>>
>> v3:
>>
>> - Rebased on v6.2-rc4
>>
>> - Fixed a bug about non-LRU folio migration.  Thanks Mike!
>>
>> - Fixed some comments.  Thanks Baolin!
>>
>> - Collected reviewed-by.
>>
>> v2:
>>
>> - Rebased on v6.2-rc3
>>
>> - Fixed type force cast warning.  Thanks Kees!
>>
>> - Added more comments and cleaned up the code.  Thanks Andrew, Zi, Alistair, Dan!
>>
>> - Collected reviewed-by.
>>
>> from rfc to v1:
>>
>> - Rebased on v6.2-rc1
>>
>> - Fix the deadlock issue caused by locking multiple pages synchronously
>>    per Alistair's comments.  Thanks!
>>
>> - Fix the autonumabench panic per Rao's comments and fix.  Thanks!
>>
>> - Other minor fixes per comments. Thanks!
>>
>> Best Regards,
>> Huang, Ying