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

[Huang, Ying]

Hesham Almatary <hesham.almatary@xxxxxxxxxx> writes:

> On 9/27/2022 12:21 PM, haoxin wrote:
>> Hi, Huang
>>
>> ( 2022/9/21 H2:06, Huang Ying S:
>>> 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.
>>>
>> As the pmbench can not run on arm64 machine, so i use lmbench instead.
>> I test case like this: (i am not sure whether it is reasonable,
>> but it seems worked)
>> ./bw_mem -N10000 10000m rd &
>> time migratepages pid node0 node1
>>
> FYI, I have ported pmbench to AArch64 [1]. The project seems to be
> abandoned on bitbucket,
>
> I wonder if it makes sense to fork it elsewhere and push the pending PRs there.
>
>
> [1] https://bitbucket.org/jisooy/pmbench/pull-requests/5

Maybe try to contact the original author with email firstly?

Best Regards,
Huang, Ying

>> o/patch w/patch
>> real  0m0.035s   real  0m0.024s
>> user  0m0.000s   user  0m0.000s
>> sys  0m0.035s    sys  0m0.024s
>>
>> the migratepages time is reduced above 32%.
>>
>> But there has a problem, i see the batch flush is called by
>> migrate_pages_batch
>>   try_to_unmap_flush
>>     arch_tlbbatch_flush(&tlb_ubc->arch); // there batch flush really work.
>>
>> But in arm64, the arch_tlbbatch_flush are not supported, becasue it
>> not support CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH yet.
>>
>> So, the tlb batch flush means no any flush is did, it is a empty func.
>>
>> Maybe this patch can help solve this problem.
>> https://lore.kernel.org/linux-arm-kernel/20220921084302.43631-1-yangyicong@xxxxxxxxxx/T/ 
>>
>>
>>
>>
>>
>>