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

[Huang, Ying]

haoxin <xhao@xxxxxxxxxxxxxxxxx> writes:

> ( 2022/9/28 H10:01, Huang, Ying S:
>> haoxin <xhao@xxxxxxxxxxxxxxxxx> writes:
>>
>>> 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
>>>
>>> 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.
>> Yes.  And should_defer_flush() will always return false too.  That is,
>> the TLB will still be flushed, but will not be batched.
> Oh, yes, i ignore this, thank you.
>>
>>> Maybe this patch can help solve this problem.
>>> https://lore.kernel.org/linux-arm-kernel/20220921084302.43631-1-yangyicong@xxxxxxxxxx/T/
>> Yes.  This will bring TLB flush batching to ARM64.
> Next time, i will combine with this patch, and do some test again,
> do you have any suggestion about benchmark ?

I think your benchmark should be OK.  If multiple threads are used, the
effect of patchset will be better.

Best Regards,
Huang, Ying