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

[Huang, Ying]

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.

> 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.

Best Regards,
Huang, Ying