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

haoxin
2022-09-27T11:23:16Z

Hi,  Huang

在 2022/9/21 下午2:06, Huang Ying 写道:
> 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.
>
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.

Maybe this patch can help solve this problem.