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

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Hi,  Huang

在 2022/9/21 下午2:06, Huang Ying 写道:
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.

Maybe this patch can help solve this problem.
https://lore.kernel.org/linux-arm-kernel/20220921084302.43631-1-yangyicong@xxxxxxxxxx/T/
		







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