在 2022/9/28 上午10:01, Huang, Ying 写道:
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 ?
Best Regards,
Huang, Ying
