Messages in this thread | | | Date | Tue, 27 Sep 2022 19:21:08 +0800 | Subject | Re: [RFC 0/6] migrate_pages(): batch TLB flushing | From | haoxin <> |
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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. https://lore.kernel.org/linux-arm-kernel/20220921084302.43631-1-yangyicong@huawei.com/T/
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