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

[Hesham Almatary]

On 9/27/2022 12:21 PM, haoxin wrote:
> 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
FYI, I have ported pmbench to AArch64 [1]. The project seems to be abandoned on 
bitbucket,

I wonder if it makes sense to fork it elsewhere and push the pending PRs there.


[1] https://bitbucket.org/jisooy/pmbench/pull-requests/5

> 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/