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

[haoxin]

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