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

[haoxin]

Hi Huang,

    This is an exciting change, but on ARM64 machine the TLB flushing 
are not through IPI, it depends on 'vale1is'

instruction，so I'm wondering if there's also a  benefit on arm64, and 
I'm going to test it on an ARM64 machine.


在 2022/9/21 下午11:47, Zi Yan 写道:
> On 21 Sep 2022, at 2:06, Huang Ying wrote:
>
> > 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.
> >
> > With the patch, the TLB flushing IPI reduces 99.1% during the test and
> > the number of pages migrated successfully per second increases 291.7%.
> Thank you for the patchset. Batching page migration will definitely
> improve its throughput from my past experiments[1] and starting with
> TLB flushing is a good first step.
>
> BTW, what is the rationality behind the increased page migration
> success rate per second?
>
> > This patchset is based on v6.0-rc5 and the following patchset,
> >
> > [PATCH -V3 0/8] migrate_pages(): fix several bugs in error path
> > https://lore.kernel.org/lkml/20220817081408.513338-1-ying.huang@xxxxxxxxx/
> >
> > The migrate_pages() related code is converting to folio now. So this
> > patchset cannot apply recent akpm/mm-unstable branch.  This patchset
> > is used to check the basic idea.  If it is OK, I will rebase the
> > patchset on top of folio changes.
> >
> > Best Regards,
> > Huang, Ying
>
> [1] https://lwn.net/Articles/784925/
>
> --
> Best Regards,
> Yan, Zi