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

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Date:   Wed, 21 Sep 2022 11:47:38 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Huang Ying <ying.huang@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yang Shi <shy828301@...il.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Oscar Salvador <osalvador@...e.de>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [RFC 0/6] migrate_pages(): batch TLB flushing

On 21 Sep 2022, at 2:06, Huang Ying wrote:

> From: "Huang, Ying" <ying.huang@...el.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.
>
> 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@intel.com/
>
> 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

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