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

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Date:   Tue, 27 Sep 2022 19:21:08 +0800
From:   haoxin <xhao@...ux.alibaba.com>
To:     Huang Ying <ying.huang@...el.com>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Zi Yan <ziy@...dia.com>, Yang Shi <shy828301@...il.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Oscar Salvador <osalvador@...e.de>,
        Matthew Wilcox <willy@...radead.org>, yangyicong@...ilicon.com,
        v-songbaohua@...o.com, 21cnbao@...il.com
Subject: Re: [RFC 0/6] migrate_pages(): batch TLB flushing

Hi,  Huang

在 2022/9/21 下午2:06, Huang Ying 写道:
> 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.
>
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/