linux-kernel - Re: [PATCH v2] riscv: enable THP

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Date:   Sat, 27 Aug 2022 21:13:03 +0000
From:   <Conor.Dooley@...rochip.com>
To:     <jszhang@...nel.org>, <paul.walmsley@...ive.com>,
        <palmer@...belt.com>, <aou@...s.berkeley.edu>
CC:     <linux-riscv@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
        <ajones@...tanamicro.com>
Subject: Re: [PATCH v2] riscv: enable THP_SWAP for RV64

Hey Jisheng,
On 27/08/2022 10:58, Jisheng Zhang wrote:
> I have a Sipeed Lichee RV dock board which only has 512MB DDR, so
> memory optimizations such as swap on zram are helpful. As is seen
> in commit d0637c505f8a ("arm64: enable THP_SWAP for arm64") and
> commit bd4c82c22c367e ("mm, THP, swap: delay splitting THP after
> swapped out"), THP_SWAP can improve the swap throughput significantly.
> 
> Enable THP_SWAP for RV64, testing the micro-benchmark which is
> introduced by commit d0637c505f8a ("arm64: enable THP_SWAP for arm64")
> shows below numbers on the Lichee RV dock board:
> 
> thp swp throughput w/o patch: 66908 bytes/ms (mean of 10 tests)
> thp swp throughput w/ patch: 322638 bytes/ms (mean of 10 tests)

I know the original commit message contains this, but it's a little
odd. If the patch /enables/ THP then how would there be THP swap
prior to the patch?

> 
> Improved by 382%!

I could not replicate the after numbers on my nezha, so I suspect
I am missing something in my config/setup. zswap is enabled and is
working, TRANSPARENT_HUGEPAGE is enabled etc. Not that it matters
for acceptance of the patch though.

I gave it a try and nothing went up in flames while using zswap so:
Reviewed-by: Conor Dooley <conor.dooley@...rochip.com>

> 
> Signed-off-by: Jisheng Zhang <jszhang@...nel.org>
> Reviewed-by: Andrew Jones <ajones@...tanamicro.com>
> ---
> Since v1:
>  - collect reviewed-by tag
>  - make ARCH_WANTS_THP_SWAP rely on HAVE_ARCH_TRANSPARENT_HUGEPAGE
>    instead
> 
>  arch/riscv/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index ed66c31e4655..79e52441e18b 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -45,6 +45,7 @@ config RISCV
>  	select ARCH_WANT_FRAME_POINTERS
>  	select ARCH_WANT_GENERAL_HUGETLB
>  	select ARCH_WANT_HUGE_PMD_SHARE if 64BIT
> +	select ARCH_WANTS_THP_SWAP if HAVE_ARCH_TRANSPARENT_HUGEPAGE
>  	select BINFMT_FLAT_NO_DATA_START_OFFSET if !MMU
>  	select BUILDTIME_TABLE_SORT if MMU
>  	select CLONE_BACKWARDS