[PATCH v2] riscv: enable THP_SWAP for RV64

[Jisheng Zhang]

On Sat, Aug 27, 2022 at 09:13:03PM +0000, Conor.Dooley at microchip.com wrote:
> Hey Jisheng,

Hi Conor,

> 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?

hmm, it's swap I'll send a v3 to correct the description.

> 
> > 
> > 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

swap on zram rather than zswap ;)

> 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 at microchip.com>
> 
> > 
> > Signed-off-by: Jisheng Zhang <jszhang at kernel.org>
> > Reviewed-by: Andrew Jones <ajones at ventanamicro.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