[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20230724-next-shifter-3ec32114b34e@wendy>
Date: Mon, 24 Jul 2023 08:49:27 +0100
From: Conor Dooley <conor.dooley@...rochip.com>
To: Jisheng Zhang <jszhang@...nel.org>
CC: Paul Walmsley <paul.walmsley@...ive.com>,
Palmer Dabbelt <palmer@...belt.com>,
Albert Ou <aou@...s.berkeley.edu>,
<linux-riscv@...ts.infradead.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/2] riscv: allow kmalloc() caches aligned to the
smallest value
On Tue, Jul 18, 2023 at 11:22:13PM +0800, Jisheng Zhang wrote:
> Currently, riscv defines ARCH_DMA_MINALIGN as L1_CACHE_BYTES, I.E
> 64Bytes, if CONFIG_RISCV_DMA_NONCOHERENT=y. To support unified kernel
> Image, usually we have to enable CONFIG_RISCV_DMA_NONCOHERENT, thus
> it brings some bad effects to coherent platforms:
>
> Firstly, it wastes memory, kmalloc-96, kmalloc-32, kmalloc-16 and
> kmalloc-8 slab caches don't exist any more, they are replaced with
> either kmalloc-128 or kmalloc-64.
>
> Secondly, larger than necessary kmalloc aligned allocations results
> in unnecessary cache/TLB pressure.
>
> This issue also exists on arm64 platforms. From last year, Catalin
> tried to solve this issue by decoupling ARCH_KMALLOC_MINALIGN from
> ARCH_DMA_MINALIGN, limiting kmalloc() minimum alignment to
> dma_get_cache_alignment() and replacing ARCH_KMALLOC_MINALIGN usage
> in various drivers with ARCH_DMA_MINALIGN etc.[1]
>
> One fact we can make use of for riscv: if the CPU doesn't support
> ZICBOM or T-HEAD CMO, we know the platform is coherent. Based on
> Catalin's work and above fact, we can easily solve the kmalloc align
> issue for riscv: we can override dma_get_cache_alignment(), then let
> it return ARCH_DMA_MINALIGN at the beginning and return 1 once we know
> the underlying HW neither supports ZICBOM nor supports T-HEAD CMO.
>
> So what about if the CPU supports ZICBOM or T-HEAD CMO, but all the
> devices are dma coherent? Well, we use ARCH_DMA_MINALIGN as the
> kmalloc minimum alignment, nothing changed in this case. This case
> can be improved in the future.
>
> After this patch, a simple test of booting to a small buildroot rootfs
> on qemu shows:
>
> kmalloc-96 5041 5041 96 ...
> kmalloc-64 9606 9606 64 ...
> kmalloc-32 5128 5128 32 ...
> kmalloc-16 7682 7682 16 ...
> kmalloc-8 10246 10246 8 ...
>
> So we save about 1268KB memory. The saving will be much larger in normal
> OS env on real HW platforms.
>
> Link: https://lore.kernel.org/linux-arm-kernel/20230524171904.3967031-1-catalin.marinas@arm.com/ [1]
>
> Signed-off-by: Jisheng Zhang <jszhang@...nel.org>
Reviewed-by: Conor Dooley <conor.dooley@...rochip.com>
Thanks,
Conor.
Download attachment "signature.asc" of type "application/pgp-signature" (229 bytes)
Powered by blists - more mailing lists