Re: [PATCH 5/6] riscv: allow kmalloc() caches aligned to the smallest value

[Conor Dooley]

On Sat, May 27, 2023 at 12:59:57AM +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 for 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.
> 
> 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 and 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.
> 
> [1] Link: https://lore.kernel.org/linux-arm-kernel/20230524171904.3967031-1-catalin.marinas@xxxxxxx/
> 
> Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxx>

Fails to build chief, with loads of:
linux/dma-mapping.h:546:19: error: redefinition of 'dma_get_cache_alignment'

And for 32-bit there's also a rake of:
include/linux/slab.h:239:9: warning: 'ARCH_KMALLOC_MINALIGN' macro redefined [-Wmacro-redefined]

At the very least, reproducable with rv32_defconfig.

Cheers,
Conor.

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