Re: [PATCH v2 1/2] riscv: allow kmalloc() caches aligned to the smallest value

[Conor Dooley]

Hey Jisheng,

On Mon, Jul 17, 2023 at 12:51:46AM +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 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/

In the future,

Link: https://lore.kernel.org/linux-arm-kernel/20230524171904.3967031-1-catalin.marinas@xxxxxxx/ [1]

> Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxx>
> Change-Id: Ica249d0f8058a02bd4bc6543b4ffc2946a4734a2

How come this has ended up with a Change-ID? Checkpatch says this is
something to do with Gerrit & needs to be removed.

> ---
>  arch/riscv/include/asm/cache.h      | 14 ++++++++++++++
>  arch/riscv/include/asm/cacheflush.h |  2 ++
>  arch/riscv/kernel/setup.c           |  1 +
>  arch/riscv/mm/dma-noncoherent.c     |  8 ++++++++
>  4 files changed, 25 insertions(+)
> 
> diff --git a/arch/riscv/include/asm/cache.h b/arch/riscv/include/asm/cache.h
> index d3036df23ccb..2174fe7bac9a 100644
> --- a/arch/riscv/include/asm/cache.h
> +++ b/arch/riscv/include/asm/cache.h
> @@ -13,6 +13,7 @@
>  
>  #ifdef CONFIG_RISCV_DMA_NONCOHERENT
>  #define ARCH_DMA_MINALIGN L1_CACHE_BYTES
> +#define ARCH_KMALLOC_MINALIGN	(8)
>  #endif
>  
>  /*
> @@ -23,4 +24,17 @@
>  #define ARCH_SLAB_MINALIGN	16
>  #endif
>  
> +#ifndef __ASSEMBLY__
> +
> +#ifdef CONFIG_RISCV_DMA_NONCOHERENT
> +extern int dma_cache_alignment;
> +#define dma_get_cache_alignment dma_get_cache_alignment
> +static inline int dma_get_cache_alignment(void)
> +{
> +	return dma_cache_alignment;
> +}
> +#endif
> +
> +#endif	/* __ASSEMBLY__ */
> +
>  #endif /* _ASM_RISCV_CACHE_H */
> diff --git a/arch/riscv/include/asm/cacheflush.h b/arch/riscv/include/asm/cacheflush.h
> index 8091b8bf4883..c640ab6f843b 100644
> --- a/arch/riscv/include/asm/cacheflush.h
> +++ b/arch/riscv/include/asm/cacheflush.h
> @@ -55,8 +55,10 @@ void riscv_init_cbo_blocksizes(void);
>  
>  #ifdef CONFIG_RISCV_DMA_NONCOHERENT
>  void riscv_noncoherent_supported(void);
> +void __init riscv_set_dma_cache_alignment(void);
>  #else
>  static inline void riscv_noncoherent_supported(void) {}
> +static inline void riscv_set_dma_cache_alignment(void) {}
>  #endif
>  
>  /*
> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> index 971fe776e2f8..027879b1557a 100644
> --- a/arch/riscv/kernel/setup.c
> +++ b/arch/riscv/kernel/setup.c
> @@ -311,6 +311,7 @@ void __init setup_arch(char **cmdline_p)
>  	if (IS_ENABLED(CONFIG_RISCV_ISA_ZICBOM) &&
>  	    riscv_isa_extension_available(NULL, ZICBOM))
>  		riscv_noncoherent_supported();
> +	riscv_set_dma_cache_alignment();
>  }
>  
>  static int __init topology_init(void)
> diff --git a/arch/riscv/mm/dma-noncoherent.c b/arch/riscv/mm/dma-noncoherent.c
> index d51a75864e53..811227e54bbd 100644
> --- a/arch/riscv/mm/dma-noncoherent.c
> +++ b/arch/riscv/mm/dma-noncoherent.c
> @@ -11,6 +11,8 @@
>  #include <asm/cacheflush.h>
>  
>  static bool noncoherent_supported __ro_after_init;
> +int dma_cache_alignment __ro_after_init = ARCH_DMA_MINALIGN;
> +EXPORT_SYMBOL(dma_cache_alignment);

Why is this not EXPORT_SYMBOL_GPL()?

Otherwise, this is generally good to me, thanks.

Conor.

>  
>  void arch_sync_dma_for_device(phys_addr_t paddr, size_t size,
>  			      enum dma_data_direction dir)
> @@ -78,3 +80,9 @@ void riscv_noncoherent_supported(void)
>  	     "Non-coherent DMA support enabled without a block size\n");
>  	noncoherent_supported = true;
>  }
> +
> +void __init riscv_set_dma_cache_alignment(void)
> +{
> +	if (!noncoherent_supported)
> +		dma_cache_alignment = 1;
> +}
> -- 
> 2.40.1
>

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