From: Jisheng Zhang Date: Tue, 18 Jul 2023 15:22:13 +0000 (+0800) Subject: riscv: allow kmalloc() caches aligned to the smallest value X-Git-Url: http://git.maquefel.me/?a=commitdiff_plain;h=2926715163cfacea481d218f9151d091f5c0c27a;p=linux.git riscv: allow kmalloc() caches aligned to the smallest value 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 Reviewed-by: Conor Dooley Link: https://lore.kernel.org/r/20230718152214.2907-2-jszhang@kernel.org Signed-off-by: Palmer Dabbelt --- diff --git a/arch/riscv/include/asm/cache.h b/arch/riscv/include/asm/cache.h index d3036df23ccbb..2174fe7bac9ac 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 8091b8bf4883f..c640ab6f843b9 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 971fe776e2f8b..027879b1557ad 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 d51a75864e531..7270b4d8c05b4 100644 --- a/arch/riscv/mm/dma-noncoherent.c +++ b/arch/riscv/mm/dma-noncoherent.c @@ -11,6 +11,8 @@ #include static bool noncoherent_supported __ro_after_init; +int dma_cache_alignment __ro_after_init = ARCH_DMA_MINALIGN; +EXPORT_SYMBOL_GPL(dma_cache_alignment); 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; +}