From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933269AbdDGCGm (ORCPT ); Thu, 6 Apr 2017 22:06:42 -0400 Received: from mail-io0-f193.google.com ([209.85.223.193]:33584 "EHLO mail-io0-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932325AbdDGCGc (ORCPT ); Thu, 6 Apr 2017 22:06:32 -0400 MIME-Version: 1.0 In-Reply-To: <20170406155825.GA7705@e104818-lin.cambridge.arm.com> References: <1458120743-12145-1-git-send-email-opensource.ganesh@gmail.com> <20160321171403.GE25466@e104818-lin.cambridge.arm.com> <10fef112-37f1-0a1b-b5af-435acd032f01@codeaurora.org> <4525901c-45d4-6bd8-eec6-ae92977f16d1@codeaurora.org> <20170406155825.GA7705@e104818-lin.cambridge.arm.com> From: Ganesh Mahendran Date: Fri, 7 Apr 2017 10:06:31 +0800 Message-ID: Subject: Re: [PATCH] Revert "arm64: Increase the max granular size" To: Catalin Marinas Cc: Imran Khan , open list , linux-arm-kernel@lists.infradead.org, "open list:ARM/QUALCOMM SUPPORT" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 2017-04-06 23:58 GMT+08:00 Catalin Marinas : > On Thu, Apr 06, 2017 at 12:52:13PM +0530, Imran Khan wrote: >> On 4/5/2017 10:13 AM, Imran Khan wrote: >> >> We may have to revisit this logic and consider L1_CACHE_BYTES the >> >> _minimum_ of cache line sizes in arm64 systems supported by the kernel. >> >> Do you have any benchmarks on Cavium boards that would show significant >> >> degradation with 64-byte L1_CACHE_BYTES vs 128? >> >> >> >> For non-coherent DMA, the simplest is to make ARCH_DMA_MINALIGN the >> >> _maximum_ of the supported systems: >> >> >> >> diff --git a/arch/arm64/include/asm/cache.h b/arch/arm64/include/asm/cache.h >> >> index 5082b30bc2c0..4b5d7b27edaf 100644 >> >> --- a/arch/arm64/include/asm/cache.h >> >> +++ b/arch/arm64/include/asm/cache.h >> >> @@ -18,17 +18,17 @@ >> >> >> >> #include >> >> >> >> -#define L1_CACHE_SHIFT 7 >> >> +#define L1_CACHE_SHIFT 6 >> >> #define L1_CACHE_BYTES (1 << L1_CACHE_SHIFT) >> >> >> >> /* >> >> * Memory returned by kmalloc() may be used for DMA, so we must make >> >> - * sure that all such allocations are cache aligned. Otherwise, >> >> - * unrelated code may cause parts of the buffer to be read into the >> >> - * cache before the transfer is done, causing old data to be seen by >> >> - * the CPU. >> >> + * sure that all such allocations are aligned to the maximum *known* >> >> + * cache line size on ARMv8 systems. Otherwise, unrelated code may cause >> >> + * parts of the buffer to be read into the cache before the transfer is >> >> + * done, causing old data to be seen by the CPU. >> >> */ >> >> -#define ARCH_DMA_MINALIGN L1_CACHE_BYTES >> >> +#define ARCH_DMA_MINALIGN (128) >> >> >> >> #ifndef __ASSEMBLY__ >> >> >> >> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c >> >> index 392c67eb9fa6..30bafca1aebf 100644 >> >> --- a/arch/arm64/kernel/cpufeature.c >> >> +++ b/arch/arm64/kernel/cpufeature.c >> >> @@ -976,9 +976,9 @@ void __init setup_cpu_features(void) >> >> if (!cwg) >> >> pr_warn("No Cache Writeback Granule information, assuming >> >> cache line size %d\n", >> >> cls); >> >> - if (L1_CACHE_BYTES < cls) >> >> - pr_warn("L1_CACHE_BYTES smaller than the Cache Writeback Granule (%d < %d)\n", >> >> - L1_CACHE_BYTES, cls); >> >> + if (ARCH_DMA_MINALIGN < cls) >> >> + pr_warn("ARCH_DMA_MINALIGN smaller than the Cache Writeback Granule (%d < %d)\n", >> >> + ARCH_DMA_MINALIGN, cls); >> >> } >> >> >> >> static bool __maybe_unused >> > >> > This change was discussed at: [1] but was not concluded as apparently no one >> > came back with test report and numbers. After including this change in our >> > local kernel we are seeing significant throughput improvement. For example with: >> > >> > iperf -c 192.168.1.181 -i 1 -w 128K -t 60 >> > >> > The average throughput is improving by about 30% (230Mbps from 180Mbps). >> > Could you please let us know if this change can be included in upstream kernel. >> > >> > [1]: https://groups.google.com/forum/#!topic/linux.kernel/P40yDB90ePs >> >> Could you please provide some feedback about the above mentioned query ? > > Do you have an explanation on the performance variation when > L1_CACHE_BYTES is changed? We'd need to understand how the network stack > is affected by L1_CACHE_BYTES, in which context it uses it (is it for > non-coherent DMA?). network stack use SKB_DATA_ALIGN to align. --- #define SKB_DATA_ALIGN(X) (((X) + (SMP_CACHE_BYTES - 1)) & \ ~(SMP_CACHE_BYTES - 1)) #define SMP_CACHE_BYTES L1_CACHE_BYTES --- I think this is the reason of performance regression. > > The Cavium guys haven't shown any numbers (IIUC) to back the > L1_CACHE_BYTES performance improvement but I would not revert the > original commit since ARCH_DMA_MINALIGN definitely needs to cover the > maximum available cache line size, which is 128 for them. how about define L1_CACHE_SHIFT like below: --- #ifdef CONFIG_ARM64_L1_CACHE_SHIFT #define L1_CACHE_SHIFT CONFIG_ARM64_L1_CACHE_SHIFT #else #define L1_CACHE_SHIFT 7 endif --- Thanks > > -- > Catalin