Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933413AbdDFP6h (ORCPT ); Thu, 6 Apr 2017 11:58:37 -0400 Received: from foss.arm.com ([217.140.101.70]:45074 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933324AbdDFP63 (ORCPT ); Thu, 6 Apr 2017 11:58:29 -0400 Date: Thu, 6 Apr 2017 16:58:25 +0100 From: Catalin Marinas To: Imran Khan Cc: "\"Chalamarla, Tirumalesh\" ; Ganesh Mahendran" , open list , linux-arm-kernel@lists.infradead.org, "open list:ARM/QUALCOMM SUPPORT" Subject: Re: [PATCH] Revert "arm64: Increase the max granular size" Message-ID: <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4525901c-45d4-6bd8-eec6-ae92977f16d1@codeaurora.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3667 Lines: 82 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?). 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. -- Catalin