From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-it0-f70.google.com (mail-it0-f70.google.com [209.85.214.70]) by kanga.kvack.org (Postfix) with ESMTP id E1F3B6B7887 for ; Thu, 6 Sep 2018 07:24:24 -0400 (EDT) Received: by mail-it0-f70.google.com with SMTP id 20-v6so11254523itb.7 for ; Thu, 06 Sep 2018 04:24:24 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id k3-v6sor2438804iop.330.2018.09.06.04.24.23 for (Google Transport Security); Thu, 06 Sep 2018 04:24:23 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <1534907237-2982-1-git-send-email-jia.he@hxt-semitech.com> References: <1534907237-2982-1-git-send-email-jia.he@hxt-semitech.com> From: Ard Biesheuvel Date: Thu, 6 Sep 2018 13:24:22 +0200 Message-ID: Subject: Re: [PATCH v11 0/3] remain and optimize memblock_next_valid_pfn on arm and arm64 Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: Jia He Cc: Russell King , Catalin Marinas , Will Deacon , Mark Rutland , Andrew Morton , Michal Hocko , Wei Yang , Kees Cook , Laura Abbott , Vladimir Murzin , Philip Derrin , AKASHI Takahiro , James Morse , Steve Capper , Gioh Kim , Vlastimil Babka , Mel Gorman , Johannes Weiner , Kemi Wang , Petr Tesarik , YASUAKI ISHIMATSU , Andrey Ryabinin , Nikolay Borisov , Daniel Jordan , Daniel Vacek , Eugeniu Rosca , linux-arm-kernel , Linux Kernel Mailing List , Linux-MM , Jia He On 22 August 2018 at 05:07, Jia He wrote: > Commit b92df1de5d28 ("mm: page_alloc: skip over regions of invalid pfns > where possible") optimized the loop in memmap_init_zone(). But it causes > possible panic bug. So Daniel Vacek reverted it later. > > But as suggested by Daniel Vacek, it is fine to using memblock to skip > gaps and finding next valid frame with CONFIG_HAVE_ARCH_PFN_VALID. > > More from what Daniel said: > "On arm and arm64, memblock is used by default. But generic version of > pfn_valid() is based on mem sections and memblock_next_valid_pfn() does > not always return the next valid one but skips more resulting in some > valid frames to be skipped (as if they were invalid). And that's why > kernel was eventually crashing on some !arm machines." > > About the performance consideration: > As said by James in b92df1de5, > "I have tested this patch on a virtual model of a Samurai CPU with a > sparse memory map. The kernel boot time drops from 109 to 62 seconds." > Thus it would be better if we remain memblock_next_valid_pfn on arm/arm64. > > Besides we can remain memblock_next_valid_pfn, there is still some room > for improvement. After this set, I can see the time overhead of memmap_init > is reduced from 27956us to 13537us in my armv8a server(QDF2400 with 96G > memory, pagesize 64k). I believe arm server will benefit more if memory is > larger than TBs > OK so we can summarize the benefits of this series as follows: - boot time on a virtual model of a Samurai CPU drops from 109 to 62 seconds - boot time on a QDF2400 arm64 server with 96 GB of RAM drops by ~15 *milliseconds* Google was not very helpful in figuring out what a Samurai CPU is and why we should care about the boot time of Linux running on a virtual model of it, and the 15 ms speedup is not that compelling either. Apologies to Jia that it took 11 revisions to reach this conclusion, but in /my/ opinion, tweaking the fragile memblock/pfn handling code for this reason is totally unjustified, and we're better off disregarding these patches. > Patch 1 introduces new config to make codes more generic > Patch 2 remains the memblock_next_valid_pfn on arm and arm64,this patch is > originated from b92df1de5 > Patch 3 optimizes the memblock_next_valid_pfn() > > Changelog: > V11:- drop patch#4-6, refine the codes > V10:- move codes to memblock.c, refine the performance consideration > V9: - rebase to mmotm master, refine the log description. No major changes > V8: - introduce new config and move generic code to early_pfn.h > - optimize memblock_next_valid_pfn as suggested by Matthew Wilcox > V7: - fix i386 compilation error. refine the commit description > V6: - simplify the codes, move arm/arm64 common codes to one file. > - refine patches as suggested by Danial Vacek and Ard Biesheuvel > V5: - further refining as suggested by Danial Vacek. Make codes > arm/arm64 more arch specific > V4: - refine patches as suggested by Danial Vacek and Wei Yang > - optimized on arm besides arm64 > V3: - fix 2 issues reported by kbuild test robot > V2: - rebase to mmotm latest > - remain memblock_next_valid_pfn on arm64 > - refine memblock_search_pfn_regions and pfn_valid_region > > Jia He (3): > arm: arm64: introduce CONFIG_HAVE_MEMBLOCK_PFN_VALID > mm: page_alloc: remain memblock_next_valid_pfn() on arm/arm64 > mm: page_alloc: reduce unnecessary binary search in > memblock_next_valid_pfn > > arch/arm/Kconfig | 1 + > arch/arm64/Kconfig | 1 + > include/linux/mmzone.h | 9 +++++++++ > mm/Kconfig | 3 +++ > mm/memblock.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++++++ > mm/page_alloc.c | 5 ++++- > 6 files changed, 69 insertions(+), 1 deletion(-) > > -- > 1.8.3.1 >