From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f199.google.com (mail-pf1-f199.google.com [209.85.210.199]) by kanga.kvack.org (Postfix) with ESMTP id AE57F6B0003 for ; Fri, 12 Oct 2018 17:34:07 -0400 (EDT) Received: by mail-pf1-f199.google.com with SMTP id 14-v6so13139302pfk.22 for ; Fri, 12 Oct 2018 14:34:07 -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 i15-v6sor2293243pgm.66.2018.10.12.14.34.06 for (Google Transport Security); Fri, 12 Oct 2018 14:34:06 -0700 (PDT) Date: Sat, 13 Oct 2018 00:33:59 +0300 From: "Kirill A. Shutemov" Subject: Re: [PATCH v2 2/2] mm: speed up mremap by 500x on large regions Message-ID: <20181012213359.qpvq3obugbvy73bg@kshutemo-mobl1> References: <20181012013756.11285-1-joel@joelfernandes.org> <20181012013756.11285-2-joel@joelfernandes.org> <20181012113056.gxhcbrqyu7k7xnyv@kshutemo-mobl1> <20181012125046.GA170912@joelaf.mtv.corp.google.com> <20181012131946.zoab2lpfmrycmuju@kshutemo-mobl1> <20181012165719.GE223066@joelaf.mtv.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181012165719.GE223066@joelaf.mtv.corp.google.com> Sender: owner-linux-mm@kvack.org List-ID: To: Joel Fernandes Cc: linux-kernel@vger.kernel.org, kernel-team@android.com, minchan@kernel.org, pantin@google.com, hughd@google.com, lokeshgidra@google.com, dancol@google.com, mhocko@kernel.org, akpm@linux-foundation.org, Andrey Ryabinin , Andy Lutomirski , Borislav Petkov , Catalin Marinas , Chris Zankel , Dave Hansen , "David S. Miller" , elfring@users.sourceforge.net, Fenghua Yu , Geert Uytterhoeven , Guan Xuetao , Helge Deller , Ingo Molnar , "James E.J. Bottomley" , Jeff Dike , Jonas Bonn , Julia Lawall , kasan-dev@googlegroups.com, kvmarm@lists.cs.columbia.edu, Ley Foon Tan , linux-alpha@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-hexagon@vger.kernel.org, linux-ia64@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-mips@linux-mips.org, linux-mm@kvack.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, linux-snps-arc@lists.infradead.org, linux-um@lists.infradead.org, linux-xtensa@linux-xtensa.org, Max Filippov , nios2-dev@lists.rocketboards.org, openrisc@lists.librecores.org, Peter Zijlstra , Richard Weinberger , Rich Felker , Sam Creasey , sparclinux@vger.kernel.org, Stafford Horne , Stefan Kristiansson , Thomas Gleixner , Tony Luck , Will Deacon , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , Yoshinori Sato On Fri, Oct 12, 2018 at 09:57:19AM -0700, Joel Fernandes wrote: > On Fri, Oct 12, 2018 at 04:19:46PM +0300, Kirill A. Shutemov wrote: > > On Fri, Oct 12, 2018 at 05:50:46AM -0700, Joel Fernandes wrote: > > > On Fri, Oct 12, 2018 at 02:30:56PM +0300, Kirill A. Shutemov wrote: > > > > On Thu, Oct 11, 2018 at 06:37:56PM -0700, Joel Fernandes (Google) wrote: > > > > > Android needs to mremap large regions of memory during memory management > > > > > related operations. The mremap system call can be really slow if THP is > > > > > not enabled. The bottleneck is move_page_tables, which is copying each > > > > > pte at a time, and can be really slow across a large map. Turning on THP > > > > > may not be a viable option, and is not for us. This patch speeds up the > > > > > performance for non-THP system by copying at the PMD level when possible. > > > > > > > > > > The speed up is three orders of magnitude. On a 1GB mremap, the mremap > > > > > completion times drops from 160-250 millesconds to 380-400 microseconds. > > > > > > > > > > Before: > > > > > Total mremap time for 1GB data: 242321014 nanoseconds. > > > > > Total mremap time for 1GB data: 196842467 nanoseconds. > > > > > Total mremap time for 1GB data: 167051162 nanoseconds. > > > > > > > > > > After: > > > > > Total mremap time for 1GB data: 385781 nanoseconds. > > > > > Total mremap time for 1GB data: 388959 nanoseconds. > > > > > Total mremap time for 1GB data: 402813 nanoseconds. > > > > > > > > > > Incase THP is enabled, the optimization is skipped. I also flush the > > > > > tlb every time we do this optimization since I couldn't find a way to > > > > > determine if the low-level PTEs are dirty. It is seen that the cost of > > > > > doing so is not much compared the improvement, on both x86-64 and arm64. > > > > > > > > I looked into the code more and noticed move_pte() helper called from > > > > move_ptes(). It changes PTE entry to suite new address. > > > > > > > > It is only defined in non-trivial way on Sparc. I don't know much about > > > > Sparc and it's hard for me to say if the optimization will break anything > > > > there. > > > > > > Sparc's move_pte seems to be flushing the D-cache to prevent aliasing. It is > > > not modifying the PTE itself AFAICS: > > > > > > #ifdef DCACHE_ALIASING_POSSIBLE > > > #define __HAVE_ARCH_MOVE_PTE > > > #define move_pte(pte, prot, old_addr, new_addr) \ > > > ({ \ > > > pte_t newpte = (pte); \ > > > if (tlb_type != hypervisor && pte_present(pte)) { \ > > > unsigned long this_pfn = pte_pfn(pte); \ > > > \ > > > if (pfn_valid(this_pfn) && \ > > > (((old_addr) ^ (new_addr)) & (1 << 13))) \ > > > flush_dcache_page_all(current->mm, \ > > > pfn_to_page(this_pfn)); \ > > > } \ > > > newpte; \ > > > }) > > > #endif > > > > > > If its an issue, then how do transparent huge pages work on Sparc? I don't > > > see the huge page code (move_huge_pages) during mremap doing anything special > > > for Sparc architecture when moving PMDs.. > > > > My *guess* is that it will work fine on Sparc as it apprarently it only > > cares about change in bit 13 of virtual address. It will never happen for > > huge pages or when PTE page tables move. > > > > But I just realized that the problem is bigger: since we pass new_addr to > > the set_pte_at() we would need to audit all implementations that they are > > safe with just moving PTE page table. > > > > I would rather go with per-architecture enabling. It's much safer. > > I'm Ok with the per-arch enabling, I agree its safer. So I should be adding a > a new __HAVE_ARCH_MOVE_PMD right, or did you have a better name for that? I believe Kconfig option is more cononical way to do this nowadays. So CONFIG_HAVE_ARCH_MOVE_PMD, I guess. Or CONFIG_HAVE_MOVE_PMD. An arch that supports it would select the option. > Also, do you feel we should still need to remove the address argument from > set_pte_alloc? Or should we leave that alone if we do per-arch? > I figure I spent a bunch of time on that already anyway, and its a clean up > anyway, so may as well do it. But perhaps that "pte_alloc cleanup" can then > be a separate patch independent of this series? Yeah. The cleanup makes sense anyway. -- Kirill A. Shutemov