From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx172.postini.com [74.125.245.172]) by kanga.kvack.org (Postfix) with SMTP id 24A346B005D for ; Thu, 16 Aug 2012 12:17:05 -0400 (EDT) Date: Thu, 16 Aug 2012 18:16:47 +0200 From: Andrea Arcangeli Subject: Re: [PATCH v3 6/7] mm: make clear_huge_page cache clear only around the fault address Message-ID: <20120816161647.GM11188@redhat.com> References: <1345130154-9602-1-git-send-email-kirill.shutemov@linux.intel.com> <1345130154-9602-7-git-send-email-kirill.shutemov@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1345130154-9602-7-git-send-email-kirill.shutemov@linux.intel.com> Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: linux-mm@kvack.org, Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org, Andi Kleen , Tim Chen , Alex Shi , Jan Beulich , Robert Richter , Andy Lutomirski , Andrew Morton , Johannes Weiner , Hugh Dickins , KAMEZAWA Hiroyuki , Mel Gorman , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-mips@linux-mips.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org Hi Kirill, On Thu, Aug 16, 2012 at 06:15:53PM +0300, Kirill A. Shutemov wrote: > for (i = 0; i < pages_per_huge_page; > i++, p = mem_map_next(p, page, i)) { It may be more optimal to avoid a multiplication/shiftleft before the add, and to do: for (i = 0, vaddr = haddr; i < pages_per_huge_page; i++, p = mem_map_next(p, page, i), vaddr += PAGE_SIZE) { > cond_resched(); > - clear_user_highpage(p, addr + i * PAGE_SIZE); > + vaddr = haddr + i*PAGE_SIZE; Not sure if gcc can optimize it away because of the external calls. > + if (!ARCH_HAS_USER_NOCACHE || i == target) > + clear_user_highpage(page + i, vaddr); > + else > + clear_user_highpage_nocache(page + i, vaddr); > } My only worry overall is if there can be some workload where this may actually slow down userland if the CPU cache is very large and userland would access most of the faulted in memory after the first fault. So I wouldn't mind to add one more check in addition of !ARCH_HAS_USER_NOCACHE above to check a runtime sysctl variable. It'll waste a cacheline yes but I doubt it's measurable compared to the time it takes to do a >=2M hugepage copy. Furthermore it would allow people to benchmark its effect without having to rebuild the kernel themself. All other patches looks fine to me. Thanks! Andrea -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org