From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with SMTP id A5BEC6B003D for ; Sat, 28 Mar 2009 05:53:51 -0400 (EDT) Message-ID: <49CDF3CB.8070000@redhat.com> Date: Sat, 28 Mar 2009 12:54:19 +0300 From: Avi Kivity MIME-Version: 1.0 Subject: Re: [PATCH 1/2] x86/mm: maintain a percpu "in get_user_pages_fast" flag References: <49CD37B8.4070109@goop.org> <49CD9E25.2090407@redhat.com> <49CDAF17.5060207@goop.org> In-Reply-To: <49CDAF17.5060207@goop.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Jeremy Fitzhardinge Cc: Nick Piggin , Linux Kernel Mailing List , Linux Memory Management List , the arch/x86 maintainers List-ID: Jeremy Fitzhardinge wrote: >>> @@ -255,6 +260,10 @@ int get_user_pages_fast(unsigned long start, >>> int nr_pages, int write, >>> * address down to the the page and take a ref on it. >>> */ >>> local_irq_disable(); >>> + >>> + cpu = smp_processor_id(); >>> + cpumask_set_cpu(cpu, in_gup_cpumask); >>> + >> >> This will bounce a cacheline, every time. Please wrap in CONFIG_XEN >> and skip at runtime if Xen is not enabled. > > Every time? Only when running successive gup_fasts on different cpus, > and only twice per gup_fast. (What's the typical page count? I see > that kvm and lguest are page-at-a-time users, but presumably direct IO > has larger batches.) Databases will often issue I/Os of 1 or 2 pages. But not regressing kvm should be sufficient motivation. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain. -- 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