Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757692AbZC1FBY (ORCPT ); Sat, 28 Mar 2009 01:01:24 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751661AbZC1FBO (ORCPT ); Sat, 28 Mar 2009 01:01:14 -0400 Received: from gw.goop.org ([64.81.55.164]:34983 "EHLO mail.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751438AbZC1FBN (ORCPT ); Sat, 28 Mar 2009 01:01:13 -0400 Message-ID: <49CDAF17.5060207@goop.org> Date: Fri, 27 Mar 2009 22:01:11 -0700 From: Jeremy Fitzhardinge User-Agent: Thunderbird 2.0.0.21 (X11/20090320) MIME-Version: 1.0 To: Avi Kivity CC: Nick Piggin , Linux Kernel Mailing List , Linux Memory Management List , the arch/x86 maintainers 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> In-Reply-To: <49CD9E25.2090407@redhat.com> X-Enigmail-Version: 0.95.6 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1678 Lines: 40 Avi Kivity wrote: > Jeremy Fitzhardinge wrote: >> get_user_pages_fast() relies on cross-cpu tlb flushes being a barrier >> between clearing and setting a pte, and before freeing a pagetable page. >> It usually does this by disabling interrupts to hold off IPIs, but >> some tlb flush implementations don't use IPIs for tlb flushes, and >> must use another mechanism. >> >> In this change, add in_gup_cpumask, which is a cpumask of cpus currently >> performing a get_user_pages_fast traversal of a pagetable. A cross-cpu >> tlb flush function can use this to determine whether it should hold-off >> on the flush until the gup_fast has finished. >> >> @@ -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.) Alternatively, it could have per-cpu flags and the other side could construct the mask (I originally had that, but this was simpler). J -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/