From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail203.messagelabs.com (mail203.messagelabs.com [216.82.254.243]) by kanga.kvack.org (Postfix) with ESMTP id E046D6B003D for ; Sat, 28 Mar 2009 01:01:12 -0400 (EDT) Message-ID: <49CDAF17.5060207@goop.org> Date: Fri, 27 Mar 2009 22:01:11 -0700 From: Jeremy Fitzhardinge 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> In-Reply-To: <49CD9E25.2090407@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Avi Kivity Cc: Nick Piggin , Linux Kernel Mailing List , Linux Memory Management List , the arch/x86 maintainers List-ID: 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, 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