From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754708AbZC1Mif (ORCPT ); Sat, 28 Mar 2009 08:38:35 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752882AbZC1MiZ (ORCPT ); Sat, 28 Mar 2009 08:38:25 -0400 Received: from casper.infradead.org ([85.118.1.10]:44103 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751686AbZC1MiY (ORCPT ); Sat, 28 Mar 2009 08:38:24 -0400 Subject: Re: [PATCH 1/2] x86/mm: maintain a percpu "in get_user_pages_fast" flag From: Peter Zijlstra To: Eric Dumazet Cc: Jeremy Fitzhardinge , Avi Kivity , Nick Piggin , Linux Kernel Mailing List , Linux Memory Management List , the arch/x86 maintainers In-Reply-To: <49CDD7B4.4020701@cosmosbay.com> References: <49CD37B8.4070109@goop.org> <49CD9E25.2090407@redhat.com> <49CDAF17.5060207@goop.org> <49CDD7B4.4020701@cosmosbay.com> Content-Type: text/plain; charset="UTF-8" Date: Sat, 28 Mar 2009 13:31:58 +0100 Message-Id: <1238243518.4039.725.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.26.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 2009-03-28 at 08:54 +0100, Eric Dumazet wrote: > Jeremy Fitzhardinge a écrit : > > 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.) > > If I am not mistaken, shared futexes where hitting hard mm semaphore. > Then gup_fast was introduced in kernel/futex.c to remove this contention point. > Yet, this contention point was process specific, not a global one :) > > And now, you want to add a global hot point, that would slow > down unrelated processes, only because they use shared futexes, thousand > times per second... Yet another reason to turn all this virt muck off :-) I just wish I could turn off the paravirt code impact, it makes finding functions in the x86 code a terrible pain. > > Alternatively, it could have per-cpu flags and the other side could > > construct the mask (I originally had that, but this was simpler). > > Simpler but would be a regression for legacy applications still using shared > futexes (because statically linked with old libc) Still doesn't help those apps that really use shared futexes.