From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with SMTP id 5A3D06B003D for ; Wed, 18 Mar 2009 19:04:46 -0400 (EDT) Message-ID: <49C17E22.9040807@redhat.com> Date: Thu, 19 Mar 2009 01:05:06 +0200 From: Avi Kivity MIME-Version: 1.0 Subject: Re: Question about x86/mm/gup.c's use of disabled interrupts References: <49C148AF.5050601@goop.org> <49C16411.2040705@redhat.com> <49C1665A.4080707@goop.org> <49C16A48.4090303@redhat.com> <49C17230.20109@goop.org> <49C17880.7080109@redhat.com> <49C17BD8.6050609@goop.org> In-Reply-To: <49C17BD8.6050609@goop.org> Content-Type: text/plain; charset=ISO-8859-1; 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 , Xen-devel , Jan Beulich , Ingo Molnar List-ID: Jeremy Fitzhardinge wrote: > Avi Kivity wrote: >>> Hm, awkward if flush_tlb_others doesn't IPI... >>> >> >> How can it avoid flushing the tlb on cpu [01]? It's it's >> gup_fast()ing a pte, it may as well load it into the tlb. > > xen_flush_tlb_others uses a hypercall rather than an IPI, so none of > the logic which depends on there being an IPI will work. Right, of course, that's what we were talking about. I thought optimizations to avoid IPIs if an mm never visited a cpu. > >>> Simplest fix is to make gup_get_pte() a pvop, but that does seem >>> like putting a red flag in front of an inner-loop hotspot, or >>> something... >>> >>> The per-cpu tlb-flush exclusion flag might really be the way to go. >> >> I don't see how it will work, without changing Xen to look at the flag? >> >> local_irq_disable() is used here to lock out a remote cpu, I don't >> see why deferring the flush helps. > > Well, no, not deferring. Making xen_flush_tlb_others() spin waiting > for "doing_gup" to clear on the target cpu. Or add an explicit notion > of a "pte update barrier" rather than implicitly relying on the tlb > IPI (which is extremely convenient when available...). Pick up a percpu flag from all cpus and spin on each? Nasty. You could use the irq enabled flag; it's available and what native spins on (but also means I'll need to add one if I implement this). -- 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