Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754241Ab3H1RLL (ORCPT ); Wed, 28 Aug 2013 13:11:11 -0400 Received: from mx1.redhat.com ([209.132.183.28]:49279 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753104Ab3H1RLJ (ORCPT ); Wed, 28 Aug 2013 13:11:09 -0400 Message-ID: <521E2F03.6040806@redhat.com> Date: Wed, 28 Aug 2013 13:10:27 -0400 From: Rik van Riel User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130625 Thunderbird/17.0.7 MIME-Version: 1.0 To: Peter Zijlstra CC: Don Morris , Mel Gorman , Srikar Dronamraju , Ingo Molnar , Andrea Arcangeli , Johannes Weiner , Linux-MM , LKML Subject: Re: [PATCH -v3] sched, numa: Use {cpu, pid} to create task groups for shared faults References: <1373901620-2021-1-git-send-email-mgorman@suse.de> <20130730113857.GR3008@twins.programming.kicks-ass.net> <20130731150751.GA15144@twins.programming.kicks-ass.net> <51F93105.8020503@hp.com> <20130802164715.GP27162@twins.programming.kicks-ass.net> <20130828164100.GS10002@twins.programming.kicks-ass.net> In-Reply-To: <20130828164100.GS10002@twins.programming.kicks-ass.net> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1898 Lines: 55 On 08/28/2013 12:41 PM, Peter Zijlstra wrote: > On Fri, Aug 02, 2013 at 06:47:15PM +0200, Peter Zijlstra wrote: >> Subject: sched, numa: Use {cpu, pid} to create task groups for shared faults >> From: Peter Zijlstra >> Date: Tue Jul 30 10:40:20 CEST 2013 >> >> A very simple/straight forward shared fault task grouping >> implementation. >> >> Signed-off-by: Peter Zijlstra > > So Rik and me found a possible issue with this -- although in the end it > turned out to be a userspace 'feature' instead. > > It might be possible for a COW page to be 'shared' and thus get a > last_cpupid set from another process. When we break cow and reuse the > now private and writable page might still have this last_cpupid and thus > cause a shared fault and form grouping. > > Something like the below resets the last_cpupid field on reuse much like > fresh COW copies will have. > > There might be something that avoids the above scenario but I'm too > tired to come up with anything. I believe this is a real bug. It can be avoided by either -1ing out the cpupid like you do, or using the current process's cpupid, when we re-use an old page in do_wp_page. Acked-by: Rik van Riel > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -2730,6 +2730,9 @@ static int do_wp_page(struct mm_struct * > get_page(dirty_page); > > reuse: > + if (old_page) > + page_cpupid_xchg_last(old_page, (1 << LAST_CPUPID_SHIFT) - 1); > + > flush_cache_page(vma, address, pte_pfn(orig_pte)); > entry = pte_mkyoung(orig_pte); > entry = maybe_mkwrite(pte_mkdirty(entry), vma); > -- All rights reversed -- 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/