Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753834AbaJVHJV (ORCPT ); Wed, 22 Oct 2014 03:09:21 -0400 Received: from mail-pd0-f181.google.com ([209.85.192.181]:53651 "EHLO mail-pd0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750769AbaJVHJU (ORCPT ); Wed, 22 Oct 2014 03:09:20 -0400 Message-ID: <5447580E.3030603@gmail.com> Date: Wed, 22 Oct 2014 15:09:02 +0800 From: Wanpeng Li User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: Yasuaki Ishimatsu , mingo@redhat.com, peterz@infradead.org CC: riel@redhat.com, tkhai@yandex.ru, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3] sched/fair: Care divide error in update_task_scan_period() References: <54475703.8000505@jp.fujitsu.com> In-Reply-To: <54475703.8000505@jp.fujitsu.com> Content-Type: text/plain; charset=ISO-2022-JP Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 10/22/14, 3:04 PM, Yasuaki Ishimatsu: > While offling node by hot removing memory, the following divide error > occurs: > > divide error: 0000 [#1] SMP > [...] > Call Trace: > [...] handle_mm_fault > [...] ? try_to_wake_up > [...] ? wake_up_state > [...] __do_page_fault > [...] ? do_futex > [...] ? put_prev_entity > [...] ? __switch_to > [...] do_page_fault > [...] page_fault > [...] > RIP [] task_numa_fault > RSP > > The issue occurs as follows: > 1. When page fault occurs and page is allocated from node 1, > task_struct->numa_faults_buffer_memory[] of node 1 is > incremented and p->numa_faults_locality[] is also incremented > as follows: > > o numa_faults_buffer_memory[] o numa_faults_locality[] > NR_NUMA_HINT_FAULT_TYPES > | 0 | 1 | > ---------------------------------- ---------------------- > node 0 | 0 | 0 | remote | 0 | > node 1 | 0 | 1 | locale | 1 | > ---------------------------------- ---------------------- > > 2. node 1 is offlined by hot removing memory. > > 3. When page fault occurs, fault_types[] is calculated by using > p->numa_faults_buffer_memory[] of all online nodes in > task_numa_placement(). But node 1 was offline by step 2. So > the fault_types[] is calculated by using only > p->numa_faults_buffer_memory[] of node 0. So both of fault_types[] > are set to 0. > > 4. The values(0) of fault_types[] pass to update_task_scan_period(). > > 5. numa_faults_locality[1] is set to 1. So the following division is > calculated. > > static void update_task_scan_period(struct task_struct *p, > unsigned long shared, unsigned long private){ > ... > ratio = DIV_ROUND_UP(private * NUMA_PERIOD_SLOTS, (private + shared)); > } > > 6. But both of private and shared are set to 0. So divide error > occurs here. > > The divide error is rare case because the trigger is node offline. > This patch always increments denominator for avoiding divide error. > > Signed-off-by: Yasuaki Ishimatsu Reviewed-by: Wanpeng Li > --- > v2: > - Simply increment a denominator > > kernel/sched/fair.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 0b069bf..f3b492d 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -1520,7 +1520,7 @@ static void update_task_scan_period(struct task_struct *p, > * scanning faster if shared accesses dominate as it may > * simply bounce migrations uselessly > */ > - ratio = DIV_ROUND_UP(private * NUMA_PERIOD_SLOTS, (private + shared)); > + ratio = DIV_ROUND_UP(private * NUMA_PERIOD_SLOTS, (private + shared + 1)); > diff = (diff * ratio) / NUMA_PERIOD_SLOTS; > } > -- 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/