Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752787AbaJTHsN (ORCPT ); Mon, 20 Oct 2014 03:48:13 -0400 Received: from fgwmail6.fujitsu.co.jp ([192.51.44.36]:59541 "EHLO fgwmail6.fujitsu.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752582AbaJTHsI (ORCPT ); Mon, 20 Oct 2014 03:48:08 -0400 X-SecurityPolicyCheck: OK by SHieldMailChecker v2.0.1 X-SHieldMailCheckerPolicyVersion: FJ-ISEC-20120718-3 Message-ID: <5444BE13.8050603@jp.fujitsu.com> Date: Mon, 20 Oct 2014 16:47:31 +0900 From: Yasuaki Ishimatsu User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: , CC: , , , Subject: Re: [PATCH v2] sched/fair: Care divide error in update_task_scan_period() References: <543F945F.4020303@jp.fujitsu.com> In-Reply-To: <543F945F.4020303@jp.fujitsu.com> Content-Type: text/plain; charset="ISO-2022-JP" Content-Transfer-Encoding: 7bit X-SecurityPolicyCheck-GC: OK by FENCE-Mail Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Could you review this patch? (2014/10/16 18:48), Yasuaki Ishimatsu wrote: > 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. > By this patch, when both of private and shared are set to 0, > denominator is set to 1 for avoiding divide error. > > Signed-off-by: Yasuaki Ishimatsu > CC: Wanpeng Li > CC: Rik van Riel > CC: Peter Zijlstra > --- > kernel/sched/fair.c | 11 ++++++++++- > 1 file changed, 10 insertions(+), 1 deletion(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index bfa3c86..580fc74 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -1466,6 +1466,7 @@ static void update_task_scan_period(struct task_struct *p, > > unsigned long remote = p->numa_faults_locality[0]; > unsigned long local = p->numa_faults_locality[1]; > + unsigned long total_faults = shared + private; > > /* > * If there were no record hinting faults then either the task is > @@ -1496,6 +1497,14 @@ static void update_task_scan_period(struct task_struct *p, > slot = 1; > diff = slot * period_slot; > } else { > + /* > + * This is a rare case. total_faults might become 0 after > + * offlining node. In this case, total_faults is set to 1 > + * for avoiding divide error. > + */ > + if (unlikely(total_faults == 0)) > + total_faults = 1; > + > diff = -(NUMA_PERIOD_THRESHOLD - ratio) * period_slot; > > /* > @@ -1506,7 +1515,7 @@ 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, (total_faults)); > 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/