linux-kernel - Re: [PATCH v3] sched/fair: Care divide error in update_task_scan

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Date:	Wed, 22 Oct 2014 15:09:02 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	mingo@...hat.com, peterz@...radead.org
CC:	riel@...hat.com, tkhai@...dex.ru, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] sched/fair: Care divide error in update_task_scan_period()

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  [<ffffffff810a7081>] task_numa_fault
>    RSP <ffff88084eb2bcb0>
>
> 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 <isimatu.yasuaki@...fujitsu.com>

Reviewed-by: Wanpeng Li <wanpeng.li@...ux.intel.com>

> ---
> 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;
>  	}
>

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