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

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Message-ID: <54352545.8060605@gmail.com>
Date:	Wed, 08 Oct 2014 19:51:33 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	mingo@...hat.com, peterz@...radead.org
CC:	linux-kernel@...r.kernel.org, riel@...hat.com, tkhai@...dex.ru
Subject: Re: [PATCH] sched/fair: Care divide error in update_task_scan_period()


于 10/8/14, 2:43 PM, Yasuaki Ishimatsu 写道:
> 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.
>   By this patch, when both of private and shared are set to 0, diff
>   is just set to 0, not calculating the division.
>
> Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
> ---
>  kernel/sched/fair.c | 30 +++++++++++++++++++-----------
>  1 file changed, 19 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index bfa3c86..fb7dc3f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1496,18 +1496,26 @@ static void update_task_scan_period(struct task_struct *p,
>  			slot = 1;
>  		diff = slot * period_slot;
>  	} else {
> -		diff = -(NUMA_PERIOD_THRESHOLD - ratio) * period_slot;
> +		if (unlikely((private + shared) == 0))
> +			/*
> +			 * This is a rare case. The trigger is node offline.
> +			 */
> +			diff = 0;
> +		else {
> +			diff = -(NUMA_PERIOD_THRESHOLD - ratio) * period_slot;
>
> -		/*
> -		 * Scale scan rate increases based on sharing. There is an
> -		 * inverse relationship between the degree of sharing and
> -		 * the adjustment made to the scanning period. Broadly
> -		 * speaking the intent is that there is little point
> -		 * scanning faster if shared accesses dominate as it may
> -		 * simply bounce migrations uselessly
> -		 */
> -		ratio = DIV_ROUND_UP(private * NUMA_PERIOD_SLOTS, (private + shared));
> -		diff = (diff * ratio) / NUMA_PERIOD_SLOTS;
> +			/*
> +			 * Scale scan rate increases based on sharing. There is
> +			 * an inverse relationship between the degree of sharing
> +			 * and the adjustment made to the scanning period.
> +			 * Broadly speaking the intent is that there is little
> +			 * point scanning faster if shared accesses dominate as
> +			 * it may simply bounce migrations uselessly
> +			 */
> +			ratio = DIV_ROUND_UP(private * NUMA_PERIOD_SLOTS,
> +							(private + shared));
> +			diff = (diff * ratio) / NUMA_PERIOD_SLOTS;
> +		}
>  	}

How about just

ratio = DIV_ROUND_UP(private * NUMA_PERIOD_SLOTS, (private + shared + 1));


Regards,
Wanpeng Li

>  	p->numa_scan_period = clamp(p->numa_scan_period + diff,

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